Re-capped MacPlus analog board - no vertical sweep

[shirsch]

In the large I guess this points to the wisdom of "if it ain't broke, don't fix it". While I had my properly functioning Mac Plus apart I decided to recap the analog board. I've been through similar exercises a half-dozen times with no issues. I have many years of bench experience and everything seemed to go smoothly. Unfortunately, the unit now has no vertical drive to the yoke. I am getting vsync pulses at the LS38 in the vertical circuit input. The portion of the '38 that drives transistor Q4 is working and I can see periodic -12V pulses at the collector of Q4. The drive to FET Q5 (2N5485) looks fine. I pulled and checked both Q1 and Q2 (vertical amplifier) and both measure as valid bipolar transistors. All diodes in the circuit show appropriate forward voltage drop and none are shorted. I replaced the LM324 used to generate the sweep ramp. Even after all this there is no hint of activity anywhere in the LM324 integrator circuit, and as a result no drive to Q1 and Q2. Almost every node in the integrator measures around 5V, which seems odd but analog circuits are not my forte. I did notice the LM324 is powered with -12 and +12 rails with no ground reference. This is not shown on the schematics I have but I'll assume it's correct and intended.

Finally, I swapped the original caps back into the vertical circuit. No joy.

I'm about out of ideas and am hoping someone familiar with the circuit can give me a hint as to where the issue lies.

 

[Snipey]

My 50cts on it:

Did you check all this with an oscilloscope? Or with just a multimeter?

The amplitude of the signal from the LM324 is actually quite low.

I recently fixed a 512K with vertical sweep issue.

In this case; With my scope I noticed ok base-signal but NOK emitter-signal from Q1.
Measuring Q1 with a multimeter and a component-tester indicated all good for this transistor.
Nevertheless it was certainly defective. Perhaps this defect only appeared under load and/or at certain frequency.
Replacing Q1 was all it took to fix the problem.

So are you sure the Q1 and Q2 are good?

 

[shirsch]

I should have followed up sooner - apologies. It turned out to be a physically broken height control. I had accidentally swiped with a pair of pliers while pulling glue away from one of the caps. At the time it looked fine so I didn't give it any further thought. Because of the circuit topology I was measuring a reasonable amount of resistance across the pot and that really led me into the boonies.. I finally found the problem after noticing that vertical drive returned when I laid my finger across the PCB lands. The pot still looked fine, but it neatly separated into two pieces when I removed it. Problem solved!