Testing a Flyback Transformer (Leader LBO-302 Oscilloscope)

[Smoky]

My old Leader oscilloscope's CRT went dark a few years ago.

I recently opened up the case to take a look inside.

A test point on on the main board should measure -1500v, but it doesn't, it tops out at -675v.

I'm hoping to find a way of testing the Flyback transformer to clear it of any wrongdoing.

Here's what I've done so far:

I tested the the two Primary windings of the transformer with a Sencore Z-meter for Inductance value and the ringing test.

The Primary Yellow and Green wires measured 2.39 milihenrys (uH) and the ringing test display read 5-7.

The Primary Red and Orange wires measured 1.8 microhenrys (mH) and the ringing test display read 2-4.

Next was the Secondary winding. Since there is a capacitor in-line in the Secondary winding circuit, I imagine any resistance test would fail with a DMM, and it does. I next did a capacitance value test.

I attached one of the Sencore test leads to the HV Red output wire and the other to the input leg of the only visible component of the flyback, which is a 22K resistor close to ground. The Value read 3020pf. I imagine the Sencore can see the three 1000pf caps in the Secondary circuit.

Am I doing the tests the right way? And are there any other tests that I can perform to confirm the diodes are Ok?

All of the components of the Flyback transformer are within the dotted lines of the schematic and are encased in epoxy, except for that lonely resistor.

I appreciate the help!
 

[strawberry]

short circuit will give near zero inductance
check load circuit components - capacitors.. ..

[james_s]

Put together a 555 based fkyback driver, a single transistor and feed it with 12V or so, see if you get output. You can also just connect the primary to a signal generator and give it 10-20kHz.

[Smoky]

Thank you James and Strawberry!

James, I set the signal generator to 1 Vrms and 10 Khz  and connected it to the primary winding (the Green and Yellow wires).

The DMM read -21.1 volts DC on the HV Red wire.

I then raised the amplitude of the generator to 2 Vrms and the voltage went beyond -90 volts DC.

...and if I raised the frequency, the flyback output voltage will continue to climb.

How confident are you, from 1-10, that this transformer is Ok?

[james_s]

It could still be arcing at full voltage but I think you'd probably know if that happened. I'd say there's a good chance that the transformer is ok.

[bsfeechannel]

I'll hazard a guess. If C406 is shorted out or leaking (electronically speaking), you don't have a voltage doubler anymore, which would explain -675V instead of the nominal -1500V.

I estimate the frequency of this circuit to be about 70KHz, which is the frequency where the impedance of C420 is 2.2k ( << R428's 22k ).

[Smoky]

Let me throw this out here.

Take a look at these pictures. When I said that the flyback components were encased in epoxy, that's not correct.

All of the coil wires are outside of that black plastic "tray" the flyback is attached to but that red material is a soft rubber or even silicone.

I see three wires going into that "tray" from below, the HV Red, a black shielded wire from the coil, and a brown shielded wire going to ground. Also, the "rubber" material doesn't stick well to the rim of the tray as you can see in the photo. I can easily peel it away.

In the last picture, you can see the black wire from the coil going into the tray. Notice how the rubber doesn't even stick to the wires shielding.

What I'm saying is, that if need be, I'm almost 100% positive I can expose those hidden components fairly easily.

Wouldn't that be something? possibly replace the bad components and seal it back up.

I absolutely love this kind of stuff

[strawberry]

transformer output current is in some uA range, it is easy to load it with multimeter and show less than it suposed to.
brightness and focuss wont be consistent, if there is arcing somrwhere
1k is not HV but 10k is and will blow holes in silicone.. if there is a weak spot

[Smoky]

Thank you Strawberry.

Being this is a hobby of mine, I'm going to spend the next few evenings and spare time to re-cap this little guy.

I'll also go through and test transistors and resistors.

I have NOS power transistors to freshen this thing up too.

From there, and I'm guessing it may be best, to power up the scope with the flyback installed but without the CRT connected.

That way I can check all of the test points throughout the machine for proper voltages.

I'll report back as progress is made.

[floobydust]

Any Marcon oil capacitors will have failed. They are common in the HV section on Japanese scopes. Possibly C417 2,000WV 0.022uF (LBO301).

I would never run it with the CRT disconnected unless you think the HV multiplier has a bad diode. Then the CRT anode capacitance just loads down the flyback due to AC.
You could disable the flyback portion to check the main power supply voltages and regulators, maybe using a variac. The +150V reg is important and a shorted pass transistor there can damage many deflection transistors. A CRT is tough and voltages being off is not a worry.

But a flyback+multiplier operating with no load, the voltage will climb up until something arcs.

[Smoky]

Thank you FloobyDust!

I purchased a 3Kv Kemet 5% metallized film capacitor to replace that 2Kv "oil" cap. It's physically the same size too.

Would you have a link to one of those Flyback multipliers?

I will follow your advice.

Also, I took voltage readings from the test points on the main board before I dissected the scope. The second picture shows the differences from what the service manual states. The +150v and +130v test points read high.

[floobydust]

I don't have the PSU schematic for an LBO-302. The 150V rail might be unregulated? I would confirm the input voltage switch is set to 120VAC, not 100VAC for Japan.
The Korean's copied the Japanese scopes, and they have a regulator on the 160V rail.

For a scope HV multiplier adventure: https://www.eevblog.com/forum/repair/tektronix-sc504-hv-multiplier/

Troubles on 160V (deflection) rail: https://www.eevblog.com/forum/repair/meguro-mo-1252a-25mhz-analog-oscilloscope-nothing-on-crt/

[Smoky]

FloobyDust,

Here is a snapshot of the schematic right off of the power transformer.

Let me say this, the selector on the back of the scope is for 117 volts.

I have been monitoring the voltage at the wall in my house for the past few months. It averages between 120-122 volts.

Looking at the schematic, would I need to adjust the resistors that are near the test points to bring down the voltage?

As for the three 50uf 250v capacitors in that circuit, they are enclosed in one canister. I purchased individual 47uf 350v replacement capacitors.

Would the process be, replace the caps, check the test point, then adjust the resistors?

[strawberry]

without crt tube there is no current for dropout resistors
old large size electrolytic capacitors sometimes could be fine
manual 105..125V~ is recomended input voltage range

[Smoky]

Check this out!

Do you see R316 the 470 Ohm resistor in the power transformer schematic above?

It goes to "R" in a diamond shaped box.

That 3 watt resistor was super-hot and remains disconnected and was the indicator to me that a short is somewhere in this scope.

Can you believe this? look where "R" leads to...

...to the HV Oscillator board feeding the Flyback transformer!

The Oscillator board is being fed possibly a higher voltage (~176.5v) instead of 150v.

Now look at the broader picture of the schematic of the Flyback transformer circuit below. "R" comes in from the upper left.

Please tell me we're on to something here.

Anyway, I never assembled anything similar to a Heathkit so trying to fix this older equipment is my chance to practice my soldering and troubleshooting

Let me get these capacitors swapped-out and everything back together. I'll slowly bring it up on the variac too.

[strawberry]

C302 should be 180VDC
150V 90mA resistor disipate 2.7W
130V 22mA resistor disipate 1.1W

[Smoky]

So if I could turn this whole thread back around, here were the symptoms prior to any disassembly of this scope:

No CRT display.

Led above on/off switch illuminates.

Extremely hot R316 470 Ohm 3 watt resistor on the main board just beyond the power transformer and before the HV Oscillator board.

Voltages read at test points.

Majority of transistors tested using diode setting on DMM and they pass (power transistors tested out of circuit).

And from the comments, voltages could be high due to CRT being inoperative. But, -675v read at -1500v test point (TP402) could point to C406 voltage-doubler failure in Flyback circuit.

The Sencore LC53 read 3020pf when performing a capacitance test on the Secondary flyback winding.

Is it possible to do a 600v "leakage" test on the Secondary winding? If so, I can.

Thank you Strawberry.

The original .022uf 2Kvdc "oil" capacitor will be removed and tested in a few minutes too.

** OK, here are the results from the Sencore Z meter. The original .022uf "oil" capacitor tested good at 22,800pf (.0228uf) and showed no leakage at 600 volts. The new Kemet capacitor read 22,300pf (.0223uf) also with no leakage at 600 volts. Both capacitors are well within 5% of value too.

And the last picture shows the Kemet capacitor installed, and also you can see the large 3 watt 470 Ohm metal oxide resistor (R316) that gets really hot on the main board.

[james_s]

Even if it was potted in epoxy stuff is still occasionally repairable. I repaired a couple dozen epoxy potted HeNe laser power supplies. Initially I was doing a more complete de-potting but then on those with a common fault resulting from cracked solder joints I worked out that I could mill a small pocket and dig out a thin layer of epoxy to expose the joints.

Those used a TO-3 HOT for its HV rating but at a low current so no heatsinking is required. They installed the transistor about 1/2" above the PCB with some components under it and thermal expansion of the epoxy would eventually crack the joints.

[Smoky]

Thank you James.

You're building up the confidence!

Here's a picture showing how soft the material is. My Exacto knife easily squishes it

This hobby is so much fun, I'm loading up my truck for work tomorrow in between trips to my solder table!

[james_s]

Yeah that looks like silicone potting, it's quite easy to dig that stuff out. If you need to I can xray it for you, I do that sometimes to avoid slicing into anything, otherwise you just have to be careful and go slowly.

[Smoky]

Thank you James!

I wholeheartedly feel that this potting stuff will come out very easily too.

It doesn't stick to what appears to be a black fiberglass tray and it doesn't stick to the PVC jacketed wires either.

I think odds are it doesn't stick well to components unless they're porous.

I'm tempted to start uncovering what's in there if you guys think this is where the problem lies.

I think the potting material that is poured back into the tray is available in a clear formula too.

[Smoky]

I got as far tonight as to remove the can that housed the three 47uf 250v capacitors.

It's a Nippon Chemi-Con branded capacitor.

The Sencore Z meter thoroughly measured each of the capacitor cores:

A- 61.7uf and ~210ua of leakage
B- 61.8uf and ~190ua of leakage
C- 59.4uf and ~180ua of leakage

The leakage test was done at the 200 volt level, so I image going higher would have made the leakage numbers worse.

Anyway, I didn't expect to see such high values for 47uf caps nor the higher leakage numbers below rated voltage.

The leakage button was depressed until the numbers went flat (~60 seconds).

The new 47uf 350v Nichicon UCY capacitors have zero leakage and average 43uf in value.

Interesting too is that the rest of the smaller electrolytic capacitors are also the Nippon Chemi-Con brand type "SL". I went onto their website and the SL 85C version is now discontinued.

[bsfeechannel]

I couldn't find a schematic for the LBO-402, so I'll use the one that's available on the web for the LBO-401, which, I presume, must be similar, regarding HV.



Below we have a block diagram of the HV circuit.



Q406 generates a constant current that flows through R in the block diagram, which corresponds to R420 + VR404 + R419 in the schematic. This creates a constant voltage across R. The voltage on the base of Q403 (point A in the block diagram) will be around 2V.

If the voltage at the HV output gets more positive than -1500V, point A will tend to be more positive which will make the oscillator increase its amplitude, making the HV more negative until it returns to -1500V. If it gets more negative than -1500V, point A will tend to be more negative, decreasing the amplitude of the oscillator.

What would cause the HV be more positive than -1500V? Anything with a path to ground or to a more positive voltage. C408 and C417 should be seen as suspects. It seems that C417 has already been replaced. Q405 should also be inspected, as well as the CRT itself. There could be a short circuit between the filament or the cathode and other more positive element, like the astig or focus anodes.

Finally, the transformer and its potted voltage doubler. C406 could be the culprit but also D401 and D402, if they are leaky.

What I would do? I would remove the CRT, Q405 and the jumper between R417 and D406. This will isolate the HV circuit from its loads. Then, before turning on the oscilloscope, I would turn VR401 so that the voltage at the base of Q406 would be +27V when the scope is powered. This will make the HV be at its minimum, perhaps even at zero volts, since Q402 and Q403 would be cut off.

I would then turn VR401 gradually until, say, 325V at the TP 402. That would give exactly 100µA through R. Then I'll measure the voltage across R417. It should be 39V. It it is more than that, especially much more than that, C408 is leaky and should be replaced.

If C408 is OK, I'd be turning VR401 until the voltage across R430 (attached to the emitter of Q406) reaches 15V (noticed how R430 is approx. R = 3256000 ohms divided by 100?). That should give you around 1500V at TP-402. If it doesn't, then to check the forward path between the base of Q403 (point A) and TP-402. If Q401-Q403 and the components around them are OK, its time to seriously consider to check the voltage doubler.

Since R427 is not potted, it is easy to check if one of the capacitors are leaking. Nothing must pass between ground and the point of contact between the resistor and the transformer. If the capacitors are OK, it is possible to check the diodes by applying some negative voltage between the thick red wire and ground and measuring the current. This test however measures the two diodes in series and can only detect if both are cactus. If one is OK and the other is shot, then only surgery on that potted doubler would tell.

[strawberry]

isn't C403 resonating with transformer winding. if oscilator frequency change and votage will change

[Smoky]

I can't thank you guys enough for the help with this scope

Yesterday after work, I was able to replace 11 small electrolytic capacitors.

I tested every single Nippon Chemi-con "SL" capacitor as they were removed, 1uf, 22uf, 4.7uf, 10uf... every cap showed leakage and they also took at least 35 seconds for uA numbers to recede during the leakage test. Also, their values have been between 15-35%. I know that this may not be the problem but re-capping sure should help. Plus the values on the new capacitors are closer to the schematic's stated values.

In my short career in electronics (~9 months) I rely on using only the following 105C electrolytic capacitor families from Digi-Key when working on projects: Nichicon UCY, UHE, UPM, and UPW, along with Rubycon YXF, YXJ, ZLG, and ZLJ.

I have 12 more capacitors to replace this afternoon and some are very difficult to access. There are no connectors between boards, only wires are used. Many notes and pictures are taken.

I will also post an ad in Raleigh's Craigslist in hopes of finding someone able to test the CRT

**Capacitor C601** is a 3.3uf at 50v on the schematic. What I found installed and that I removed were two capacitors, one on each side of the board and in parallel, a 2.2uf and a 4.7uf. I did purchase 3.3uf capacitors for this location. Is this the right move? This capacitor is located in the trigger sweep section on the main board.

I will probably install the 3.3uf first and go by the schematic, if something acts up, I will return the combination of the 2.2uf and 4.7uf capacitors.

