You'd have to ask yourself just WHY do they get so hot?
THEY ARE CONDUCTING TOO MUCH CURRENT!
Why?
Well, you've had her going for a while then problems emerge....gotta be heat related.
Suspect more faulty caps. ![Rant :rant:](https://www.eevblog.com/forum/Smileys/default/rant.gif)
So where to from here?
How are you determining/measureing PSU ripple?
I hope not with a DMM, this is fine for a quick check, but because of their limited frequency response, you must use a scope, ESPECIALLY for SMPS.
Having a look at a 465 SM, there is a procedue to isolate where loading of the PSU occurs.
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Also there is a fault finding flow chart
Sure swap out the bridges, but check for a faulty diode in them too.
It was 3 am when I started it back up and did the freeze thing, so yes, I did use the DMM just for quick check of ripple. I definitely intend on checking with the scope today. After getting some rest, reading these posts, and thinking about it a little, of course MarkL is correct. If the problem was specifically in the PSU, it would affect more than channel two. So what is happening is the CH2 issue is pulling so much current off the +15 and -8 rails that it's really heating those two bridge's up a lot. It's pulling extra voltage off the +110, but the current is likely normal. Thus when I cool the rectifiers and get them back into normal spec, it's likely limiting the current that can pass thru them and stabilizing CH2 and the +110V rail a bit temporarily.
Do you mean the 468 SM? Volume 1 I assume..... I will have a look at it.
I'm thinking it may be a bad cap or resistor that's allowing more than it should be allowing to pass thru into the ground plane..
What I'm saying is I wouldn't give it a very high likelihood that it's related to the root cause. I don't think it's worth tracking it down when you have an unexplained, major symptom of the vertical jumps staring at you.
The power supply needs to be verified. That would be a good next step (actually should always be the first step).
The power supply was the first thing that I checked, but I overlooked actually checking it with the scope. I checked the rail voltages and ac ripple with a DMM, adjusted +55 to exact, and moved on.. I should have been a bit more thorough there. Now I know. The reason I adjusted the +55 is because when I was working on my 465, I had a lot of ripple on the rails - and when I adjusted the +55V all the ripple went away. So I thought it would be worth a shot here.
If everything checks out with the power supply, you could take a look at the final vertical stage that's driving the vertical plates on the CRT. The jumpy vertical will be visible there. Guaranteed.
Take a look at TP38 and TP39.
The problem is happening in an earlier stage, but hopefully looking at it on the plates will give you an idea what to search for and allow you to experiment with how to correctly set up your triggering on the 465.
Once you can confidently observe the jumping, then go backwards through the vertical signal path to find where it starts.
Keep in mind that the 468 is running fine with the exception of the jumpy Ch2. If something was wrong with the power supply it would likely be affecting more than just Ch2. Unless one of voltages or ripple is way out of spec, don't let it drag you off on a tangent.
The vertical output amp is part of my planned check-out for the power supply section. That and the CRT circuit. It's accessible and I figure I might as well just check it while I'm checking the PSU section anyway.
I will certainly go thru your suggested checks.
My thoughts so far are:
1- It's pulling excess current off of +15, -8, and possibly some from +5
2- It only affects CH2 vertical
3- It affects the trigger circuitry - specifically A and B gate
4- It has caused B gate's waveform to collapse, but only causes A gate to 'jitter'
The B gate problem could be unrelated, and probably is, so I am excluding it from my current task. I will deal with repairing it once CH2 is stable.
Good point MarkL - I need to focus on finding the CH2 'jitter' problem and solving it. I now know it's likely pulling more current than it should be. There's nothing wrong with the PSU. If there was, a lot more issues would be obvious than just CH2 and the B gate collapsed output. Like I mentioned, I am excluding B gate issue for now since it is most likely unrelated.