Testing a Flyback Transformer (Leader LBO-302 Oscilloscope)

[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.