[floobydust]

Nobody can test a scope CRT. You need a lot of support signals and best to just do it in the scope, operating.
It will be good to replace the old electrolytics and see if anything changes or improves. Takes patience and pictures to remember how the old ones were  installed in case you get distracted. Phone rings and I forget which cap was where.

Your one pic has a 47 ohm resistor that looks a little burnt, which one is it.

[Smoky]

Here is Capacitor C408.

Since it is near the electrolytics that I'm changing-out, I removed it from the board and tested it on the Sencore.

It reads 1230pf and showed no leakage at 600 volts.

I'm guessing that it is a 20% tolerance capacitor (a Panasonic)?

Since I have a small order to place with Digi-Key, and I also see C410 and C418 right behind it (470pf 3Kv), I was going to order new 5% X7R ceramics to replace the three.

[Smoky]

I just want say one thing about this Sencore LC53 Z Meter.

Last month I opened it up and cleaned every switch and every contact. I checked every wire that had a crimp terminal in a connector too. I followed that up by coating every contact and switch with Deoxit GX5.

I then replaced every single electrolytic capacitor.

I did not touch any calibration knobs. I felt that this machine was calibrated prior to being put on the store shelves 30 years ago and when its internal parts were new

I purchased two capacitors from Digi-Key, a 100pf 1% Mica and a 1uf 10% X7T ceramic.

Every time I use the Z-meter, I test these caps. The 1uf cap always reads 1.03-1.05uf and the 100pf cap reads between 99 and 100pf.

...now back to work!

[Smoky]

FloobyDust, are you speaking of this one?

If so, this is R316 , a 3 watt 470 Ohm resistor in the power supply circuit.

This is the resistor that indicated to me a problem. It gets really hot!

I disconnected it.

It is located just before the HV Oscillator board.

[Smoky]

I think I found something very important

I desoldered two electrolytic capacitors from the main board. I looked on the schematic and I couldn't find their location by value.

So I went back and plotted where these traces were. The locations were for capacitors C311 and C315 in the Power Supply.

I went to the Parts List in the service manual. The service manual identified these capacitors as both being 0.1uf 50v 10% plastic film capacitors.

What do you think was soldered into those spots?  3.3uf 50 volt electrolytics!

Take a look on the schematic below where these capacitors are located (C311 & C315).

These caps were tied to the emitters of Q301 and Q304 (a pair of 2SD315's).

I marked the (-) symbol on the PC board to show you how the caps were installed since one is tied to the -27v rail and the other to the +27v rail.

Maybe if they needed a higher value capacitor, wouldn't using a film capacitor be better than an electrolytic since the original design called for such?

I want to quit my day job and do this stuff from now on

[bsfeechannel]

FloobyDust, are you speaking of this one?

If so, this is R316 , a 3 watt 470 Ohm resistor in the power supply circuit.

This is the resistor that indicated to me a problem. It gets really hot!

I disconnected it.

It is located just before the HV Oscillator board.

R316 is running hot perhaps because the oscillator is probably working like crazy to lower the voltage at TP 402 from -675V to -1500V. Q402 and Q403 are probably running saturated.

This indicates that something is either pulling TP 402 up, or preventing the oscillator to provide the adequate voltage.

I will also post an ad in Raleigh's Craigslist in hopes of finding someone able to test the CRT

As floobydust hinted, there are no CRT testers out there. The best way to test a CRT is using the scope itself. However, if you want to test it outside the scope, you can perform some simple tests yourself.

The most important one is the filament continuity test. If your CRT is the same as used in the LBO-301, you should see continuity between pins 1 and 14. Apart from those pins no other pin must present any continuity with any other pin. So, start with pin 1 and test it for continuity with all other pins from 2 to 13. Then take pins 2 and all others from 3 to 13, and so on.

Then, you can perform the "emissions" test. Just connect pins 5, 6, 7, 11, 12, 13 together, and 1, 2, 3 also together and apply 1500V between them like in the picture below. Leave pin 4 disconnected. You should see a bright spot with a diameter of 1cm or so on the screen. But I guess that won't be necessary, once you solve the issue with the HV supply.

I want to quit my day job and do this stuff from now on

Oh, my! You need to talk to these people. They may help you with your job-quitting ideation. Maybe not. 

[Smoky]

Thank you for your awesome help BsFeeChannel!

The part number on the CRT is 75ARB1.

I did your quick continuity test, and it passes

Here are a couple close-up pictures of the CRT:

[floobydust]

FloobyDust, are you speaking of this one?

This one 47 ohms looks discoloured compared to its neighbor.

For the (schematic) 0.1uF caps C311, C315 are for the voltage regulator stability. If they were factory upsized, I would leave them at 3.3uF or even 10uF is fine there. Going back to 0.1uF I would not recommend.

[Smoky]

...and get a load of this original 47uf/16v capacitor I removed just moments ago!

I'm not bothering to measure leakage anymore

[Smoky]

FloobyDust,

Here's another shot of where that discolored resistor is, it's tied to the same transistor (2SD315) that is affected by that leaking 3.3uf Lytic capacitor.

That's why I thought it would be best to go back to the schematic requiring the 0.1uf plastic film cap used there instead.

Doesn't a film capacitor give you better insurance of not opening to ground than a electrolytic?

I will lift both of those resistors and test them in a minute.

**Update** both of those resistors measure 47 Ohms. They are R306 and R312 (1/2 watt).

That's it for me tonight, I'll be back tomorrow evening to continue this great adventure!

[james_s]

Analog scope CRTs are really fascinating devices, a whole class of engineering that is now dead.

[Smoky]

BsFeeChannel,

 I came on late tonight because I was just ready to hit the rack, and I thought to myself, let me do another continuity test on the CRT.

Pins 1 and 14 show continuity

...but so do Pins 2 and 9!

But when you look through the glass, you can clearly see Pins 2 and 9 lead to the metal band that encircles the ceramic core surrounding the filament wires.

Please tell me this is so!

...I'm going to lose what little hair I have left

[bsfeechannel]

Oh, I'm sorry. 

I forgot to tell you that pins 2 and 9 are connected internally to the control grid, at least for the 75AKB1, which is the CRT for LBO-301. You can check it in the pin connections section of the service manual.

[james_s]

This is not uncommon, multiple pins going to the same element for mechanical reasons.

[Smoky]

All is going well

I was able to get about two and a half hours in tonight.

I'm down to the last three capacitors, these would be the three 47uf 250v's (C301, C302, C303) in the power supply.

They need to fit into the same space the original canister did.

We should be firing this scope back up in a day or two once I go over everything with a fine tooth comb.

I do have a Tenma 72-1095 Isolation Transformer to bring it back to life slowly.

What will be a good indicator for me is the temperature of resistor R316.

[Smoky]

BsFeeChannel,

 I pulled NPN transistor Q405 (2SC515A) and tested it out of circuit. I checks-out fine.

 I was also thinking of not connecting the flyback circuit upon initial restart of the oscilloscope so to be able to check test point voltages on the main board and compare them to my previous readings.

 Also, here is a picture of the HV Oscillator Board.

 The yellow wire coming in directly centered at the top of the board comes from resistor R316 (3w 470 Ohm from the main board).

 The orange, black, red, green, and yellow wires at the bottom go to the flyback transformer.

[bsfeechannel]

BsFeeChannel,

 I pulled NPN transistor Q405 (2SC515A) and tested it out of circuit. I checks-out fine.

 I was also thinking of not connecting the flyback circuit upon initial restart of the oscilloscope so to be able to check test point voltages on the main board and compare them to my previous readings.

One of the secrets of troubleshooting is to isolate and eliminate variables until you find the culprit.

[Smoky]

Just to let you know what I'm up to besides the Leader LBO-302 oscilloscope.

I've been cleaning, re-capping, and repairing my Sencore SC61 Waveform Analyzer.

About six months ago, I noticed that the screen would flash, and at the same time, the digital voltage display would momentarily read a higher voltage than the circuit I was testing, so I decided to open it up and do the same thing I'm doing now to the Leader scope.

I'm not going to go into much detail but I want you to see the Sencore's HV 11000 Board.

This board generates three legs of -1500 volts without any components encased in potting material.

Not only that, but one -1500v leg goes to a multiplier board which generates -4500 volts, again without potting material.

Both the HV and the multiplier board utilize EDI DL800 8Kv rectifiers.

Can and would the Leader scope's flyback circuit be designed to utilize these same parts for an open-air design?

I love my Sencore SC61, it can swallow almost 3kv with its Lo Cap probes.

...and the bottom picture is the SC61 capturing the "ringing" in my mono tube amplifiers that I repaired last Fall

So I want it to live a long and happy life too!

[Smoky]

The Leader oscilloscope is being brought up on the isolation transformer as I type.

Here are the basics on this restart:

The wires leading to the flyback transformer are disconnected from the HV oscillator board.

No other wires are disconnected.

The socket to the CRT is disconnected.

The scope has been brought up to 120VAC gradually over 10 minutes.

Amperage draw on the Tenma isolation transformer reads .02 amps.

LED above power on switch is illuminated.

**Update**

I took pictures of the voltage results since it is easier to compare.

Next, I will connect the flyback transformer, the HV red wire, and the CRT.

I will be monitoring the R316 resistor to see if it gets warm.

[David Hess]

Tektronix used essential the same circuit in their oscilloscopes.  It is not a flyback converter but more like half of a Royer converter.  Usually the failure is in the voltage multiplier.

[Smoky]

Thank you Dave.

I just finished the second test with the flyback transformer (Royer Converter) and the CRT installed.

I felt resistor R316 getting warm as I ramped-up the isolation transformer.

But this time, I kept a closer eye on test point TP402 (-1500v) and my Fluke DMM.

As I quickly brought the scope up to 120VAC and back down again before the resistor could get warm, the DMM read -785 VDC.

Open Heart Surgery on the Flyback transformer is slated for 7:00 PM Eastern Standard Time tomorrow!

Stay tuned

[Smoky]

 

[Smoky]

Check this out, capacitor C406 was crammed into the corner of the potting tray. You can tell that the parts were soldered above the rim of the tray and then stuffed in afterwards.

The leads were badly kinked, and at the solder joint of C406, only one strand of wire from the coil was barely holding on for dear life! It separated when I lifted it up.

I checked the continuity between that end of the coil (the black wire) to R427 on the opposite end, and it tests good!

All of the remaining solder joints within the potting tray are rock solid.

Would anyone know or have replacement rectifiers for D401 and D402 (3HT-5)? It's a 5000v rectifier. I do not know the detailed specifications.

I have new capacitors and resistors on the way.

Thank you all!

[Smoky]

OMG, both 3HT-5 diodes failed a diode test too!

One diode has 62 stamped on it, the other 64.

Don't tell me these are from the early 1960's

[bsfeechannel]

Can and would the Leader scope's flyback circuit be designed to utilize these same parts for an open-air design?

In theory, yes. But I'm not sure why they decided to pot the voltage doubler. Perhaps it was outsourced and the potting is there to protect the internal parts against improper handling. Or the HV supply is installed in the scope next to other sensitive components. I have no further details about it than those that you showed us.

The scope has been brought up to 120VAC gradually over 10 minutes.

Amperage draw on the Tenma isolation transformer reads .02 amps.

LED above power on switch is illuminated.

**Update**

I took pictures of the voltage results since it is easier to compare.

Next, I will connect the flyback transformer, the HV red wire, and the CRT.

I will be monitoring the R316 resistor to see if it gets warm.

Just a reminder, these waveforms are to be taken when the triggering is set to AUTO. Otherwise you won't have triggering pulses (unless you have a triggered signal coming from the vertical amplifier).

Tektronix used essential the same circuit in their oscilloscopes.  It is not a flyback converter but more like half of a Royer converter.  Usually the failure is in the voltage multiplier.

The circuit on the primary side of the transformer is called tuned-collector oscillator. But, yeah, technically this is not a flyback converter. I guess we can grant the OP some "poetic" license, though. 

The leads were badly kinked, and at the solder joint of C406, only one strand of wire from the coil was barely holding on for dear life! It separated when I lifted it up.

That could have been one of the sources of failure: poor contact.

Would anyone know or have replacement rectifiers for D401 and D402 (3HT-5)? It's a 5000v rectifier. I do not know the detailed specifications.

Origin? A former subsidiary of Philips?

OMG, both 3HT-5 diodes failed a diode test too!

One diode has 62 stamped on it, the other 64.

Don't tell me these are from the early 1960's

I guess it won't be difficult to find replacements for those diodes from the usual suspects (Digikey, Mouser, etc.). Don't quote me on that but I think any high voltage (above 3KV) diode, 10mA+ forward current, less-than-1µS-recovery-time rectifier would do.

Anyway, I'm enjoying your approach to this restoration. I am learning a lot. Looking forward to the next step. 

[floobydust]

I use 2CL70 6kV 5mA Vf=23V, fast recovery 100nsec. HV rectifier diode. You know the forward voltage is really high due to the diode stack, so they fail a multimeter diode-test.
For an autopsy, you can disassemble the 3HT-5 diode stack by pulling the end caps off.

The potting compound is also used to stop corona and ozone buildup, which degrades the local air's dielectric. You can buy small tube of MG Chemicals silicone potting compound.

Stay with carbon comp 22k resistors because of the surge currents and voltages. For the amount of hassle, I would replace all the parts. Even disc capacitors age and go low value.

[bsfeechannel]

I use 2CL70 6kV 5mA Vf=23V, fast recovery 100nsec. HV rectifier diode. You know the forward voltage is really high due to the diode stack, so they fail a multimeter diode-test.
For an autopsy, you can disassemble the 3HT-5 diode stack by pulling the end caps off.

Yeah, very important observation. How did the 3HT-5 fail? If they failed "open", chances are the there was not enough forward voltage from the ohmmeter to turn them on.

[Smoky]

Alright, here's a list of what parts I have or what's on order from Digi-Key:

...and FloobyDust, I couldn't find any diodes matching your spec's or that part number yet, so if you have a supplier near you, please send me their phone number or link, and I'll keep checking though. Thanks!

For the 3Kv 1000pf capacitors, C406, C407, C420, and C408 on the main board, I ordered Kemet's GoldMax 5% 125C X7R's (C330C102JHR5HA). They seem to be built pretty stout (I'll attach a picture of their construction, kind'a neat).

For the 3Kv 470pf capacitors, C410 and C420 on the main board, I ordered TDK's CK45 Series 10% 105C ceramics (CK45-B3FD471KYGNA). They're not potted and easy to get to.

