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.