What I can do / afford for now (barely...) , is buy a new can of contact cleaner to clean all those push buttons, as well as replace all the crappy black/red Sprague caps, and that ancient looking, paper shrouded " Cornell Dubilier " cap as well.....and see where that gets me. It needs to be done anyway, it's basic stuff. So let's do that first, and go from there...
Stay tuned....
Vince, I wouldn't automatically replace the red lettered Sprague caps - they are di-film and in my experience (and from things I've read online) likely good. The yellow lettered ones are paper and almost always leaky. The rule of thumb I use to remember is a twist on traffic lights - yellow means caution (leaky, replace on sight) and red is stop - don't replace unless found to be bad.
-Pat
Hmmm...didn't know that. I learned about these old caps 5 years ago on Carlson's YT channel, he just said to scrap them all, didn't talk about lettering colours.... or maybe he did and I don't remember it....
At that time I was working on my first glowing Teks scopes, and I kept running into the red ones being cracked, ballooned, or exploded, so I figured he was right about them.
But now that you are adding some nuance to the problem, I guess I should add that these failed only in one particular spot in the scopes : the HV oscillator circuit for the CRT, that drives the little CRT transformer. I don't recall seeing any bad red Sprague any where else in the scope. Only the CRT oscillator.... maybe this circuit stressed the caps too much and/or they were underspecced.
OK won't replace them for now... also because I have looked at them in more detail in the 180A, and realized that half of them are in the PSU section which I have tested good. the other half are in the divider chain, but only in the 6 slowest / last dividers, hence can't possibly explain why all the preceding / upstream dividers misbehave as well. Also I looked these caps in the schematics, and they are not used to couple the various stages, so even if they leaked DC, they would not cause havoc like they would in an audio amp.
So for now, I will leave them alone. Less work and less expense... I like that.
After a good night of sleep, I spent some hours thinking more about the situation... might not be as disastrous as it seems :
Now that I have replaced the dead tube, it looks like basically, out of 14 dividers / outputs, the first 3 basically work, well enough for now at least (stable picture on the scope, and period is correct, and markers amplitude is about "level" / constant, just tiny bit of "ripple"/undulation). However the next/fourth stage, all of a sudden is all over the shop both with period duration and marker amplitude...
So despite the fact that this stage and the remaining 10 stages behind it are misbehaving badly, hence it feels overwhelming... the fact is, it's just a long chain, it's simple in concept. So if the 4thy stage spits out garbage, next stage gets that in so of course is going to throw garbage out, next stage same things etc, the garbage introduced in the 4th stage just ripples all the way down the chain.
So, plan of attack is to completely ignore staged 5 to 14, and only concentrate on Stage 4, where the crap is being introduced, because as long as this is not fixed, clearly the other stages can not possibly work properly, and also.. with some luck stage 4 is the ONLY problem here and once fixed everything will suddenly start working good. If not, I will then just fixed the next stage, and the next, in order, until the entire chain works properly. But mass replacing tubes is not necessary I think, any more. At least not at this stage of the troubleshooting.
So, now that I have decided to put blinders and narrow it down to a single stage, the 4th one... I looked at the schematic in more detail. All stages are copy/paste. so studying one will be a good investment to help fix others stages later if need be.
Below an example schematic for one of the stages, they are all the same.
It does not look so complicated any more. I read bit the theory of operation as well.
So basically a stage is made of 3 tubes, all dual-something. So in total we have 6 elements : 2 diodes and 4 triodes.
There are one input and two outputs. One output goes to the front panel for the user enjoyment. The other output is wired for internal use only. It's the one that triggers/feeds the next stage.
Each output has a cathode follower. So right there that's two of the triodes easily explained.
The two diodes are not essential to the understanding of the stage either.
No, the core of the business revolves only around the remaining two triodes (both part of the same tube).
This is all it takes to divide the frequency by N.
The manual explains how it wokrs. It's delightfully clever and simple, I love it.
Basically the triodes make up a monostable multivibrator.
It gets triggered by a given pulse, say pulse #1, then it "runs" for a duration equivalent to 4 pulses, so it ignores them. Then it resets and starts all over again.
Genius...
So you get a simple RC network to set the duration of the monostable, hence set ' N '.
The trim pot I see ni all stages in the instrument is just that, it helps fine tune the RC network.
Nice thing is, from what I understand, that although it is analog in design, it is perfectly accurate... it can't divide by any random number, 3.7 or 6.4 or whatever.
As long as you set the time constant ball park in between two pulses, to make sure it's unlikely to trigger unwillingly on the wrong/adjacent pulse, then all is well.
This means the trim pot doesn't even need to be set that accurately. So I am sure they are all just fine and the problem is not there. So I will not touch any of them for now, until proven otherwise.
So... that's it then. Let's start work on that 4th stage see if I can get it to work !
Oh forgot. dirty switches are going to affect the operation of the chain, since the signal does not propagate through the switches, it's hard wired internally from one stage to the next. So, I don't even need to wait until I restock on contact cleaner to start working on the thing. I can just go right at it !
Stay tuned
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Topic: Test Equipment Anonymous (TEA) group therapy thread (Read 18843743 times)