For the 22K 1/2 watt resistors R427 and R428, I have a choice of several different ones on-hand:

Vishay 22K 5% 155C 100ppm Metal film 1/2 watt
Dale 22K 2% 175C 200ppm Metal film 1/2 watt
Stackpole 22K 5% 155C 500ppm Carbon film 1/2 watt
TE Connectivity 22K 1% 155C 100ppm Metal film but in a 1 watt size.

For the 470 Ohm 3 watt resistor R316, I ordered a Panasonic 5% 235C Metal oxide film (ERG-3SJ471).

And FloobyDust, the 47 Ohm 1/2 watt resistors R306 and R312, I have Vishay 47 Ohm 2% 150 C 100ppm Metal film or Stackpole 47 Ohm 5% 400ppm Carbon film. Probably doesn't need to be anything special.

I totally agree, I don't want to use any of these old parts. So the delay will be the diodes.

I measured the volume of the potting tray and it is 1.6 cubic inches (26.22ml), so I'll get searching for that Silicone potting stuff (MG Chemicals). It comes in a clear formula by some, so it would be cool to see the components floating in it

I did use my Fluke 8024B DMM to test those original diodes and the user's manual states it uses 3.5 volts or less for the diode test.

And just one other thing that I noticed, power transistor Q401 on the HV Oscillator board, it's supposed to be an NPN 2SC1059, but what's installed is a 2SC1168 X. Who knows, but I do have a new Motorola gold-pin JAN2N3739. Do these transistors happen to interchange?

Fun stuff!

...now the weeds need pulled, the lawn needs mowed, the garage...

Happy Fourth of July!

[floobydust]

Digikey and Mouser stock nothing for high voltage rectifiers. I haven't seen any from North American distributors so I've been buying them off eBay or Aliexpress, under $0.50 each. The Tesla coil fanboy shops also have them.
It looks like the white cartridge type HV rectifier diodes are selenium.

The hardest part with silicone potting is getting rid of air bubbles. But at 2kV this is not super critical. Just keeping part leads apart and the capacitor plates away.

[bsfeechannel]

I discovered the manufacturer of those diodes. It's Origin Company, Limited. They have a subsidiary in the US called Origin Electric America Co., Ltd.. They still produce high voltage rectifiers.

Studying their datasheets, I concluded that their part numbers follow the pattern Voltage(kV)-Speed-Current. So, if 3HT-5 follows their standard, 3 means 3kV, H means high speed, which for them is t_rr<= 500ns, and 5 may signify 5mA, or perhaps 500mA. I'm not sure about T. However, I guess they can be contacted about it if someone gets that curious.

I initially estimated the oscillator to run at around 70kHz, but now that I know that it is a tuned-collector oscillator, given that C403 is 10n and the primary coil is 1.8mH (as measured by the OP), we have a frequency f=1/(2πSQRT(10nF·1.8mH)) ≃ 37.5 kHz. So T= 1/(37.5 kHz) = 27 µs, way over 500ns.

If we go to Mouser, we'll find 3,865 pieces in stock for immediate shipping of a 3kv, 500ns rectifier (R3000F-T) for mere $0.18 a pop.

Of course we'd have to experiment with those diodes to see if they work fine for this application.

[strawberry]

6kV 5mA Vf=23V  rather selenium rectifier
something 20 Si diodes in series sounds silly

MOUSER : BAS170W

[floobydust]

For low current, fast recovery HV rectifier diodes, Asian manufacturers seem to have taken over. I would try get samples.
Microwave oven parts are higher current and capacitance, majority are slower switching, so I would not use them.

I have some extra 2CL77's 20kV 5mA and Vf measures around 34V. PM me and I can send some.
You want no sharp edges at soldered joints and the entire side of a disc capacitor is at one potential, so it would not be against the opposite potential. You have to plan the sandwich for lowest corona.

edi electronic devices inc. NY.
Dean Technology Dallas, TX.
All Electronics surplus GP260 6kV 200mA (R6000) but maybe not fast recovery. Tesla Coil forums found them around 100-120nsec.

[bsfeechannel]

For low current, fast recovery HV rectifier diodes, Asian manufacturers seem to have taken over.

One possible explanation is that after the fall of RCA, the Asians took over the production of TVs and monitors. Those diodes were primarily used around CRT circuits. In fact I suspect that the T in 3HT-5 means TV, because you find that kind of designation with other manufacturers as well.

[Smoky]

Let me show you this.

Here are two pictures of the CRT multiplier board inside of my Sencore SC61 Waveform Analyzer.

This board takes -1500 volts and ramps it up to -4500 volts using EDI DL800 diodes and 6Kv capacitors.

The Sencore also uses several of these diodes to generate the three -1500 volt legs coming out of the HV supply board too.

No potting, just a small metal cover that I unscrew from the chassis.

Someone on Ebay has these DL800 diodes for sale.

What do you think?

...and thank you so much for the help!

[floobydust]

Take a listen to the Sencore CRT multiplier or look at in the dark and you will see why potting is preferred. You don't have to have it but it's insurance. Dirt dust and humidity can be a problem without a cover.

From Paschen's curves, air breakdown is 3kV/mm at sea level and you design HV air spacings for around 10X greater. Some HV diodes are stubby, their lead spacing is designed for them to be potted. The EDI datasheet is saying if the devices operate over 10kV/inch (400V/mm) it needs to be encapsulated.

The DL800 diodes look good to use, to 4kV in air and 8kV potted. I think the HV capacitors have much less spacing though.

[Smoky]

Here's what I did, I ordered 5 2CL70's (6Kv) and 5 DL800's (8Kv)

I was thinking too how close the components are on this little thing, coils, caps, diodes, etc.

One thing's for sure, these diodes will be smaller, so more room to work with.

BSFeeChannel, I followed up on your remarks about selenium rectifiers and I read how they were prone to failure and that they also have a "service life."

I didn't think much about the lead spacing of the capacitors when I bought them, but I did read a warning that they had on the 1000pf caps about market volatility. Can you believe this, the cost of those capacitors almost doubled since I bought them Tuesday!!!

I also carefully cleaned and stripped the two fine strands of wire coming into the tray from the coil. They look good.

I noticed the way the components were installed. Their leads were turned into loops and dropped over the wire and soldered.

[Smoky]

First design of rectifier circuit using 2CL70 diodes:

Btw, all of the 1000pf capacitors measure 1020-1050pf on the Sencore.

[Smoky]

Second design using DL800 diodes:

[Smoky]

I made custom potting trays out of my business cards

I intend to make epoxy "modules" with just the three outer leads exposed. That way, if a rectifier fails, I can quickly un-solder and change it out.

[Smoky]

I purchased MG Chemicals 832HD Black Epoxy potting compound ($10) since I own a small Loctite 50ml dispensing gun.

It should arrive on Thursday.

I intend to solder-up and pour at least three "modules" to make use of the potting material since it will go bad.

[Smoky]

I replaced capacitors C410 and C418 (470pf 3kv's) and C408 (1000pf 3kv) on the main board.

A matched 47uf 250v capacitor bundle replaces the "three-cap-can" (C301,C302,C303) in the power supply and a new 470 Ohm 3 watt resistor replaced the burnt C316.

...and I cleaned all of the switches in the scope too!



And I failed to mention that the old "voltage-doubler" capacitor (C406) measured 1440pf on the Sencore.

[bsfeechannel]

And I failed to mention that the old "voltage-doubler" capacitor (C406) measured 1440pf on the Sencore.

This may indicate that the capacitor is leaky. The higher value means that the capacitance meter took longer to charge the capacitor, due to the leakage resistance.

[Smoky]

Check this out

Someone on YouTube demonstrated how to test a high voltage diode.

I first set my power supply to 25 volts DC at 200ma:



Next, attach a 270 Ohm resistor to the anode side of the diode under test and connect it to the (+) side of the power supply.

Then, ground the cathode side of the diode.

Use your DMM to read across just the diode.

Here are the results of the original "Origin" diode tested both ways:





Here is the result of a new 2CL70 diode:



And here is a DL800 diode:



I read power supply voltage both ways on the original Origin diodes! They're "open."

Now notice the difference in the voltage drops between the new 2CL70 and the DL800 diodes.

This is awesome!

[Smoky]

Last but not least...

...the little epoxy "mudcake" modules poured fresh tonight!

We hit the switch tomorrow night



I'll be trying the DL800 diode design first.

That diode has up to six times the "average and surge" current capacity than the 2CL70 and "maybe" sacrificing 50 nanoseconds of switch time (150 vs 100).

The DL800 lead diameter matched the resistor's lead diameter making the assembly tougher. The 2CL70's leads are thinner.

The new 22k Stackpole carbon film resistor measured 21,800 (under 1%).

[floobydust]

 

[bsfeechannel]

 

[Smoky]

Just home from work and I'm ready to put this thing back together!

Now I did put a small blob of type II silicone over the solder joints, so start up may be an hour or so from now.

[Smoky]

Unfortunately, resistor R316 still gets super-hot as I ramp up the isolation transformer.

What's nice though, I have my Sencore SC61 Waveform Analyzer probe hooked on test point TP402 and it kept an eye on what should be -1500.

It reached -914vdc or so before I felt high heat on the resistor and rolled her down.

No visible lighting in the tube.

So there is still a malfunction somewhere.

Maybe it's in the CRT itself?

I may try a couple power transistor swaps next since I have them.

Thank goodness I don't do this for a living. I'd be feeling really hungry

[bsfeechannel]

But if you disconnect the CRT and try to bring the scope up, what voltage do you get at TP402 before R316 starts "chernobyling"?

[Smoky]

BSFeeChannel,

I disconnected the CRT and read TP402. I'm getting just 2 volts now.

I reconnected the CRT and it reads only 2 volts.

Whatever the problem is, I think it's taking out the rectifier circuit with ease.

[bsfeechannel]

Don't lose hope. With the CRT disconnected, turn VR401 all the way up to +27V. This will cut Q406 off. The voltage on TP402 would be the next to zero. Now bring the scope up on the variac keeping an eye on R316.

With your sencore hooked up to TP401, and a meter to TP402, turn VR401 gradually until you see the waveform below.

[floobydust]

You disconnected the multiplier? Uh I wouldn't do it because the HV can shoot up and damage something. It really needs a load. I use old CCFL tube salvaged from LCD monitors with a ballast resistor. Or just a resistor if you have a few megs at a watt or so for short term use. Technically, the HV should have regulated due to feedback on R419, R420 etc.
The HV regulator Q406, Q402/Q403 I would check for shorted transistors which might supply too much base current to the flyback transistor Q401.

TP401 (on HV OSC board) tells you the health of the flyback oscillator, looking at current in Q401. If should be 1.7Vpk across R403 10R, so 0.17Apk. This will give some idea if R316 is getting too much. A small series light bulb i.e. nightlight 120VAC 7-10W might help if you need to run things for longer to make more measurements.

The only way I have killed HV multiplier is with arcs or partial discharges. The surge current of a 1,000pF cap dumping into a rectifier can usually kill it. That's why there are 22k resistors, to limit the current. Did it die, to only give 2V now?

I wouldn't blame the CRT just yet. It takes several seconds for the filaments to warm up, and during that "cold" time, the HV sees no load.

[Smoky]

Thanks for the help guys!

I want to get back into that transformer tonight because my first concern are the primary and secondary windings.

Let me do that first and check those resistors too.

What beats me up is that I don't own a transistor tester. I usually just take them off the board and do a basic diode test on them.

That's the next purchase for my toolbox

And on a side note, I'm not sure what the little neon lamps do on the main board but I noticed that they didn't exhibit any light or flickers either.

BSFeeChannel, I also have a restored Tektronix TDS420 that functions flawlessly since I rebuilt it last Fall. The scope does great with low voltages using the "zoom" feature.

[floobydust]

Try not to get struck on ideas, like it's the windings or something. If there is a crack or extra air-gap, poor fit loose screws in the fragile delicate ferrite core, it can cause problems like high current.
A waveform from TP301 will reveal a lot.

[Smoky]

BSFeeChannel, I can get no reading off of TP401. I tried every combination with CRT, w/o CRT, HV red wire from transformer connected, not connected, transformer connected to HV Oscillator board, not connected to HV Oscillator board, etc, etc...

And even using the DMM to read transistor Q401 on the HV Oscillator board I read maybe .83 volts on the base, .15 volts on the emitter, and .46 volts on the collector.

I was able to bring 27 volts to the base of Q406 too, just as requested, and still nothing. My scopes, as in multiples, could find something more emitting from my fingertips than TP401     I raised the Variac while turning VR401 too.

But this again, I think is because the rectifier circuit has been taken out and so things should be screwy?

After scratching my bald head, here's what I did after all of that testing madness:

I took a brand new rectifier module and installed it on the transformer. I hooked everything back up, the CRT, the Flyback transformer, the HV Oscillator board, everything but...

...the HV red wire. I attached it to the Sencore SC61.

I slowly brought up the Variac and watched the digital display on the Sencore.  The voltage reading climbed to about -1650 volts and it remained there for about 20 seconds, and that's when the Sencore display dropped to zero.

Does this indicate the current going into the rectifier circuit is above the threshold of the 2CL70 and DL800 diodes or resistors?

Anyway, this is fun stuff to me, and I want this to be the third oscilloscope that is fixed in my house this year

And to top it off, and really blows my mind, is that the original fuse on the back of the scope is still good!

And another observation while testing, resistor R404 on the HV Oscillator board, on the schematics it reads it as a 820 Ohm resistor, but what is installed is a 470 Ohm resistor.

[Smoky]

And here is my trivia question for tonight:

Can a 2N3739 NPN transistor replace a 2SC1059 (Q401) NPN transistor?



Also, reading a little more about transistors, I came across several articles about 2SC458's, and their failure rate. These would be used as Q402 and Q403 in the LBO-302. They say that they can fail in circuit but test good on a transistor tester.

Transistor BC549 was recommended as their replacements.

Is anyone familiar about that? And would they work as replacements in this scope?

Nothing like reading and modifying my posts in bed when I should be sleeping

[bsfeechannel]

And here is my trivia question for tonight:

Can a 2N3739 NPN transistor replace a 2SC1059 (Q401) NPN transistor?

Yes. According to these guys.

[floobydust]

The mysterious 20 second delay, is it due to the CRT warming up and then presenting a load?
I know the anode connection (red wire) is off, but the other CRT grids are there and can be a load. I only have the LBO-301 schematic and it's clipped off, missing the CRT portion, might be the pdf viewer in Firefox. You can get unwanted screen-grid current when the anode is not catching electrons.
Or something else heats up after 20 seconds.

My rules- don't run it open-loop (no voltage reg feedback), don't run it not connected to any load. Just to avoid damaging the insulation on something with HV being too high.

