Author Topic: TDS 620A Display Problem (Read 2614 times)

mwbarth36 · « **on:** April 12, 2020, 01:40:00 am »

I recently acquired a Tektronix TDS 620A oscilloscope. It has the brightest, sharpest, clearest displays of the dozen or so 620's I have seen over the years. That is for the first half hour of operation. After about 30 minutes of operation, the display starts to fade, and fades to a completely dark screen within a minute or so. During the fading the image stays in focus, does not distort to any degree, simply fades away. A monitor hooked up to the VGA output still performs perfectly. Almost certain to be a component overheating, but which one. The driver board is buried with no possible access while operating. Cannot even access sufficiently to use cooling spray to determine the bad component or components that way. I did disassemble enough to look at the component side of the driver board. It looks pristine, no sign of overheating of any sort. Cannot imagine something overheating to cause this issue and not showing some signs of the overheated component, but that is the case. Let the scope cool down for 15 minutes and it is good to go for another 30 minutes or so.

Has anyone seen this problem before? Any ideas as to what the cause might be will be greatly appreciated. As far as I can tell the schematics for the board are not available, if anyone knows differently, that would help (maybe). Even ideas as to how I might track down the bad component not being able to access the board while operating the scope would help. I am hoping the someone may have seen this problem in the past.

Again, any suggestions will be appreciated!

shakalnokturn · « **Reply #1 on:** April 12, 2020, 07:54:48 am »

I had a TDS540 behave in a similar way, it would last about 90 minutes from a cold start, the fade out was quite fast once it had started. There was a very little image zooming with the fade.
Similar times as yours for the cool / play again.
The transistor or regulator responsible for the 21V would run hot.

After many hours poking around it was decided and later confirmed that the HV flyback transformer was defective.

You can run the display disassembled, you need to be a little creative and watch out to not short anything, don't forget to maintain a CRT ground!

mwbarth36 · « **Reply #2 on:** April 12, 2020, 04:16:34 pm »

Thanks, I was afraid that might be the case. How were you able to confirm that the HV transformer was defective? I have a chance to buy a replacement board on ebay at a reasonable price..................believe I will go that route. What you saw with your scope is almost exactly what I see. What is your guess as to the problem with the transformer? Shorted turn.....?

mwbarth36 · « **Reply #3 on:** April 12, 2020, 04:27:59 pm »

Wondering if you know if the schematic for this board is available. I found one for the '30' board used with the 544, etc. Do not find one for the '20' board, used with the monochrome display.

shakalnokturn · « **Reply #4 on:** April 12, 2020, 06:31:35 pm »

I think it is around, I'd have to check my files...
IIRC it would be in the TDS520 schematic.

mwbarth36 · « **Reply #5 on:** April 12, 2020, 09:47:46 pm »

Probably is the same a a 520A, maybe an earlier revision. The revision is -05 in this scope.

shakalnokturn · « **Reply #6 on:** April 12, 2020, 10:10:20 pm »

Quote from: mwbarth36 on April 12, 2020, 04:16:34 pm

Thanks, I was afraid that might be the case. How were you able to confirm that the HV transformer was defective? I have a chance to buy a replacement board on ebay at a reasonable price..................believe I will go that route. What you saw with your scope is almost exactly what I see. What is your guess as to the problem with the transformer? Shorted turn.....?

I skipped over that...
Monitoring current on the 21V supply showed that it would rise slowly over a long period then very rapidly before losing display.
The zoom effect along with loss of brightness were pointing towards a sagging HV output, once I had ruled-out the LVPS itself and all diodes around the HV flyback I did a few pre-heat test on the transformer itself.
That showed that it would have less run time, by then I was confident enough although it is not the most common failure mode for FBT's.

At the time there were no cheap enough parts on eBay, instead I noted all the values from the original FBT while it was working, then put some effort into building an adapter board for a different FBT pulled from a videotext terminal.

My conclusion was that the HV winding's insulation had gone leaky, maybe I should have gone to the trouble of at least breaking the HV diode free.

james_s · « **Reply #7 on:** April 12, 2020, 11:01:13 pm »

I've seen this before in a CRT monitor caused by the G2 voltage dropping due to a bad focus/screen divider assembly. G2 (or other electrode) presents almost no load so it acts like a capacitor and sitting open circuit the voltage will slowly drift. You can usually measure it at the CRT socket, most small magnetic deflection tubes have the same pinout. Normally I'd expect G2 to be in the range of 50-500V depending on the monitor.

shakalnokturn · « **Reply #8 on:** April 13, 2020, 01:15:55 am »

