bill koski

After reading about the resurrection of a 575 tracer. I thought I'd ask about my 556 scope.
I've had it for about 15 years now and use it from time to time. And it has worked fine so far. When I first got it, I tested all the tubes replaced a couple weak ones but most were still original and in great shape.
Other than that I've not recapped it or done anything else to it. I also have a number of plugins which all seem to be working well and other than checking tubes and maybe some switch cleaning have done nothing else to those either. And most stuff seems to be in reasonably good calibration, at least close enough for most things I do. I know the main frame is only rated 50 MHz but I can lock and see signals to 100MHz (at greatly reduced amplitude) so it seems very stable.

That in itself is a testament to the build quality of Tek products of the era. The fact that its working so well is part of the reason I've not really messed with anything beyond that so far.
That said I know that at this age I should consider going through it at some point. Of course all the lytics would be suspect at this point. It's been a while since I had the case open but I seem to remember a number of can caps In the power supply area. I imagine that to be a pretty big undertaking to rebuild in itself.
Beyond that are there things specific the 556 that are known problems areas that I should take a look at?
Also any tips that might help making a rebuild easier?
Anything is absolutely avoid messing with that might permanently throw it out of whack?
If I'm going to do this I'd like to do it once and be done for the next 50 years!

I do have the complete factory 556 manual.
And have downloaded all the manuals for all the plugins that I have and recently gotten an extension so I can run any plugin out of the bay so servicing them should be no problem.

I am a BSEE and have extensive experience in tube gear (love tube audio, and no I don't harvest tubes from Tek gear for audio!!!) and also have extensive experience doing calibration of all our equipment at work for decades.

However a 556 is not a Leader scope or a Dynaco ST70. So I value any guidance from the experts here.
Thanks for any helpful info everyone!

Sean Turner

One comment I'll make is you should test the scope with the "weak" tubes before you condemn them...the Tek manuals for all these scopes emphasize that performance in circuit is the only reliable measure of whether a tube needs to go or not (other than shorts). If they don't perform properly in circuit, then replace them.


Bruce Atwood

My recommendation would be to, after due deliberation, decide if you are going to Repair, Rebuild, or Restore. They are very very different in terms of how long you and your 'scope will be out of commission.

John Williams

Hi Bill. As owner of some 556 scopes I have had some experiences during their restoration. They are pretty well engineered and I would say ultra reliable. The only area I would caution about is the tunnel diode sweep triggering. I had one unit that I just could not get to trigger and wound up parting it. The other thing might be to get some spare tubes. There are a couple of unusual ones in there that are unique to this scope. A final word of advice would be not to drop it on your foot. Otherwise good luck.

Morris Odell

The old adage "if it ain't broke don't fix it" applies here. I wouldn't go recapping it or rebuilding the scope unless it was absolutely necessary. The only problem I had with my 556 was the dreaded HV transformer rot. The 556 has two of them and they are located where they are not easy to remove and replace. Once you've dealt with that you can consider yourself invincible!


bill koski

First the Repair, Rebuild or Restore issue. While it is very nice cosmetically with just a little road rash from 50+ years of use, it's going to continue to be used and not sent to a museum. And it is currently working well so that puts it pretty much in the rebuild category. The question is how much to touch. The, "If it ain't broke" has its strong points. But there's the counter argument, "You can pay me now or you can pay me later!"
So I'm looking at do what's necessary but do no harm.
Old lytics can fail at any time. Depending on where it can take a transformer winding or maybe a hard to find regulator transistor with it.
So I'm inclined to consider replacing all the electrolytics which in itself I'm sure is no small undertaking. At the absolute very least I should at least get a look at the bottom of all the can caps and look for any bulging or leakage. And check any of the dropping resistors in the supply for correct value and signs of heat fatigue and replace and upgrade wattage if necessary. My thinking is that I at least want the power supply section solid and reliable as failures there can cause chain reactions that could doom the unit.

Then I would like to go through and at least check all the carbon comp resistors I can for proper value. I know especially higher values in HV situations are are most prone to drifting.
So these are the things I'm considering and my mindset is, "Do what seems necessary but do no harm."

As far as tubes go, I checked one at a time and returned each to its original spot. I only replaced a couple that had really low emissions and I had to replace one vertical output 8608 tubes because the trace was jumping around and I could see arcing in the tube.
I checked one at a time and returned each to its original spot too.
And yes that's a rare one I should get a spare or 2 of that one! Thanks!

Now as far as trouble spots. The unit triggers fine on anything I've thrown at it so I assume that I have no tunnel diode problems at the moment. I also assume that I don't have HV problems as I have bright sharp traces. I'm not familiar with the HV transformer rot problem. Is there any preventative measures for working ones? I assume there is a thread or 3 somewhere here dealing with this problem?