You could isolate it by putting a dummy load on the (flyback) CRT filament winding 75AKB1 CRT 6.3VAC 600mA, so a 10R load. Or winding a few turns on the core and using that to give it some load, some work to do.

These one transistor oscillators can be finicky. This is a strange repair where the oscillator would not start: https://www.eevblog.com/forum/repair/tek-model-464-repair/msg1704728/#msg1704728

P.S. Found the press release for LBO-302 in March 1974 $700 ($3,640 today). LBO-301 was 1971.

[Smoky]

Thank you guys!

FloobyDust, the best I could do about the CRT schematic was to take several pictures of it. You'll need to piece them together.

Over the next few weeks, I'm going to acquire new transistors and resistors. Some I have, some are obscure and would need some alternative. But before any new parts are installed, I'm going to check all of the resistor values on the boards, do tests on diodes, etc.

But what adds frustration is finding different value resistors and capacitors on the schematic than what's installed.

I can only imagine, at the factory, there were points on each board where voltages and currents needed to be, they pass or fail. The schematics show voltages, but when components don't match the schematics, then what?

So when I read -1650 volts, that was without a load. The tolerance on the schematic is +/- 100 volts. I'm sure the CRT would have brought it down and the VR would fine tune that number.

Is it possible to bring those test point voltages to service manual specs then work the voltages back up when the loads are applied?

Anyway, this project will be kept on the little bench that it rests on for however long.

...and if you have any parts or boards to this machine, I'll buy 'em

[floobydust]

I'll bet there's nothing wrong with the HV power supply now. Just a theory in my head I'm liking.
The problem is the CRT is overloading it, once warmed up. This is the 20 second delay.
Vacuum tubes are normally full-on devices (once the cathode is warmed up) like a depletion-mode FET. They need a negative-bias voltage from cathode wrt control grid to lessen or cutoff current. This is what the intensity control is doing, as well as blanking pulse.
The control-grid CRT pin 7 or maybe screen-grid could be acting as the anode, due to the absence of grid-bias or the control grid is stuck near 0V.

The only hole in my theory is the HV DC should not go so low to 2V, the CRT could take it down to 100V min. as a guess.
Can you test Q405. Shorted C408 would light up the two NE-2 neon lights.
I will stare at the schematics more and see if I am out to lunch or this is a possibility.

[bsfeechannel]

I can only imagine, at the factory, there were points on each board where voltages and currents needed to be, they pass or fail. The schematics show voltages, but when components don't match the schematics, then what?

So when I read -1650 volts, that was without a load. The tolerance on the schematic is +/- 100 volts. I'm sure the CRT would have brought it down and the VR would fine tune that number.

The CRT is irrelevant for the HV supply because the supply is regulated. And we regulate a power supply to make its output voltage independent of the load.  Besides, the CRT is "absent" anyway for the first half a minute, every time you turn the scope on. So disconnect it. Preferably remove it from the scope and keep it in a safe place. It is one variable less. And an unobtainium one.

Now, instead of changing out every component, let's troubleshoot. It's fun, I promise.

When you bring 27 volts to the base of Q406, you cut it off. The oscillator should stop. Q401 should be also cut off and R316 should be as cool as a cucumber in this condition. If it is not, there's something fishy going on. So let's investigate.

With Q406 cut off, you should read around zero volts on its collector, which is connected to the base of Q403.

Q403, and Q402 will also be cut off. This means that you should read next to zero volts at the base of Q401.

Now, please, turn VR401 to 27V, bring the scope up until bearable by R316. Then, please, measure these voltages and report them to us.

Q406 Vb, Vc
Q403 Ve, Vc
Q402 Vb, Ve
Q401 Vb, Vc, Ve

This is what I expect to see

Q406 Vb = 27V, Vc = 0V (or less)
Q403 Ve = 0V, Vc = 27V
Q402 Vb = 0V, Ve = 0V
Q401 Vb = 0, Vc = whatever is there on TP-301, Ve = 0V

The aim here is to see if there's something wrong with the reference Q406, the control circuit Q403 and Q402, and the oscillator Q401. If nothing's wrong, rule them out. If there's something strange somewhere, watch it under a magnifying lens.

[Smoky]

Here's what I read from the transistors with everything connected and Q406 having +27 volts at the base:

Q406

B +27.1
E +26.9
C -16.74

Q403

B  -.13
E +.38
C +14.69

Q402

B +.318
E  -.48
C  +14.69

Q401

B -.55
E +.98
C +189.5

TP 301 +185.8

Measurements were taken twice.

Everything is connected BSFeeChannel because I just took readings with everything connected after FloobyDust made some comments. So FloobyDust, my reply to you is next...

[Smoky]

FloobyDust, after you made your comments, I connected everything back up on the scope.

I wanted to go through all of the tests points with everything active. I toggled through the Variac to come up with these results:

*Test point TP402 -1056 volts

*Test point TP301 +136  (it should be +150, it's now low)

*Test point TP302 +118.6 (it should be +130, it's now low)

HV Red wire entering main board -1118 volts.

The unregulated test points TP301 and TP302 are now low, before, these test points were ~30 volts high.

I think things are improving.

...and here is a shot of the scope from test point TP401 on the HV Oscillator board:



Q401

B +.7
E +1.29
C +130

So something is drawing-down the voltage to the transformer.

No adjustment to VR401 will raise the test point TP402 any higher than -1056 (it should be -1500v at this test point).

BSFeeChannel, things are strange

[bsfeechannel]

Here's what I read from the transistors with everything connected and Q406 having +27 volts at the base:

Q406

B +27.1
E +26.9
C -16.74

Q403

B  -.13
E +.38
C +14.69

Q402

B +.318
E  -.48
C  +14.69

Q401

B -.55
E +.98
C +189.5

TP 301 +185.8

Measurements were taken twice.

Wonderful. Now we're getting somewhere. I forgot that R402 and R429 form a voltage divider, so the voltage at the collector of Q403 and Q402 should be 14.7V, which, your measurements show, are spot-on.

There's a discrepancy is the voltage at the collector of Q406, which your measurements show -16.74V and the base of Q403, which is at -0.13V. They should be exactly at the same voltage, since they are directly connected.

Another problem is that the emitter of Q402 is negative, -0.48V. There's no path for negative voltage there. Same thing with Q401, which has a negative voltage at its base, -0.55V, and a positive voltage at its emitter (+0.98V).

Without changing anything, see if you have some waveform at TP-401 and report to us what you see, please. After you check that, you're going to investigate the suspect voltages.

EDIT: Since you edited your post and showed TP-401 with an oscillation, now it is time to investigate why the collector of Q406 and the base of Q403 do not have the same voltage. Please do a visual inspection on the board. See if there is any cracked solder joint, raised tracks, or shorts. Measure the resistance between the two terminals.

Everything is connected BSFeeChannel because I just took readings with everything connected after FloobyDust made some comments. So FloobyDust, my reply to you is next...

That's OK. It won't change our investigation. Leave it the way it is.

[Smoky]

I apologize, I didn't realize that I had to wait for the numbers to climb when I took the initial voltage tests. Some of the transistors took longer than others to top-off.

So I decorated resistor R316 like a Christmas Tree with heat sinks and waited for the voltage numbers to come up and stabilize on the transistors.

I didn't do this in one swoop, so slight deviations can appear due to bringing the Variac up several times. It's not a digital one.

Q406

B +27.1
E +26.9
C -16.74

Q403

B  -16.74
E  -8.8
C +14.69

Q402

B  -8.8
E  -.6
C  +14.69

Q401

B  -.7
E +1.0
C +189.9

*TP 301 +149.9

TP301 is almost spot on at +150

TP302 is also close to spot on at 130.9

[bsfeechannel]

I apologize, I didn't realize that I had to wait for the numbers to climb when I took the initial voltage tests. Some of the transistors took longer than others to top-off.

No worries.

So I decorated resistor R316 like a Christmas Tree with heat sinks and waited for the voltage numbers to come up and stabilize on the transistors.

R316 shouldn't be overheating. Without changing a thing, measure the amplitude and frequency of the waveform at TP-401, please.

I didn't do this in one swoop, so slight deviations can appear due to bringing the Variac up several times. It's not a digital one.

Q406

B +27.1
E +26.9
C -16.74

Q403

B  -16.74
E  -8.8
C +14.69

Q402

B  -8.8
E  -.6
C  +14.69

Q401

B  -.7
E +1.0
C +189.9

*TP 301 +149.9

TP301 is almost spot on at +150

TP302 is also close to spot on at 130.9

The base-emitter junctions of Q402 and Q403 are in reverse breakdown. This means a current is flowing from their emitters to their bases, not the other way around. The only DC path for that current to flow would be through the base of Q401. But that current would have to flow from its emitter to its base, which would cut it off. You'd have no oscillation, no negative voltage on TP-402 and consequently no negative voltage at the base of Q402.

So we need to discover what is providing the path for the reverse current at the base-emitter junctions of Q402 and Q403. If C405 or C404 be leaky, we have a problem. So let's check them for leakage.

[Smoky]

Thank you for your generous help BSFeeChannel!

And yes, R316 gets very hot even with +27 volts on the base of Q406.

I connected a TDS420 to TP401 for the measurements with the Variac turned up:



I will update again tonight after I test the capacitors and transistors.

*C404 tests good.

*C405 tests good.

[floobydust]

Um 197Hz     so the osc. stops and sits there and cooks? I see AC mains ripple as the only modulation there in that strange TDS420 scope trace.

I'm a little confused- when it's stalled, at TP401 you are getting steady -200mV (inverted?) with around ~80mVpk of AC?
I can't make out the units on the Tek222A snapshot, I'm assuming now that's when it's working, running at say 30kHz.
C403 is suspect, test at voltage on the Sencore, and the 150V rail filter cap 50uF 250V C302 is already small but the 150V rail should have less ripple at 20mA (-200mV/10R).

[Smoky]

HV Oscillator board:



C402 and C403:



I had to verify the accuracy of the Sencore Z Meter with my reference caps to confirm what I found and it is spot on:



1.1pf on one cap, .3pf on the other! These caps should measure .01uf or 10,000pf.

FLoobyDust, is this what was taken out when the -1650 volts went to zero?

If so, what other collateral damage could there be?

*I just attached the HV red and the black ground wire coming from the Flyback transformer to the Z meter, it reads only 2180pf. I tend to believe a capacitor was taken out in the rectifier module.

[bsfeechannel]

Thank you for your generous help BSFeeChannel!

My pleasure.

And yes, R316 gets very hot even with +27 volts on the base of Q406.

Certainly, something is drawing too much current for this resitor to overheat. Please measure again the voltage on Q401 and then on both leads of R316 like in the picture below (I'm using lbo-301's schematic).

I connected a TDS420 to TP401 for the measurements with the Variac turned up:

As floobydust pointed out, this screen capture is confusing. It is not possible to conciliate it with the measurement you took with the meter +0.98V.



The frequency is ridiculously low (around 200Hz), when it should be in the range of 30 to 40KHz.

Can you please clarify what is in the screen?

I will update again tonight after I test the capacitors and transistors.

*C404 tests good.

*C405 tests good.

So, please, turn the scope off. Wait for the capacitors to discharge, then measure the resistance as indicated in the figure below. Both ways, I mean, since the ohmmeter has polarity. We need to know if at that point there is a path for a current to flow both ways.



After that, test Q401 and C405.

[floobydust]

What I think is going on now, the oscillator is just weak and stops, leaving Q401 on and R316 cooks?

C403, C402, C404 are critical for the flyback oscillator tuning and gain. If C403 is open, that is a problem. I would say their failure is not from anything going on here, for a wound film cap to go open, they must have corrosion inside and died of old age. I'm really glad we found something.
If you replace C403, C402 that will change everything I think, as far as it staying running and not cooking up R316, unless the multiplier has croaked.

The second issue would be what happened to the new HV multiplier? Is it alive or did it go open?
What part number are the 1,000pF caps, they look smallish and can they take 4kV? With no load they would see more.

[Smoky]

Here's what I have so far:

Q401 tests good in circuit (diode test).

C404 and C405 test good with no leakage.

Resistance across C404 and C405 Is over-limit (OL) nothing.

FloobyDust, those 1000pf caps in the rectifier circuit were and are 3kv

[Smoky]

BSFeeChannel, you can see things are still apart.

Should the film caps C402 and C403 be replaced before the scope is fired up again?

I'm searching now for a 600+ volts .01 caps as I type.

As it stands at the moment, the CRT is out because it shrouded access to Q406, Q403, Q402, C405, and C401.

[Smoky]

Here's something else I noticed.

Q402 and Q403 should be 2SC458 B or C's by the schematic.

What's installed are two 2SC979's.

[floobydust]

Should the film caps C402 and C403 be replaced before the scope is fired up again?
I'm searching now for a 600+ volts .01 caps as I type.

I would not run it with open capacitors, they also tame the leakage inductance of the flyback transformer and we don't want to make Tesla coil here.
Stay with a metal film cap 0.01uF 630V +/-20% (service manual) like Kemet R73 PP with long leads Digikey 399-12529-ND. The same part is in a few other locations in the scope, to keep in mind if it is a high failure rate item.

[Smoky]

FloobyDust, I was just going to post a picture of the other ones I see in the scope!

There are also different value film caps in the same grey jacket too.

And C801 is a large one.

...and another will have me taking the scope totally back apart.

I love it, I'm going to write a book when this is done.

Wait a minute, we are writing a book

BSFeeChannel, I will describe the settings on the Tektronix scope later on tonight.

**I'm also looking to buy a transistor tester for the types we're using here. Something that could read gain and such, and not just if they're open or shorted as in a basic diode test. Let me know what's best.

...and thanks again for all of the help guys!

[bsfeechannel]

Should the film caps C402 and C403 be replaced before the scope is fired up again?

C402 and C403 should be in the circuit. Don't run the scope without them.

Q402 and Q403 should be 2SC458 B or C's by the schematic.

What's installed are two 2SC979's.

2SC979 is better rated (Vceo = 50V) than 2SC458 (Vceo = 30V). "Pinout" is different though. We'd better check it since they're not "pin-to-pin" compatible.

[Smoky]

Back after dinner, and four in a row, gone!



These two came from the Vector Scope board T-376.

C415 and C419.

And here's another NTK capacitor, C801, this time a .1uf cap in the Horizontal Amplifier section:



It should read 100,000pf!  I never imagined seeing this in film capacitors.

FloobyDust, I don't think an automotive style cap will work as replacements. Some of these caps need to fold down flat because they are so close to the lids of the case. This .01uf Panasonic cap appears to be identical in size and lead spacing, P12110-ND.

