I tacked in some 51-ohm resistors in series with R4307 and R4309 which puts the total resistance very close to 549 ohms. With that in place, I found that, in addition to the horizontal compression when the BEAM FINDER is working, the voltage at the collectors of Q4371/Q4356 and Q4395/Q4366 are now very close to the values shown on the schematic. I didn't expect that. Also the voltages at the collector of Q4351 and the collector of Q4361 are closer to the values on the schematic. I need to reread/study the circuit description and hopefully better understand those circuits.
Thanks,
Barry - N4BUQ
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I found the problem. R4307 and R4309 are supposed to be 549-ohm 1% but those
have been replaced with 499-ohm 1%. I put some 56-ohm resistors in series with
each of them and the horizontal compression when the BEAM FINDER is pressed
looks great now.
I've noticed that in one of the manuals, those are spec'd at 357-ohms 1% for
lower serial numbers but that doesn't explain this.
One thing about this scope is that it is supposed to have Option 9 installed;
however, from what I read, that option only involved a slightly different
calibration adjustment and not a parts replacement but maybe...?
Now I don't know whether to replace those with proper resistors or what.
Thanks,
Barry - N4BUQ
I set that up with the 7A13 and that does give a different view. The waveform
at the collectors of Q21/Q31 are definitely not like what I see across those
two transistors in a properly-working scope. I think that gives me something
to go on. I'd previously tried swapping those transistors out with the
"working" pair but apparently the problem isn't in the transistors.
Thanks, Benjamin.
Barry - N4BUQ
I've been looking at the waveforms at the points shown in the manual between a
properly-working scope and this one. I can see where the baseline of the
waveforms are changed but the magnitude of the sweep isn't reduced as much as
it is in the properly-working scope and, so far, I can't find anything that
explains that but I can keep looking.
BTW, what's the advantage of a differential measurement here? I have two 7A13
plugins so that facilitates that process just a bit but I'm not understanding
what that will show in the way of additional information here. Can you
elaborate?
Also, I'm still wondering if (or how much) the higher-than-expected voltages
that I was asking about have on the horizontal compression of the trace.
Thanks,
Barry - N4BUQ
Signal tracing, if you haven't yet tried that.
On another oscilloscope, add channel 1 and channel 2. Invert channel two. So
(CH1)+(-(CH2))=CH1-CH2. This will give you differential probing on your scope
without a differential probe. Don't connect the ground anywhere in the
horizontal amplifier. I'm sure you already know how to do this.
Start from the bases of Q4331 and Q4321. Check if that is affected by the beam
find. Then check the collectors. Here, attenuation should be normal or as
expected. Keep going down the chain until something unexpected or wrong
happens.
