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The same vertical amplifier drives the display for both trace and characters.? If, while displaying a triangle wave, you do not see any non-linearity when the trace is within the zone of the readouts, then it's the readouts.
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Looking at J3405 on the Y readout connection to the vertical amplifier suggests that if Q4470 on the vertical amplifier is odd, then you've got some problems.? What I would suspect is non-linearity either around Q4470? (possible, check waveforms on good and bad scopes), or back in the Y readout.? I'd think that Q4479 and Q4485 might be well behaved, although they are not in the signal path except for the readout.? One presumes that the capacitors C4427, C4424, and pots R4427, R4423 are well behaved. On the readout board, I'd hope that U3480 is ok, that's the stroke generator.? That leaves you with checking Q3486 and Q3489 for bad biasing.? If you have a good board (I think you do), then comparison between boards is going to be a key, also looking at waveforms. Q3486 is an example of a grounded base system, where the voltage gain has to do with the emitter resistor to +15 and collector resistor (effective) to -15.? Q3493 looks to be more of an emitter follower than anything else, so it generally gets non-linear if it's leaky or it runs out of headroom.? Again, voltage and waveform comparisons are likely the key. Hope this helps a bit. Harvey On 3/2/2024 12:59 PM, n4buq wrote:
I have monitored the signal at that point and cannot see any difference there when positioning the trace at the top or bottom. Yes, fully CW or CCW is most likely overdriving the vertical amplifier and the stretching is at its worst at those settings; however, the stretching is quite noticeable even when a trace is positioned at the top or bottom of the screen. The stretching is worse on the characters at the top when the trace is at the bottom and vice-versa. I can see a very small bit of this effect on the characters in my other 7704A but it is much worse in this one. |