I have a list with Digi-Key started and I will let you guys know what I purchase.

[bsfeechannel]

Wait a minute, we are writing a book

Indeed.

What do we have up to now?



Q406, Q403 and Q402 are doing their jobs, working their butts off (literally) to bring the base of Q401 down and stop the oscillator.

There's a current of about 300µA reversely flowing through the base-emitter junction of Q402 and Q403. If the resistance between the emitter of Q402 and ground is OL, where is this current coming from?

The voltage at Q401 indicates it is cut off, yet R316 is on fire. Where is the current through it going to?

Don't miss the next episode.

BSFeeChannel, I will describe the settings on the Tektronix scope later on tonight.

Looking forward.

[floobydust]

While you are in there, can you test C410 470pF 3kV. If it has any leakage it would bias the CRT into grid current.

[Smoky]

BSFeeChannel,

On the TDS420, the "arrow" represents zero volts. I raised it up while the Variac was increasing AC voltage to the scope to allow the waveform to make its way on to the screen. As the Variac was climbing, the waveform was changing shape as the variac approached 120 AC.

My initial voltage measurement with the DMM was negative (-) and under 1 volt so the volts per division was scrolled to 100mv.

As the waveform stabilized, I turned the horizontal scale until the sampling rate showed enough repetitions of the waveform to trigger the measurements.

I then snapped the picture and rolled down the Variac.

When I use the scope, I run Signal Path Compensation tests and usually revert back to factory settings.

I just checked the scope and it is set on DC coupling.

I like the smaller 222A because it has an Auto-Level button that brings the waveform quickly to the screen. And it even reflects .1 volt per division.

The tests between the two scopes weren't at the same time or while the LBO-302 was in the same condition.

Does that help

FloobyDust, C410 and C418 were replaced with brand new TDK 470pf 3kv ceramics. I will test them again since they were installed prior to the -1650 volt drop (AKA The Great Depression).

*C410 and C418 retested and good, 481pf and 484pf with no leakage at 600 volts.

[bsfeechannel]

Does that help

Where things don't make sense is that you measured +1.0V at the collector of Q401 and the waveform shown by the scope at TP-401 is wiggling around -200mV. It can't be both.

Also, the collector of Q401 is at around +190V, indicating that it is cut off.  But since R316 is overheating, we need you to measure the voltages like in the picture below, please.



We need them to complete our diagnostic.

[Smoky]

Absolutely brutal.

There's no doubt in my mind why so few of these scopes are around.

For instance, C416 and C540 on the Vertical Amplifier board T-575.

I disconnected 6 wires just to get the board to expose itself this much.

You can see down low, the vacant pass-through holes for C416.

There are still a dozen wires connected to this board that's 2" x 3".

Both .033uf NTK film caps were shot:



BSFeeChannel, once these caps are replaced, I'll go through the voltage and scope tests again. Things should look a little better, but...

...not until I get these two out:



C101 and C1012, .1uf NTK film capacitors sitting on top of the input BNC's. No sign of the second one so the faceplate will need to come off.

Got'em!



...and C502 and C511 on the Vertical Amplifier board T584 (notice the matched pairs of J-FET's, with sockets having gold plated soldering legs):



All of the NTK capacitors are removed. Every single one is bad (C102, C1012, C402, C403, C415, C416, C419, C502, C511, C540,and C801):



...and here's the Panasonic PP capacitors I ordered from Digi-Key to replace them:

.01uf    ECW-F6103JL 5% 630 volt (P12110-ND)
0.1uf    ECW-F6104JL 5% 630 volt (P12158-ND)
.033uf  ECW-F4333JL 5% 400 volt (P12068-ND)

They should be here by Thursday.

I had a little time to detail the knobs and to apply new white paint!

[floobydust]

Your patience is a model to me. Looking forward to the parts coming in.
It's too bad the Nitsuko film capacitors have all gone bad, we should claim warranty.

Japanese products can have a real mechanical puzzle to them, to take apart. You have to think many steps and even then it can just be a bear. Toyota saying to remove the entire fender to get access to the master cylinder bolts. That one had me screaming. Instead I bought a flex-joint wrench and contorted to undo them, but there were obviously no thoughts about repairs.

One of my first scopes was a Leader, like a LBO-506 but single trace and uncal sweep. Had a lot of fun using it. I remember scoping 600VDC but AC coupled, and when I disconnected the probe and switched to DC coupling, the scope made a ka-blam sound and sparks flew out of the switch. It was the input coupling cap discharging 600V. Scope still worked fine, but I learned not to do that on any scope.

[Smoky]

Thank you FloobyDust!

With the scope powered off and waiting for parts, I spent some time checking transistors on the main board. I think I found something.

The oddities are in the Q301 and Q304 area near where FloobyDust noticed an over-heated resistor, R306:



I am getting continuity from the collector to the emitter of Q304 (NPN).

I am getting continuity from the collector to the emitter of Q305 (PNP).

I am getting continuity from the emitter to collector of Q302 (NPN).



Should I pull each transistor out, one at a time, and check the traces to see if the continuity remains, then move on to the next one?

Again, no power is applied to the scope.

[floobydust]

Any overload on the +27V (or +12V Q512) rail would cook R306. The transistors are OK if you measured +/-27V, they are regulating as they should, which I thought you got? In-circuit there is a fair bit of stuff to give false readings. Some power transistors have a C-E reverse diode built-in.

Usually fairly hot carbon resistors discolour brown, and big overloads turn them black and they smoke.
I don't like the blackened R306 for +27V, if it's still 47R then I would leave it alone and come back to it later once other problems are first cleared up. A new part might just cook.
Something must have loaded down the +27V rail and R306 smoked a bit. I suspect that problem might still exist. The flyback oscillator could not overload the +27V rail, it has R402 and R430 to limit that. A thermal IR camera can tell you right away who is hogging current.

Leader probably had some production issues. R306, R312 is 47R only 1/4W on the BoM but they upgraded to 1/2W parts on the backside. Not sure if a part is also on the pcb top side in parallel.
I would say they normally run hot for the +/-27V as 100mA is 1/2W dissipation for each.

[Smoky]

Yes, the +27 and -27 test point voltages were measured and are spot on.

I have a package of new Radio Shack 47 Ohm 1/2 watt carbon film resistors but they appear even smaller than what's installed.

The installed 47 Ohm resistors measure good, so we'll leave them for now.

Anyway, I found a box on my porch when I came home for lunch



I just turned on the soldering iron. Let's get it going!





Back home in the jungle



These caps are so tight in tolerance, they're matched pairs. The two BNC .1uf caps are .099uf, the two .033uf are both 33200pf, and the .01uf caps for C502 and C511 are 10200pf.

[Smoky]

Guys, I've been super-careful the entire time.

So I roll the chassis to gain access to one of the small boards and my hand touches one of the neons. It detaches from its legs!

I may have touched the neons a few times, but nothing that would break it.

I now look closer at the legs stuck to the board, they literally look burnt. I've seen copper get to this state before in older home wiring. Brittle and dry. One leg looks like it broke up in the glass somewhat.

Anyway, would this cause a -1500 volt disturbance or excessive current draw?

And where would I find two new fresh neons?

The part's list describes them as 50 volt NE-2's.

It just keeps getting better and better, and I love it!

[xavier60]

Likely just corrosion and possibly accelerated by the presence of the high voltage producing some ozone.
The first thing I would do is to do some tests on an intact one to get some idea of the break down voltage.
From what I know about CRT TVs, the neon bulbs are used to clamp high voltage spikes in case of CRT internal flashovers.

[xavier60]

It seems that the 5AH type is the most suitable but is difficult to source, https://books.google.com.au/books?id=5-NEmE1EhYQC&pg=PA130&lpg=PA130&dq=.neon+bulb+for+crt+protection&source=bl&ots=kO5ckNbyQG&sig=ACfU3U2MTovYwwy4cjLaB5E0p8nPNzkFEA&hl=en&sa=X&ved=2ahUKEwjggbiD373jAhVNeH0KHQ5JC3EQ6AEwFHoECAgQAQ#v=onepage&q&f=false

[Smoky]

Thank you Xavier60!

I took a picture of the neon leads sticking out from the circuit board.

It looks like copper-clad steel. The one end almost looks shiny like a welding puddle.

There's some NOS stuff on Ebay. I few of the descriptions read: 120 VAC / 60 VDC, along with a few different wattages.

[floobydust]

The two series NE-2 neon lamps protect the CRT from internal arcs due to excessive heater-cathode voltage. You get static discharges when you power up/down the scope, whenever the HV goes up or down (as part of the crackling sound) it's normal to see them flash. The CRT can handle say 300V H-K difference there so the neon lamps keep it below that.

I would say any vanilla NE-2 would work as a replacement, it's not critical. They usually fire at 90-135VDC (in light). Not the little short neon lamps from crappy power bars though, they are low current.
Extra low firing voltage neon lamps have a radioactive isotope in the gas, to lower and stabilize firing voltage.  That era has past now, nobody makes opto-couplers out of them with a CdS cell. Some have argon but I think for higher firing voltage. GDT tubes can also be used.
VCC, CML make neon lamps. Mouser and Digikey carry them, A1A is 1" for A1A-ND.

The lead failure might be from defects in the metal from the high temperature crimp of the glass.

[Smoky]

Hey Guys, let me know if I did this test right.

I attached a 220K Ohm resistor to one leg of the working neon from the scope.

I set my DMM to the 2mA setting to read the current draw.

I then attached the Tenma isolation transformer to each end of this circuit and scrolled up the AC voltage.

When I made it to 50 volts AC, the neon came to life and the DMM read .089mA.

I continued to raise the AC voltage to 120 volts AC and the current draw read .335mA.

When I turned down the isolation transformer, the neon cut back off at 50 volts AC on the button.



This seller on Ebay has these NOS neons where he states that they kick on at ~60 volts AC:

https://www.ebay.com/itm/113642968784?ul_noapp=true

They seem to have the copper-clad legs too.

[xavier60]

The tested original fires at 50 x 1.414 = 70.7VDC
It would be about 85VDC for a 60VAC rating.
There are other aspects such as response time.
The body size of the NE2's looks right.

Extra: The diagram in Reply #83 shows the bulbs as "NE-2" anyway.

[Smoky]

Thank you Xavier60.

The part number in the service manual lists the neons also as 50v.

NE-2 A1A neon has several sub-varieties, and most start over 95v. Some are "high" and some are "low" brightness from what I've read.

The 60v version on Ebay should be fine. I'll do the same test with the new neon using the same resistor and see how well it matches.

I imagine it will be close enough.

They're out-board of the CRT and very easy to install, so this won't keep me from putting the scope back together



All of the new film capacitors are now installed.

The neons were ordered yesterday so it may be Tuesday before they arrive.

The only other thing to note was the brown wire going to the #13 pin on the CRT fell off of the Vector switch on the back panel. I reconnected it with a fresh solder job. So it must have been hanging on by a thread

Other than that, I followed every jumper wire, inspected every trace, and looked closely at every component and I couldn't find any area showing signs of a short. R316 is the only area with that "heating" problem.

I replaced resistors R306 and R312 because I didn't want to leave a "darkened" resistor on the board. Two matched Stackpole 46.5 Ohm 1/2 watt resistors took their place. I raised their legs a little higher since they do get warm.

I must say, these Leader singe-sided PC boards are durable. The copper traces are thick and lightly covered in a conformal coating. Tektronix and Sencore boards "suck" in comparison.

I also sent an email to Origin Company, Ltd. to ask them for the specs on the original 3HT-5 diodes. Maybe they'll reply? I'm wondering what the "current" capacity is of this old diode. What would you calculate the current of this rectifier circuit to be if this scope is functioning properly? This has been in the back of my mind while trying out these 2cl70 (5mA) and DL800 (25mA) diodes, could they not be up for the task? Is there a "surge" current in the scope that they must tolerate?

Tonight I'll put the knobs back on.

We've covered a lot of ground in this thing so thanks again for all of the help!

[Smoky]

I wanted to share this response from Origin Ltd. of Japan.

I spoke with Ryo Kurita of Origin Electric America Company Ltd., Chemitronics Division, in Torrance, California on Friday.

Ryo spoke with the Origin engineers in Japan and they scanned these three documents from their archives.

Thank you Ryo!

These selenium diodes were discontinued in 1980:







So for a 3HT-5 diode, it appears that the "3" represents the current in mA and the "5" represents the voltage in kV.

Also, the neons arrived!

I'll be setting up the test to see what voltage they fire at later tonight. Stay tuned

[Smoky]

Ok, here are the results of the neon tests:

The original neon fires at 50 volts AC with a current of .089 mA, and at 120 volts AC the current is .335 mA.

The new neon fires at 70 volts AC with a current of .100 mA, and at 120 volts AC the current is .338 mA.

The size of the new neon is also smaller than the original.

I plan to try the new neons in the scope.



The scope will be brought back up on the Variac tomorrow night.

Let me know if there would be any special tests that I should do as it ramps up.

Should I adjust VR401 to allow current to flow to transistors Q402 and Q403 now? If so, I should see -1500v at test point TP402 during start-up. I will be keeping a close-eye again on resistor R316 to see if it heats up, and if it does, I will adjust VR401 to cut off Q402 and Q403 and take voltage readings before and after R316 and also at the collector of Q401.

A fresh rectifier module (using 2CL70 diodes) is also installed.

Everything is connected within the scope and it is ready to be powered up.



**I just watched a video tonight of a repair of a short wave radio. In the schematics it was described that the capacitors attached to the collector of a transistor prevented RF frequencies from false-firing the transistor.  Since we discovered C402 and C403 film capacitors were bad, were they there to keep Q401 from false-firing? Just a question out of the blue because it sort of rang a bell to me because C402 and C403 are tied to the collector of transistor Q401 (our problem-child).

[xavier60]

That's quite a difference in firing voltage. I can't say for certain that the higher voltage bulbs will provide enough protection.
I would feel it safer using the good original in series with a new one for now. Another idea that should be researched is the possible use of  TVS diodes.
https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

There were some loose ends with the high voltage supply mentioned in Reply #106.

[Smoky]

Thank you Xavier60,

I'm in no hurry about turning this thing back on.

If you think I should wait, I will.

Did you happen to catch the comment I just made above about capacitors C402 and C403 and their interaction with Q401? Reply#121.

Anyway, I'd love to find 50 volt firing neons for sure.

**And here's a question that just came to me, these two neons are in series. Can I use just one so it triggers at the lower 70 volts AC level?

Anyway, it's around Midnight, and I'm about to turn into a pumpkin!

