Testing a Flyback Transformer (Leader LBO-302 Oscilloscope)

[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.