Intermittent TEK 475A issue

#189751

Hello! New guy here.

I have a 475A scope w/DM44 that I bought not working maybe 15 years ago. Replacing one of the power supply filter caps fixed it then. It's been great for occasional use since then. But several months ago it developed an intermittent problem where the display would go very bright and the fan speed would slow down for just a fraction of a second, then back to normal. Then it would be fine for a while (maybe the rest of my session, at least an hour or so), and repeat the very short glitch. One time it finally stuck in it's bad mode and I was able to troubleshoot it. All the supplies measured good (I don't have another scope to look at ripple, but no appreciable AC showed on my multi-meter). I was able to track down the issue as a shorted transistor (Q1344 in the CRT & Z axis circuit). I replaced it, and everything seemed fine again, for a few weeks. Now it is back to occasionally doing the bright display and slow fan blip.
I'm assuming this must be something in the +15v supply, since that is common to the fan and brightness circuits, and a low +15v might explain the Q1344 crapping out before. The problem lasts for such a short time, that even if I had it apart and was ready to measure things, I'd never catch it.

Does this sound like a common problem that has a known fix?
Can caps (tantalum or others) display a momentary short, then recover like this? If so, how can I identify a bad cap?
Could an aged solder joint cause this problem?
I have tried applying heat to various parts, but that does not reproduce the issue (or I haven't applied enough heat!),
It doesn't feel good just replacing parts wholesale, then waiting for maybe days for the problem to show up again, so I'd like to narrow it down to a small set of possibilities.

I have a service manual, and a 45 year old EE degree, so I ought to be able to fix this! :)

#189753

How about a wild shot from the hip...? There is a (I believe) 47 uF tantalum on the unregulated 15 supply located under the shield cover over the hv section. Maybe it is trying to fail?

Regards

Show quoted text

On 1/12/2022 5:04 PM, jim@... wrote:
Hello! New guy here.

I have a 475A scope w/DM44 that I bought not working maybe 15 years ago. Replacing one of the power supply filter caps fixed it then. It's been great for occasional use since then. But several months ago it developed an intermittent problem where the display would go very bright and the fan speed would slow down for just a fraction of a second, then back to normal. Then it would be fine for a while (maybe the rest of my session, at least an hour or so), and repeat the very short glitch. One time it finally stuck in it's bad mode and I was able to troubleshoot it. All the supplies measured good (I don't have another scope to look at ripple, but no appreciable AC showed on my multi-meter). I was able to track down the issue as a shorted transistor (Q1344 in the CRT & Z axis circuit). I replaced it, and everything seemed fine again, for a few weeks. Now it is back to occasionally doing the bright display and slow fan blip.
I'm assuming this must be something in the +15v supply, since that is common to the fan and brightness circuits, and a low +15v might explain the Q1344 crapping out before. The problem lasts for such a short time, that even if I had it apart and was ready to measure things, I'd never catch it.

Does this sound like a common problem that has a known fix?
Can caps (tantalum or others) display a momentary short, then recover like this? If so, how can I identify a bad cap?
Could an aged solder joint cause this problem?
I have tried applying heat to various parts, but that does not reproduce the issue (or I haven't applied enough heat!),
It doesn't feel good just replacing parts wholesale, then waiting for maybe days for the problem to show up again, so I'd like to narrow it down to a small set of possibilities.

I have a service manual, and a 45 year old EE degree, so I ought to be able to fix this! :)

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#189755

Jim,

Check CR1372 and CR1379 for leakage. If leaky, replace with 1N4937. When one or more are leaky, it will give a too bright trace like you have. Also check the 22 meg resistor. It is likely out of tolerance. Use Vishay VR37 to replace it. When this resistor is high, the trace will be bright at turn on then dim as it warms up over 20 minutes without touching the intensity control. C1374 should be a film type to replace the electrolytic. Check VR1416. If bad (open), use BZX85B9V1. Check the 151-0367-00 transistors for leakage from C to E for leakage. Replace leaky ones with KSP10BU (BCE pinout). If you decide to recap the supply, use long life high temperature types to give a long life. Increasing the capacitance is fine. Watch for tantalums that are bad. They can be replaced with something like Nichicon ULD, 47mfd 25V for example. The 1.5A bridges should be replaced with 4A types. A bad solder joint may be on some resistors that should be raised in wattage which there are ones that should be a higher wattage than original. I can give you a list if you want. How far you want to go is up to you.

Welcome to the group. I hope you get your scope going to your satisfaction soon.

Mark

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Intermittent TEK 475A issue