[xavier60]

Did you happen to catch the comment I just made above about capacitors C402 and C403 and their interaction with Q401?

I hadn't. Hopefully the faulty capacitors were fully responsible for the odd behavior.

[Smoky]

Ok, I have the Variac supplying the full 120 VAC to the oscilloscope.

VR401 is sending +27.0 volts to the base of Q406, supposedly, cutting off voltage to the bases of Q402 and Q403.

The high voltage test point TP402 is reading +1.646 volts DC (-1500 VDC test point).

So far, this is the first time resistor R316 does not burn while in this state. R316 has +185.1 volts DC on its incoming lead and +184.9 on its outgoing lead.

Here's where I see an issue before I go any further. On the base of Q402 I measure +.996 VDC, and on the base of Q403 I measure +1.451 VDC.

Shouldn't both of these base measurements be zero since they should've been cut off by Q406?

The Orange and Red Primary wires entering and exiting the flyback transformer read +.539.

The base of Q401 reads +.536.

Shouldn't these also be zero?

[bsfeechannel]

Now turn VR401 to reduce its voltage to around 14 volts. Do that slowly and carefully, always monitoring TP-402 for any strange voltage and R316 for overheating.

[bsfeechannel]

Here's where I see an issue before I go any further. On the base of Q402 I measure +.996 VDC, and on the base of Q403, I measure +1.451 VDC.

Shouldn't both of these base measurements be zero?

They're consistent with what you're measuring at TP402. Q406 is providing Q402's and Q403 bias.

[Smoky]

Good to see you back BSFeeChannel!

I slowly lowered the voltage to 13.9 on the base of Q406.

R316 started to get hot again but only as I scrolled downward past 16-17 volts.

TP402 measured -1204 volts while 13.9 volts was measured on the base of Q406.

[Smoky]

As I scrolled down from +27 volts on the base of Q406, the voltage on TP402 changed smoothly to -1204.

I must say, it takes a little time for R316 to get hot once I get to +14 on the base of Q406.

**Also, if I continue to turn VR401 to +10.6 on the base of Q406, TP402 easily reaches -1500 volts DC.

So at least we know the rectifier circuit we made is doing its job!

...and seeing the heater at the base of the CRT glow is a good sign too

[Smoky]

BSFeeChannel,

 I didn't see this because the scope was on its side while I was adjusting VR401.

 But when I start to turn the variable resistor, and before it even reaches +26 volts on the base of Q406 (-110 VDC on TP402), both neons illuminate.

 I carefully watched a second time and the neons fire at or around -95 volts DC.

[Smoky]

While adjusting the variable resistor, VR401, I reached over and pressed the "identify" button on the top of the scope.

I seen a green glow in the lower left corner of the CRT.

I made a quick adjustment of the "centering" and the "focus" knobs.

I couldn't believe what I seen when I ramped the VR back up the second time!

[Smoky]

So we know now the rectifier circuit and CRT function.

R316 still gets hot now, but after a much longer delay.

So there is something still that's causing R316 to react this way.

I replaced all of the electrolytic and film capacitors.

I believe only original small ceramics remain.

Anyway, what a great night

[bsfeechannel]

Good to see you back BSFeeChannel!

Thank you.

I slowly lowered the voltage to 13.9 on the base of Q406.

R316 started to get hot again but only as I scrolled downward past 16-17 volts.

TP402 measured -1204 volts while 13.9 volts was measured on the base of Q406.

As I scrolled down from +27 volts on the base of Q406, the voltage on TP402 changed smoothly to -1204.

I must say, it takes a little time for R316 to get hot once I get to +14 on the base of Q406.

**Also, if I continue to turn VR401 to +10.6 on the base of Q406, TP402 easily reaches -1500 volts DC.

So at least we know the rectifier circuit we made is doing its job!

...and seeing the heater at the base of the CRT glow is a good sign too

BSFeeChannel,

 I didn't see this because the scope was on its side while I was adjusting VR401.

 But when I start to turn the variable resistor, and before it even reaches +26 volts on the base of Q406 (-110 VDC on TP402), both neons illuminate.

 I carefully watched a second time and the neons fire at or around -95 volts DC.

I've redrawn the HV section of the LBO-302.



V302 and V402 apparently do more than just offer protection for the CRT. They act as voltage regulators to maintain the filament and the cathode with a 100V potential difference, being the filament more negative than the cathode. Normally, you'd find the filament and the cathode at the same potential, but not in this case. It seems that the cathode helps to accelerate the electrons emitted by the filament, or something like this.

So every time TP-402 goes past -110V, the neons will glow.

D405 and D409 form a voltage reference for Q405, which is a current source for the intensity circuit.

The voltage doubler output, therefore, has to deliver at least -1610V, so that, after the voltage drops across the neon lamps and the zeners we have -1500V at TP402.

Of crucial importance is C418 and C408. If they haven't been tested for leakage, they should. I'd recommend Xavier60's method: using a 10k resistor in series with the capacitor and a high voltage source (anywhere between 100 to 1000V would do, I guess).



Any leakage in those capacitors would make the HV supply to deliver an output voltage more positive than -1500V and will load the voltage doubler which will make R316 overheat.

[floobydust]

When the HV is up, measure CRT pin 2 (control-grid) and pin 3 (cathode), that will tell us a bit. Both are at high voltage, close to TP402. I think it's trouble if the neons stay lit.
Q405 is a constant-current source for controlling beam current (intensity) and that circuit might not be working. The (4V7?) zeners D405, D409 and diode D406 should be conducting through R416 and turning Q405 on to back off CRT current.  If not, then you have (too) high CRT current which would explain R316 getting hot.

I would look for open INTEN trimpot/potentiometer (does it do anything?), open Q405, open R416, open D405, D406, D409.
You said C410 470pF 3kV tests good... the 1.8MEG's I would also take a peek at.

You don't want the CRT control grid trying to go +ve or zero wrt the cathode, then it would be full on too much current.

[Smoky]

Thank you BSFeeChannel.

C408 and C418 are new and both were tested before the first rectifier module was installed, and again, after the second rectifier module was installed (I desoldered the caps and tested them for leakage at 600v out of circuit on the Z-meter).

I will remove the caps and test them again, or even replace them with another pair of new 470pf and 1000pf caps.

When you mentioned at first to turn the VR401 down to 14 volts at the base of Q406, are you saying that -1500 should have been read at TP402 (14v @ Q406= TP402 -1500)?

It wasn't until I turned VR401 down to 10.6 volts on the base of Q406 before TP402 reached -1500 VDC.

I also turned VR401 down even further and exceeded -1650 VDC on TP402.

When I stop turning VR401 and remove my plastic screwdriver, the voltage on TP402 remains rock steady.

FloobyDust:

When TP402 is at -1510 VDC, Pin #2 on the CRT measures -1190 VDC and Pin #3 measures -1500 VDC.

The variable resistor for intensity (VR403) works great.

The "intensity" pot on the front panel (J403) works great.

And BTW,  if the neons are in series on the -1500v rail and remain lit, what are they sensing?

...and I think she's smiling

[floobydust]

Ahhh cathode rays 

If you have intensity control, then the CRT bias is reasonable and all looks good so far 
Focus seems to be OK, I don't see retrace lines either.

I redrew the circuit, stared at it and realized the cathode current path is either through R416 or the neons.
My guess is leaky C417 or C408 could load down the HV and light the neons without upsetting the CRT's operation, up to a point.
Note the LBO-301 schematic, mostly similar- except there are no neon lamps.

Otherwise, you would have to scope TP401 to see what primary current looks like now, with the new caps and other parts.
The factory had some issue if they lowered R404 from 820R to 470R.

[Smoky]

This is Test Point TP401.

The LBO-302 has -1505 VDC on TP402 while TP401 is being scoped.

On the Tektronix 222A's screen, you'll see that every vertical division represents .5 volts.

In the lower left hand corner you'll see "1.70v" which represents the location of the (+) sign in the waveform. I set that because that is where the top of the waveform should be as outlined in the service manual. As you can see, the waveform reaches ~2.25 volts at its peak.

I imagine this shows the irregularity of the HV regulation. The waveform should be lower by .5 volts yet still produce -1500 volts at TP402.





I will also hook up TP401 to my TDS420 oscilloscope tonight.

BsFeeChannel, I will also get the voltage of the HV Red wire where it enters the main board of the scope so you can have the differential between it and TP402.

[floobydust]

The peak current you measure of 225mA verses service manual 170mA I would say is due to the high raw DC rail at C302.
LTSpice R316 avg. power dissipation is 5W at 165VDC and 225mA peak for Q401 (assuming flyback primary resistance is 140 ohms). R316 sees peak power dissipation over 20W.
I think current at TP401 looks reasonable.
It's a delicate circuit, even 10% change in C403 value or raw DC causes R316 to go from say 3W to 5W.

[Smoky]

Thank you FloobyDust.

I was thinking at work today about the difference between the voltage at the cathode and the control-grid of the CRT.

It reminded me of the tube amplifier repair I went through last Fall. The screen-grid to plate resistors. I recall adding a 1K Ohm resistor to keep the screen grid at a lower potential than the plate. Parasitic oscillation also comes to mind.

Should the voltage spread between the CRT's control-grid (-1190 VDC) and the plate (-1500 VDC) be so huge? I imagine that's a lot of heat to dissipate.

In those tube amplifiers the voltage differential was at most 200 volts.

So less voltage differential equaled less heat dissipation by the resistor.

And C402 and C403, those are the new .01uf 3% film caps we just installed, I don't have to think, they measured less than 1%. Almost all of them measured exactly 10200pf.

I want to adjust the components to bring the scope under less stress. Would we be looking at a larger cap for C402 and C403?

And do you recall when I mentioned that resistor R404 on the HV Oscillator board measured 470 Ohm and not the 820 Ohm listed in the service manual? Should that resistor be adjusted too?

I apologize if I'm not making any sense, I'm just so happy to see that green dot!

I'll have more voltage measurements for BsFeeChannel in a little bit too.

[bsfeechannel]

The peak current you measure of 225mA verses service manual 170mA I would say is due to the high raw DC rail at C302.
LTSpice R316 avg. power dissipation is 5W at 165VDC and 225mA peak for Q401 (assuming flyback primary resistance is 140 ohms). R316 sees peak power dissipation over 20W.
I think current at TP401 looks reasonable.
It's a delicate circuit, even 10% change in C403 value or raw DC causes R316 to go from say 3W to 5W.

I calculated the average power R316 dissipates if the voltage at TP-401 were at 1.7V peak. Its over 2W. For 3W carbon resistor, the surface temperature rise is expected to be in the range of 100°C above ambient.

You could barely touch it.

However with TP-401 at 2.25 V peak, R316 dissipates 3.6W, which exceeds its rated power. I'd spend a little more time trying to hunt down the cause of this discrepancy.

[bsfeechannel]

I redrew the circuit, stared at it and realized the cathode current path is either through R416 or the neons.

The neons are there to provide Q405 plenty of leeway for the voltage at its collector to be up to 100V more negative than the cathode. The heater is also 100V more negative to repel any emissions in its direction.

So the voltage at the control grid should not be more positive than -1500V with the maximum intensity, and not more negative than around -1600V with zero intensity.

[Smoky]

BsFeeChannel, I just measured TP402, it is right on at -1500 VDC.

The HV Red wire from the Flyback transformer where it attaches to the main board reads -1640 VDC on the nose.

Your calculations are very close.

* I also forgot to mention last night, during one of the start-ups I had turned the "Trigger Level" knob all of the way to "Auto."

I noticed that the neons would flicker randomly, but not constantly, even when Q406 was getting +27 volts at its base.

[bsfeechannel]

So, from the voltage point of view, things seem not to be out of the norm.

Check, please, R403 to see if it is not out of tolerance. Also measure the voltage across R316 now that we managed to get -1500V at TP402.

[Smoky]

BsFeeChannel,

Resistor R403 measures 10-10.1 Ohms in circuit (it is listed at 10 Ohms 5% in the parts legend too).

For resistor R316, the incoming lead measures 170 volts and the out-going lead to the HV Oscillator board measures 149 volts.

That's quite a few volts being dumped.

I'm going to measure the to legs coming from the power transformer, they should be at 128 volts.

BsFeeChannel, I'm measuring 154.1 and 154.4 volts AC going to Diodes D301 and D302. I believe it should be 128 volts AC on the schematic. I'll check again. Yes, with TP402 at -1510, Diodes D301 and D302 are receiving ~149 volts AC from the power transformer.

[bsfeechannel]

P_R316 = (170V - 149V)²/470Ω = 0.94 W.

That's even below my calculations for 1.7V peak at TP-401.

What we can do now is to measure the temperature at the surface of R316. It should be around 70 to 75°C above ambient.

[Smoky]

So, we're talking 167 degrees Fahrenheit plus another 70 degrees Fahrenheit of room temperature.

237 degrees Fahrenheit.

I'm going to check the voltages again when R316's temperature starts to rise quickly.

I don't own an IR thermometer.

[Smoky]

BsFeeChannel,

R316 remains at 170 VDC (incoming) when it starts to get hot. So the voltage doesn't change.

Now get this, R306, a 47 Ohm 1/2 watt that FloobyDust noticed that was discolored from heat a couple of weeks ago. I replaced it with another new 1/2 watt resistor two days ago for this new start-up (It's listed as a 1/2 watt in the parts legend too). It's the only other "hot" resistor.

I just measured it. There is 44.9 volts going in, 39 volts coming out. 44.8-39=5.8, 5.8 x 5.8=33.64, 33.64/47 ohms = 0.7157 watt. Wouldn't this justify a 1 watt resistor if the voltages are proper?

R306 and R316 "Feel" the same on my finger

I need to beg, borrow, or steal an IR temperature gun!

R316 does feel like piercing heat   ...and maybe 237 degrees Fahrenheit should?

As you can tell, I'm mumbling to myself!

Could this scope be that close to Ok voltage wise?

I think we're doing very well 

[bsfeechannel]

So, we're talking 167 degrees Fahrenheit plus another 70 degrees Fahrenheit of room temperature.

237 degrees Fahrenheit.

Well, I said Celsius because specs for resistors are given in that unit.

I'm going to check the voltages again when R316's temperature starts to rise quickly.

I don't own an IR thermometer.

You can use a thermocouple.

[Smoky]

Wait a minute, water boils at 212 degrees Fahrenheit!

Of course my finger will find it piercing

Another great night, and more to follow!

[bsfeechannel]

I redid my calculations.



I_MEAN = 0.42 V / 10 Ω = 42 mA

V_R316 = 42 mA * 470 Ω = 19.74 V.

