On Sat, Feb 3, 2024 at 10:47 AM, Albert Otten wrote:
After reading again the Trouble shooting section and once more studying the
schematics I very much doubt that my reasoning about integrator drift was
correct. More specifically there is a Test 9 with grounded grid and (as far as
I can see) all diodes and transistors reinserted again. Then the voltage at
test point 10A should be close to 0 V .
Albert,
I still think you're right about the effect of grounding the grid. Grounding the top of R1148 isn't quite the same as grounding the grid, but it's pretty close; it's just a couple of resistors away. I'd been thinking that grounding here just eliminated any effect from the DC Offset circuit, which it does, but it also removes any feedback from the integrator/memory. This test has always worked for me, but now I'm beginning to wonder why.
I tried this in my working 3S76. 3T77A set for free running trace. The test
point voltage went op to about 26 V! With Offset and Position at mid range the
normal trace is near the vertical CRT center and the test point voltage is
about 0.3 V. I next thought that the Miller level setting would be important
for the test. I reattached a shorting strap and DMM and switched on the
scope... and quickly off again since smoke emerged from the bridge
compartment. End of my story for this moment.
OUCH! That really hurts, but, with luck, you may not have damaged anything. The close quarters in this 3S76 have caused me two instances of inadvertent shorts but, so far, no damage was done.
I also wondered about the Miller level setting, but found that it was good when checked per the note on p. 6-4. I've noticed that the Miller setting is different, depending on whether I'm running dual or single trace. I can adjust it either way, and make it nearly perfect either way, but then it's less good in the other mode. Frankly, I'm not clear on what this is doing. Is this Miller level related to the Miller effect on something in this circuit?
The manual says to use a 4" jumper with alligator clips to make the various shorts to ground. I don't think there are any alligator clips that are small enough to fit in there without hitting the wire "next door." I've been using some mini-grabber jumpers I must have gotten many years ago from Radio Shack; the metal spring clips are tiny enough that they can't touch two adjacent components at the same time. I've also taken to attaching them "live", attaching the ground end LAST, so if I touch something I shouldn't, there's no harm. The ground end gets attached only after the difficult connection is secure.
I usually do this live, when possible, because the 3S76 warmup time is so long.
I've been wondering how much current the 10 MOhm resistors are shunting around the integrating capacitor in some of the tests. I finally realized that if I used a 10 MOhm DVM or FETVOM in place of that resistor, it should do the same thing and also give me the voltage drop, from which I can calculate the current. I haven't tried this yet, but it would allow me to compare my channel A and B currents, which might be useful.
I really hope your 3S76 didn't suffer any serious damage!