Quote from: mwbarth36 on April 12, 2020, 04:27:59 pm

Wondering if you know if the schematic for this board is available. I found one for the '30' board used with the 544, etc. Do not find one for the '20' board, used with the monochrome display.

mwbarth36 · « **Reply #9 on:** April 13, 2020, 04:07:53 pm »

You certainly did put some effort tracking this down, and have me convinced! It does seem to be a strange way for a FBT to fail. I did buy the board on ebay and am waiting to install it. Just do not believe I have the patience to repeat your work. Thanks for your excellent explanation.

mwbarth36 · « **Reply #10 on:** April 13, 2020, 04:13:54 pm »

Thanks for your response. Sounded like a possibility, so I ran a wire out to monitor the grid (do not have the display out of the chassis to monitor directly). The grid voltage does not appear to change when the display fades away. Good thought though!

mwbarth36 · « **Reply #11 on:** April 13, 2020, 04:24:51 pm »

This is, indeed, the correct board schematic. Thanks again!

james_s · « **Reply #12 on:** April 13, 2020, 04:28:46 pm »

Something I forgot to mention, does the heater glow fade out? It's possible the supply to that is dropping for some reason. Also check G1 and the cathode, unless the problem is internal to the CRT, at least one of the voltages is drifting. Assuming the size and focus are not changing I would assume that the EHT is ok.

mwbarth36 · « **Reply #13 on:** April 14, 2020, 12:47:31 am »

Thanks, I will try to monitor these voltages. Not an easy task, but I do have the schematics for board now. May take a few days before I get back to it.

james_s · « **Reply #14 on:** April 14, 2020, 04:05:02 am »

It shouldn't be too hard, there is a common pinout used on virtually all small-neck monochrome magnetic deflection CRTs.

1. G1
2. K
3. Heater 1
4. Heater 2
5. G1 (internally connected to pin 1)
6. G2
7. Focus

CaptDon · « **Reply #15 on:** April 15, 2020, 02:01:49 pm »

I have found the HV rectifier diode buried within the FBT goes leaky
with heat and then loads down the HV windings. The drive circuit
tries to compensate requiring more and more current from the 21vdc
supply until it collapses.

shakalnokturn · « **Reply #16 on:** April 15, 2020, 04:55:08 pm »

Quote from: CaptDon on April 15, 2020, 02:01:49 pm

I have found the HV rectifier diode buried within the FBT goes leaky
with heat and then loads down the HV windings. The drive circuit
tries to compensate requiring more and more current from the 21vdc
supply until it collapses.

Thanks for that, I was suspecting something of the kind...

I imagine that the HV winding having the highest resistance of all secondaries the drive is able to maintain the correct voltages on the other secondaries to some extent.

Have you had any success cutting into the end of the flyback and trying to replace the HV diode?

CaptDon · « **Reply #17 on:** April 16, 2020, 01:16:33 pm »

Haven't tried to repair the HV rectifier on any Tek stuff but
years ago at my Dad's T.V. repair shop we would add a new
rectifier in series with the HV CRT lead. Needs to be WELL
insulated however. This would fix a TV that did not use a
HV multiplier. When a multiplier went bad we just used the
ECG replacement or a factory warranty unit.

james_s · « **Reply #18 on:** April 17, 2020, 12:24:06 am »

Color TV flybacks normally have several diodes distributed between sections of winding, it reduces the voltage any one diode has to deal with. Monochrome monitors and especially small ones have significantly lower EHT so they may use a single diode. If it ends up being that the flyback is confirmed to be faulty I'd be happy to xray it and see if the diode can be located.

mwbarth36 · « **Reply #19 on:** April 17, 2020, 04:20:29 pm »

I did monitor all other voltages during the failure, and they do not appear to be the problem. Therefore I am fairly sure it is a fault in the flyback. Thanks for the input, everyone!

Rollin Hand · « **Reply #20 on:** April 18, 2020, 02:13:47 pm »

https://wonghoi.humgar.com/blog/2018/11/03/tds-color-crt-driver-repair-700-series/

this person uses a tv flyback for replacement on this article.
Someone can experiment and find a replacement.

Rollin Hand · « **Reply #21 on:** April 18, 2020, 02:16:15 pm »

sorry is for the b/w monitor but the same idea apply

https://wonghoi.humgar.com/blog/2017/12/10/tds-500800-series-monochrome-crt-driver-repair/

shakalnokturn · « **Reply #22 on:** April 18, 2020, 07:23:36 pm »

The colour TDS models should be somewhat easier to substitute as the HV transformer and H deflection have separate drives IIRC.


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Author Topic: TDS 620A Display Problem (Read 2614 times)

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