P_R316 = (42 mA)² * 470 Ω = 0.829 W.

Pretty close to what you are measuring with the multimeter and below the spec'd 3W.

[Smoky]

This has been an incredible experience for me.

Thank you BsFeeChannel!

You too FloobyDust!

Here's a shot from my Tektronix TDS420:





And one last thing for tonight, I'm measuring 154.1 and 154.4 volts AC going to Diodes D301 and D302 from the power transformer. I believe it should be 128 volts AC as read on the schematic.

Please check this because this could be the reason why R316 has to drop so much voltage.

[floobydust]

I redrew the circuit, stared at it and realized the cathode current path is either through R416 or the neons.

The neons are there to provide Q405 plenty of leeway for the voltage at its collector to be up to 100V more negative than the cathode. The heater is also 100V more negative to repel any emissions in its direction.

So the voltage at the control grid should not be more positive than -1500V with the maximum intensity, and not more negative than around -1600V with zero intensity.

Here it's a high voltage glowing zener diode as somebody at Leader intended? It still doesn't make enough sense beyond supplying (when glowing) more current for the two 5V zeners. I see no link from Q405 collector to the lamps.

Higher sweep speed CRT's and TV's use an additional acceleration (second) anode. This is where neon lamps or spark gaps are for dealing with static charges.

[floobydust]

... I'm measuring 154.1 and 154.4 volts AC going to Diodes D301 and D302 from the power transformer. I believe it should be 128 volts AC as read on the schematic.

Please check this because this could be the reason why R316 has to drop so much voltage.

How are TP301 (+150V), TP302 (+130V), TP303 (+40V), TP304 (-30V) ?
It would tell if the power transformer is hyper, or the loads on 150V, 130V are too light and causing the raw DC voltage to be high.
It's a badly distorted sine-wave at the transformer secondary, so your multimeter may not read true RMS.

My poking around in LT Spice I'm still getting high ~5W power dissipation in R316. I don't have all the flyback transformer's data (winding resistances, secondary inductance etc) but made some estimates.
170V raw I get Peak 225mA, Average 50mA, RMS 100mA current through R316 for 4.7W and a 3W part would be past the limit over 200°C.
Careful a thermocouple does not connect to the resistor if paint or ceramic is chipped, you would get a hazardous live thermocouple which is dangerous. A spot IR temperature reading might be enough.

Before going into any notion of changing the factory design, best we agree if things are normal or not for R316.

[xavier60]

Not that it really matters, I think it's not a flyback converter.
There is no damper diode across Q401 to pass reverse current for energy recovery.
I expect the Collector waveform to be rather sinusoidal and Q401 not saturating.

[Smoky]

When looking at the schematic, the power going to R316 appears not to be regulated.

There is slight discoloration on the board in the area of R316 and it's probably from 40 years of heat transfer.

I see no reason to allow this to continue.

Also, the Ebay seller that sold me the neons said that he found a few neons that fire at 59 volts AC. He put them in the mail. He said they are physically different in size.

[Smoky]

Even though I got home from work late, I'm right back at it!

Tonight, I want to do these following test points:



Here is the first one, TP601. The trigger level knob is set to "auto" and -1500 VDC is at TP402.

TP601:



Here's the problem. This screen-shot was taken only when +27 volts is applied to Q406. When I begin to turn VR401 to raise the high voltage, resistor R316 goes into meltdown. I seen vapors from R316 this time!

When the trigger level knob is in the "auto" position, heat from Hell emits from R316 when turning VR401.

I continued with the other test points only for future reference to compare. They don't look right.

[Smoky]

TP602:



I would say "not right."

[Smoky]

TP603:

[Smoky]

TP604:



It appears off too.

Btw, on page 29 of the LBO-302 Operating Instructions and Service Manual, I stand by my use of "Flyback Transformer" in the title of this EEVBlog.com thread

[bsfeechannel]

I need to find out why resistor R316 gets blazing hot.

We have discovered, by your measurements, that R316 is dissipating no more than 1W.

This is a reasonable value. If, instead of that resistor, you had a transistor, or an integrated voltage regulator, the power dissipated would be the same.

However, one thing is power dissipation, another is surface temperature.

If the resistor you chose has a high thermal resistance, it'll bake.

That's why I asked you to measure its surface temperature. You don't need a thermal camera. A thermocouple will do. Many modern multimeters have a range for temperature using thermocouples.

If the temperature is out of what is expected for a carbon film resistor, perhaps a wire-wound resistor (with a heatsink) of a higher rated power would be in order to replace the old resistor.

[Smoky]

BsFeeChannel, the leads of R316 began to melt the solder on the PC board.

The resistor is a 3 watt flameproof 470 Ohm metal oxide film made by Panasonic (ERG-3SJ471).

The service manual states it should be a metal film so I thought this resistor was a good choice, but now, maybe not.

[bsfeechannel]

This is not the first design where I see components heating up so much that they melt their own solder joints.

It's too early to discard other causes, but poor design choices is something that can always be included in the list.

For the time being, let's perform some measurements more. We could hook up a scope probe on C402 and see how good it's decoupling AC. My hint is that it is leaving quite a bit of AC for R316 to deal with, and that could explain the extra heat that our calculations are not showing up to now.

If your scope is dual-trace, we could also hook up the other probe on the incoming lead of R316, subtract the waveforms (if your scope has this feature) and see exactly what voltages R316 is really having to sustain.

By the way, it's always a good idea show more than one pulse (but no more than two or three) on the scope, so that we can calculate the frequency at which the oscillator is operating, because that helps to determine duty cycles and the impedance of reactive components (capacitors and inductors) in the circuit.

 

If you please.

[Smoky]

BsFeeChannel, it looks like there will be a delay for the waveforms since the I removed the .68uf cap from the machine.

Here's that .68uf cap:

[bsfeechannel]

And as I mentioned before, when the "Auto" trigger mode is energized, R316 goes ballistic, so I image there's a little problem with this "merger" of sections.

When you autotrigger, the beam is present all the time, which draws current through the cathode, which is fed by the HV supply, which is supplied by R316. No surprise.

I'm going to get to work on those requested waveforms tonight.

Thank you.

What made me scratch my head was floobydust's simulation on SPICE. Although he used approximated values, it hit me: we are forgetting to consider the current that goes down the drain through C402 and has to come through R316.

We need to investigate that.

[Smoky]

BsFeeChannel,

I have the .68uf 100v capacitor out of the machine from the trigger level pot. I do not want to reinstall it, it shows signs of stress. I imagine this will delay the waveform from C402 until the new capacitor arrives.

From the calculation that FloobyDust performed, which size of resistor would be preferred, a 5 watt or a 7 watt?

[Smoky]

"When you autotrigger, the beam is present all the time, which draws current through the cathode, which is fed by the HV supply, which is supplied by R316. No surprise."


I see now. That's why the heat ramps up on R316 after I start turning VR401 to reduce the voltage on Q406. The current draw increases.

I even see twinkling in the neons when auto-trigger is activated and when the base of Q406 is at +27 volts. R316 feels slightly warmer during this state also.

I may not be making sense to you, but I can recollect the different levels of warmth from R316 and when they change

[Smoky]

FloobyDust,

You mentioned that you needed to guess on some of the data about the Flyback transformer in your calculation.

Well, when I first discovered that I didn't calibrate the leads properly when measuring inductance of the Flyback transformer's windings on the Sencore LC53 Z Meter.

I retested the coils twice and here are the results:

Primary side:

   Red and Orange winding measured 6.8 uH.

   Yellow and Green winding measured 2.27 mH.

And the Secondary winding measured 720 mH.

[Smoky]

I just read something interesting about "Flameproof" and metal-oxide resistors.

Flameproof resistors may not burn but they can get "red" hot and damage nearby components.

Metal-oxide resistors can handle the power and heat of the environment they are installed in, but the resistor gets hotter because of its smaller case size.

The 3 watt 470 Ohm Panasonic's that I installed for R316 were both "Flameproof" and a "metal-oxide film" type.

Just lovely!

[bsfeechannel]

I have the .68uf 100v capacitor out of the machine from the trigger level pot. I do not want to reinstall it, it shows signs of stress. I imagine this will delay the waveform from C402 until the new capacitor arrives.

No worries. It'll be good to investigate that to be absolutely sure that we do not have any other issues.

From the calculation that FloobyDust performed, which size of resistor would be preferred, a 5 watt or a 7 watt?

Go for broke. The only caveat is to see if it fits on the PCB, or it needs some adaptation like a lug/terminal strip, etc.

I may not be making sense to you, but I can recollect the different levels of warmth from R316 and when they change

Ha! It's the old recollecting-the-different-levels-of-warmth trick.

[xavier60]

Because of the small value of C402, some of the AC power that's produced by the oscillator is being dissipated by R316 as well as what's being caused by the DC.

[Smoky]

Check out this observation.

Here's a picture of the original R316.

Since the resistor and the PC board face downward in the scope, notice how they notched an opening in the PC board to allow the heat to rise!

Leader knew this resistor was going to get hot.

[xavier60]

Look for any change in waveform at the down stream end of R316. Hopefully there might be a clue.

[Smoky]

Thank you Xavier60.

I should have screen shots of those locations Monday night. Once digested, any modification ideas are surely welcome.

BsFeeChannel, the hole spacing for R316 is 1-1/4". I was easily able to insert a .039" lead into both holes. The Dale 5 and 7 watt wirewounds are ~7/8" long and share the same lead diameter (.032"), so we're good. I'd like to see a heat sink proposal on my bench ASAP

I have heat sink compound and that aluminum rail near R316 in the picture above appears beneficial. I have small taps and dies down to #4 too.

The Oscillator board mount on the opposite side uses the aluminum rail w/compound to sink-to also.

[floobydust]

You're about to fall into the power resistor trap. Did you notice the Vishay is 250°C at rated power?
Resistor manufacturers are competing based on resistor size and power rating. They have the materials good for even 350°C operation, but this is ridiculous.

It's simply the laws of physics, where larger surface area = larger heat dissipation.
So a cooler running resistor has to be physically larger. No way around it, in convection cooling unless you want to use a fan. You have to be careful selecting a part.

If the flyback oscillator gets stuck and does not start up, because you are slowly raising the variac up, Q401 will sit there on and badly roast R316 and heat up the flyback primary winding. I would expect the mains fuse to blow. To protect the flyback I would have a fuse there, or be using a fusible resistor.
Unless it is oscillating and now R316 is running much hotter than before?
I would need the DC resistance of the flyback windings and the power transformer 128VAC windings to make the simulation better. I can post it for people to try out. It's a delicate circuit, the HV load has maybe 10% effect on current through R316. It seems tuned 0.01uF/2.27mH for 33.4kHz and you are seeing it operate at I think 24kHz. I'm not sure what these are tuned for.

[Smoky]

Thank you FloobyDust!

When the new film capacitors were installed and the scope was powered up and found to show good HV improvement in the Flyback, I stopped using the Variac to power-up the scope. I use the on/off switch.

It's only when I use VR401 to raise TP402 to -1500 VDC. About 15 seconds later, R316 starts to get screaming hot.

As for measuring the DC resistance of the Primary and Secondary windings in the Flyback and power transformer, I never did such a test before. All I can say is that my Kepco power supply can maintain very stable current from a few mA up to 3 amps. It also can produce up to 30 volts DC that remains rock steady.

Is this sufficent voltage and current to do the resistance tests?

Thanks again guys!

[floobydust]

I would just use a multimeter for the winding resistances in-circuit. Side-effects are OK just need ballpark numbers. Don't need a PSU or to take apart anything for this.

[Smoky]

There were a few comments about capacitor C402 on the HV Oscillator board.

It is a new Panasonic .01uf capacitor measuring 2% tolerance, 10,200pf.

Here's a shot of the HV Oscillator as it sits right now:



Next, I'm going to take the resistance measurements from the Flyback transformer's windings and the 128 volt supply winding from the power supply transformer.

[Smoky]

Here are the Flyback windings resistance measurements

My DMM was set on the 200 Ohm setting (the lowest range).



Yellow and Green wire winding (A & B) = 1.5 Ohms

Red and Orange wire winding (C & D) = .3 Ohms

Secondary winding (E & F) = 20.2 Ohms

[Smoky]

I am now working on the power supply transformer. On the schematic, this would be the 128 volt winding going to diodes D301 and D302. They are two Orange wires:



You guys aren't going to believe this! The Orange wires go to taps 8 and 9 on the power transformer:



No wonder why I read higher voltage at those diodes. They're attached to 140 volt taps.

There are no 128 volt taps on this transformer. Now we know my oscilloscope is built slightly different than the way it appears in the schematics.

I'm going to detach the orange wires from the diodes and read the 140 volt winding's resistance.

The resistance on the 140 volt Secondary winding is 281 Ohms.

[Smoky]

...and the test on the newest neons. A smaller package size but the current draw is the same. They fire at 52.5 Volts AC:



So the 70 VAC neons are installed, should I try a pair of the 52.5 VAC neons?

[David Hess]

Now make your neon bulb strike voltage measurements with the bulbs alternately in the light and in the darkness.

I forget what they are called but typically the bulbs they use in circuits like these have a little bit of radioactivity so that they have a consistent strike voltage whether in the darkness or the light.

[Smoky]

Thank you Dave! That is a very cool test.

Starting with the first neon in the picture above (52.5 VAC), it fired at the same voltage in light and in total darkness (these are installed in the scope).

The second neon (original 50.0 VAC) fired also at the same voltage in both light and total darkness.

The third neon (70 VAC) fired at 67.2 VAC in the light and 68.8 VAC in total darkness.

[floobydust]

I noticed the neon lamps end up in-series with the CRT (cathode), so they could be for start-up. The flyback oscillator would see little or no load until they are ionized. My opinion is it would not make a difference in changing the firing voltage between parts. edit: note the originals have crooked electrodes, close spacing could also be the reason for the lower firing voltage.

Sometime measure the power transformer's 6.3VAC filament winding so we can settle if the transformer is putting out way too much or not. Leader upped the secondary voltage from 128VAC to 140VAC it seems, as well as changing the base drive from 820R to 470R.

When you increase the HV trimpot, there is a point where the oscillator is pushed too hard and efficiency drops and Q401/R316 are just gonna cook and get real hot. It may be the HV section is fine at say -1300V but can't really pull off -1500V. So this might be the problem, you're trying to get more that it can do right now.
You can look at TP401 with different HV output settings and see when current shoots up.

[Smoky]

Sounds good FloobyDust.

