Tektronix 475 intensity problem

[MrBungle]

Having an intermittent problem on my Tektronix 475 scope. Once every 3-5 minutes, it does the following i.e. drop out intensity and focus and then return to normal:

https://www.youtube.com/watch?v=VXP7...ature=youtu.be

It does this even if the intensity is down to nothing.

Does anyone know any diagnostic steps to work through this one?

[Bazz4CQJ]

You'd have to suspect that a Tektronix engineer would start swapping out boards; don't suppose you have that option?

B

[MrBungle]

Unfortunately not. I'm not sure a Tek engineer would either; they're difficult to swap out in the 475.

[PJL]

Check out the PSU voltages first. There is a very comprehensive service manual on tek.com. If its one of the scopes with tantalum caps there is a good chance that one of them has failed and is causing the PSU to trip.

[MrBungle]

Yeah this is a possibility. I’ve already replaced one of them so far which was dead short. Might do the lot!

Edit: actually rather than playing whack a mole I will stick the digital scope on it and see if I can catch a transient on any of the rails in slow sweep mode.

[RogerEvans]

The shape of the flashes when out of focus suggests that horizontal and vertical amplifiers are still running ok. Have you got a HV probe to look at the cathode voltage on the CRT? Also look for arcs in the HV area in a darkened room. If this scope has not been used for some time, try leaving it in a warm place for a while, but that is a long shot. I guess you know that HV diodes and capacitors need to be tested at a few hundred volts or more, they can still look ok on a multimeter.

Roger

[MrBungle]

I’m without an HV probe at the moment. I have left it on to bake for a few hours to see if that cured it but alas no. Any moisture is well out now.

Will crack it open and observe around the HT this evening. Is it worth cleaning out under the HT shield? This was a monumentally dirty scope when I got it!

I’m not equipped to test the parts so I may end up just replacing the HT caps/diodes.

[RogerEvans]

I forgot to add in the earlier post that a general clean with IPA around the HT area and the PDA connections may help. After cleaning it may be easier to spot any tracking. I found a Robin electrician's insulation tester on eBay for £12, tests at 250V, 500V and 1kV and that found the dud capacitor in my 454. Incidentally I bought a non-working 454 as a parts donor for my original 454 that I have had for 30 years, I ended up fixing both of them, and that started the collecting habit!

Roger

[MrBungle]

Thanks for the tips - will try that this evening

I rather like the 454's. No horrible unobtainable ICs in them!

[WME_bill]

Intermittent intensity problem.
Without getting out my manual, I suggest that the one of the resistors on the EHT chain is failing under stress, or a capacitor is failing, and then healing itself.
Diodes do not normally heal themselves.
I would just patiently try changing each component in turn, if the darkened room peer does not help. I am always worried that my nose or glasses will get too close in the dark.
"RogerEvans" clean up sounds an excellent idea.
wme_bill

[MrBungle]

Thanks for the reply. I've posted this question in a few places and the consensus is definitely clean and then check the HT. The scope was really amazingly dirty when I got it so it's possible there's some leakage or flashover somewhere.

[The Philpott]

I think the HT is 18kv on those- watch it!

[MrBungle]

Yeah the discharge stick is coming out for this one. (One very old very long Stanley screwdriver with a croc lead strapped close to the end of it and covered in several layers of high dielectric strength kapton (good for 8kv a layer apparently!))

Also 40kv probe arrived this morning!

Just need arm pit length rubber gloves now

[MrBungle]

Well I cleaned it out and it went away.

Unfortunately when I powered it up this afternoon, the issue is back

[RogerEvans]

There is a thick film resistor pack, R1303, around the focus pot and an associated high voltage decoupling capacitor. Did these get included in the cleaning? They look highly stressed. Another common failure is the 47uF capacitor that decouples one end of the invertor transformer for the EHT. They usually go short or open rather than intermittent but I suppose it could be on the verge of something more permanent.

Roger

[MrBungle]

Yeah did all of that. Think I might go with a HV diode and capacitor replacement approach. One of the tubular caps looks a tiny bit cracked inside now I’ve stared at it under a bright light. Oh and to find some silver solder