It is written in the Service Manual that TP402 is to be -1500 Volts +/- 100 Volts. So I will bring it to -1410 the next time and see if the beam comes up on the CRT.

Should I slowly back down VR401 until it diminishes?  Then raise it back up until the beam reappears?

Ultimately, I should be able to see the CRT function without R316 overheating.

Anyway, I installed the original 3 watt resistor in the R316 location. It is huge and it should be a good indicator for temperature.

...and if the CRT works at a voltage below -1500 VDC, I'll be able to use my Tektronix P6009 HV probe (1500VDC + Peak AC) with the TDS420 to get good measurements at all test points and components.

[Smoky]

Here's the reason for the waiting for the new .68uf capacitor for the back of the trigger level knob pot.

This is a picture of the original .68uf film capacitor. The sides delaminated and I can catch the edge of the foil with my finger. The new Panasonic PP cap measures .678uf and is now installed:

               

[Smoky]

Here is the only mention of test point TP402 (-1500v) regarding the CRT circuit after reading the entire service manual:

"If the spot cannot be extinguished or made to appear with the Intensity control knob, check the following voltages and adjust as necessary. Check TP305 for +27, TP306 for -27, and TP402 for -1500v +/- 100v."

[Smoky]

FloobyDust,

The first test I did tonight was to use my DMM and measure across the CRT heater pins #1 and #14. They measure 6.58 VAC.

[Smoky]

I turned the Intensity control knob fully clockwise for max brightness.

I turned VR401 until the dot appeared on the CRT.

The voltage read -1385 VDC at test point TP402.

The voltage incoming on R316 is +170 and outbound +150.

I can hold my finger on R316 for an extended period of time but it is hot but not melting hot.

Next, I turned the Trigger Level knob to "Auto" mode and that's when R316 goes into meltdown!

So the dot appears on the CRT at -1385 VDC and the Intensity control knob can make it appear and disappear, just like it is stated in the service manual.

The question now is, why does R316 go into meltdown when the Trigger Level knob is turned to "Auto" mode?

BsFeeChannel, I will scope TP401 while TP402 is at -1390 VDC without the Trigger Level knob in the "Auto" mode.

Here it is:



*After five minutes of it running on its own, R316 hot but not burning, the main fuse blew   It is a 0.4A "slow-blow" type.

New fuses are on order!

[bsfeechannel]

The question now is, why does R316 go into meltdown when the Trigger Level knob is turned to "Auto" mode?

C410 is there both to convey the blanking pulse and to enhance the brightness when the beam crosses the screen.

It pulls the average grid voltage up when there are triggering pulses and the CRT conducts more, loading the HV supply.

BsFeeChannel, I will scope TP401 while TP402 is at -1390 VDC without the Trigger Level knob in the "Auto" mode.

Here it is:

Nice. Try to hook the scope probe on both leads of R316 (each one at a time) so that we can add the current that goes through C402.

You can measure the current through the CRT by measuring the voltage across R418. With and without auto trigger.

It'd also be a good idea if you could publish, if not the whole manual, at least the whole schematic. The more information you give us, the more we can help you.

[Smoky]

Will do BsFeeChannel.

I placed an order for the 400mA 250v 2x50mm slow-blow fuses that the scope requires.

This weekend I will scan the schematics and post them to either my website or here. One way or the other, you will have a link to them soon.

[nigelwright7557]

I have designed a few SMPS and made my own transformers.
Once I have made the transformer I need to find out primary inductance and leakage inductance.
Rather than use an LC meter, I put a capacitor in parallel with primary of transformer and put a resistor in series with both.
I then use a sig gen to apply a signal.
I move signal up and down until I find resonant frequency.
Then work out from 1/2*pi* square root (LC) the inductor inductance.

I then short out secondary and do the same again and work out leakage inductance.

Its long winded but gives accurate "real" results.

[Smoky]

I've been calling R316 a 3 watt resistor because that's what the service manual lists it as in the parts legend.

I just physically measured it with my calipers and it is .95" in length by .31" in diameter. It's huge!

Could this resistor be a 5 watt after all? ...and yet again, another factory revision? I just found this chart on the internet too, .945" = 24mm and .31" = 7.874mm. So after all of this time, it could be a 5 watt resistor that's getting blazing hot!

         

* I went back and reread some of the posts just in case I missed a request by someone for something. Floobydust, I did check the 1.8MOhm resistors (R410,R411, and R412). They are spot-on.

I also want to point out that the supply voltage at the wall outlets in my house average 122VAC. This Leader oscilloscope was originally intended to run on 115VAC. I just recently repaired a Tektronix 310 oscilloscope and I can attest to the fact that the 7VAC increase at the plug made the unregulated supplies within the Tek scope rise ~27VDC higher. Could this supply voltage increase cause an "imbalance" within this Leader oscilloscope

I'll be posting schematics tomorrow

[Smoky]

Hey guys and gals,

 I’m bringing back up a repair thread that I started a few months ago regarding this Leader oscilloscope.

 I finally was able to take apart the service manual and scan the schematics for everyone to view.

 I also have the new fuses to get her back under power.

 Each page of the schematics is ~1.8MB so it may take a few replies to upload them all (8 pages).

 BTW, I also uploaded this group of LBO-302 schematics by .ZIP file to Ebaman.com.

Smoky

[Smoky]

Power Supply        Vertical Input

[Smoky]

Vertical Amplifier      Calibrator

[Smoky]

CRT Circuit      Trigger Sweep

[Smoky]

Timing Switch           Horizontal Amplifier



The oscilloscope will be back on my bench this weekend and I'll start by testing both leads of R316 and posting the results. I really appreciate all of the guidance I've received so far regarding this restoration/repair!

Also, I was recently given this Huntron Tracker but I'm not familiar with it. Maybe it's not too useful in helping to find the fault in this repair unless you have an idea?



Here's what I've decided to do. Having had such a good time restoring that Tektronix 310 oscilloscope, this Leader is going to get the same treatment. I have already replaced all of the electrolytic and HV ceramic capacitors, rebuilt the Flyback transformer, and installed new neon lamps.  Starting with the CRT circuit, I'm going to pull every single resistor, transistor, and small capacitor. I'm going to replace every single part. There aren't that many. I wish I had a curve tracer to really get down and dirty with testing transistors in the event that the current "gain" is too high, but then again, new transistors of this era are cheap too, and some of the bolted-in ones I already have. Since some of the transistors that are installed have part numbers that differ from the schematics, I'll pull all of the data sheets and compare hFE specifications.

If you look at the parts legend, you'd see that the majority of resistors are labelled as 5%, others are 1%, and some are 0.5%. It appears that this scope was assembled with very tight tolerance parts. So, I imagine, it wouldn't take too many parts to wander and throw this machine out of whack. I really like those new Vishay metal film resistors that I used in the Tek scope, small and spot-on. So I'll invest in the Dale 0.1% and 0.5% for the entirety. And I wonder what those small low-voltage brown Mica capacitors have to say too? I'll measure each one to see if I find a fault.

There is something drawing too much current in this machine, and I'm going to find it!

Again, it's when I turn the Trigger Level knob to "Auto" mode that makes resistor R316 go into meltdown.

Time to dig into my 2019/2020 Winter project 


*Something just popped in my mind, what effect would Silicon diodes have in a Flyback transformer circuit when they are used to replace Selenium diodes? Isn't there a "voltage drop" factor to take into account?


Another observation, the power supply transformer is wired for 115VAC but I have 122VAC at the outlets in my house. That's an increase of about 5%.

Floobydust asked me to measure the filament voltage at the CRT heater pins. I read 6.58 volts, it should be 6.3. That's an increase of ~5%.

We also discovered that the transformer taps supplying R316 are 140 volts and not the 128 volts as outlined in the schematics. If this is an unregulated supply rail, I believe the voltage is higher than it should be, but I don't see a voltage reference at this point (R316) in the schematics to verify this.

I think there are errors now that are "component value" related due to the change of the power transformer's output voltages. I have used "dropping" resistors in the past to adjust filament voltages and panel lamps in my mono tube amplifiers with excellent results. But, calculating and making adjustments to higher voltage outputs and making component changes within this scope would be well above my pay-grade!

I know very little about circuits, but I can fix things mechanically pretty well. If you feed a transformer more voltage at its inputs it only seems logical that you will over-drive the regulated supplies (additional voltage dissipation required through heat along with increased stress) and you will raise the voltage of the unregulated supply rails. Maybe, a custom transformer can be made? Or can the original transformer be modified? Or a voltage regulator be added to the outputs?

Fun stuff and just thinking out loud

[bsfeechannel]

Welcome back.

Floobydust's hunch is that the 3-W resistors of yore are advertised as 5-W resistors today. My hunch is that R316 was dimensioned to operate on the limits of its power dissipation. The PCB cut-out above it is a hint. That, coupled with the fact that your line voltage is greater than the rated 115 V, makes me suspect that a modern 3-W resistor will turn your scope into a barbecue grill.

It is, however, a good idea to fully inspect your circuit to discard any other causes.

[Smoky]

So last night I stayed up to watch three videos on the winding of transformers. This was the first one, and I like the music too



Instead of messing with component testing tomorrow, I'm taking out the power supply transformer. We're going to re-wind it!

There are so many pluses here, one, I won't need all of the primary windings re-installed. I only need one, and that one will be custom-sized to 122VAC.

The secondary windings needed are four, 6.3, 10, 35, and 140V. Where the 10V winding goes is a mystery to me, I see the letter "E" below it (I'll trace it and let you know where it goes). The 6.3V wires are large green ones going straight to the CRT heater pins. Since this scope came with this transformer, those output voltages need to remain the same.

So, yes, this transformer is slightly different than the one in the schematics. But based on what I watched in the videos, the iron core size and gauge of wire are the most important factors to know when building a transformer. And if the winding has a "center tap" you divide the number of turns in half for each voltage.



Thank you for the help, I'll need it!

Whoah, what if it's possible to access just one of the primary windings? Would a new 122VAC primary winding need to have fewer "turns" than say the 100V or 115V primary windings that are in it now? If so, is it possible to maybe just shorten one of the existing primary winding without disturbing the secondary windings? And if that's the case, it looks like the primary windings have a center tap, which means that I would need to shorten the wires a little on both ends.

And the Mailman just dropped off a Digi-Key box on my porch! The parts inside are for the repair of the HP 35660A Floppy drive I have going on in another thread.

It feels like Christmas already

[bsfeechannel]

Would a new 122VAC primary winding need to have fewer "turns" than say the 100V or 115V primary windings that are in it now? If so, is it possible to maybe just shorten one of the existing primary winding without disturbing the secondary windings?

In your case the primary would have to have MORE turns. What you could do is to reduce turns of the secondary windings. But I wouldn't recommend you to do it if you're not familiar with the "useless theory" mentioned by the guy in the video you posted.

[Smoky]

I did smile when I heard him say that

We'll see how this transformer comes apart tomorrow, or if it even can.

I'm not sure what other options that there are, but we can surely say that we gave it a good run!

[bsfeechannel]

You can try to install a modern 5W resistor instead of the 3W you purchased. Then you can try to reduce the HV from 1500 to 1425 V, i. e., less 5% to put less strain on the oscillator.  You can try to install additional power resistors in series with the D301 and D302 to bring TP301 and TP302 to specs and consequently the voltage drop on R316. There are plenty of options.

I think tinkering with a power transformer, which is a critical and safety component, is not the best of strategies for the moment. But that's just what I see from my point of view. If you're having fun, that's what counts.

[Smoky]

Thank you BSFeeChannel.

The original 3 Watt R316 resistor is installed. FloobyDust ruled against using any of the new resistors.

The HV has been resting at -1400VDC for the past few start-ups.

I like the idea of adding the power resistors in line with the diodes.

Tomorrow I will  disconnect the output leads from the power transformer to the oscilloscope. I will then measure exactly what the output voltage is at each Secondary winding.

[Smoky]

Alright, I put a new fuse in the scope and hooked-up my DMM to the 140V output taps. Each measured 155.1V. The 35V output taps measured 38V.

 When calculating a dropping resistor, I wasn't sure what current value should be used in the equation, so I just used 400mA (the fuse size). At that amperage, and the needed drop being 15.1 volts, an online calculator came up with a 38 Ohm 6 Watt resistor.

 I have no idea what the current draw is of this oscilloscope when it's working properly, so would I need to experiment with different values until we find the happy medium? We know what the voltage is now and we know where it needs to be, so here are other values based on different currents:

At 150mA = 101 Ohm 2.26 Watt
At 200mA = 75 Ohm 3.05 Watt
At 250mA = 60 Ohm 3.77 Watt
At 300mA = 50 Ohm 4.53 Watt
At 350mA = 43 Ohm 5.28 Watt
At 400mA = 38 Ohm 6 Watt

Surprisingly, I see plenty of space to bolt a heatsink to the aluminum panel of the chassis.

We need to dump 11% of the voltage somewhere. Do we need a chimney

[floobydust]

I would add a small power transformer (in auto-transformer connection) to lower mains voltage. I think I see room in the chassis. A 120-12.6VAC transformer maybe 1A could shed 12.6VAC or about 10% with very little heat.
See: ]https://www.eevblog.com/forum/projects/hp-3410aac-microvoltmeter-overheats-power-supply-mods/msg2818584/#msg2818584]
Just ensure the little transformer is phased properly so it does not end up boosting +10% (instead of buck). Try it on a light bulb.

[Smoky]

Thank you FloobyDust!

So I get up this morning and I'm thinking, why not find out exactly what the input supply voltage should be that brings the transformer outputs to the correct voltages as written on its label?



So I did, I brought out the Variac and plugged in the oscilloscope. Any guesses?

...with the Variac set to 110VAC the outputs of the transformer fall in line!

   

Thank you for the "auto-transformer" idea too. I never heard of such a thing. And look how it is wired. I never imagined something like that could be done. So I type that topic in on Google, and it looks like many folks are in the same boat as I am. Old equipment/screwy voltages.

Now, do you think that there is a small 1 amp 120VAC to maybe 18-22VAC transformer available? And could the transformer be rated as low as 700mA? And maybe a product is out there that I can cannibalize its transformer from, a transformer that is sleek/small or custom-shaped so it fits better?

And the secondary winding needs to have a "center-tap" for this to work.

We need to shave 22 volts!

...now everybody get looking

[floobydust]

You could try a 24VAC transformer from a home alarm panel, a home doorbell transformer is 16VAC. But little transformers have poor regulation even 20% so they will have higher output at light loads, especially if they too are rated "115VAC" but fed the high line. Digikey has lots of different ones. It's just they might not give the exact expected results, and the power factor is not 1 for the scope I imagine.