Tektronix 2235 - Volts/Div Malfunction

[CarsonReidDavis]

I recently bought a Tektronix 2235, and most of the functions seem to be in working order.

However, something seems to be wrong with the high resolution portion of the Volts/Div adjustment on both channels.

The adjustment works as expected from 50 V to 50mV, however things get weird from 20mV to 2mV. As the scope is put into 20mV and beyond, it becomes impossible to keep the signal centered on the screen. First it moves to the top, and then it disappears completely.

Any ideas as to what might be happening and how it could be fixed?

Photos of the ongoing repair are at https://imgur.com/a/q7FfYSe.

[tautech]

Welcome to the forum.

Work your way through the Service manual adjustments section and there should be an offset/zero adjustment just dedicated to the higher sensitivity ranges. Failing that being of any use there'll be a problem in the high sensitivity input attenuator like maybe someone has over-volted the input while at high sensitivity settings.
Some study of the circuit description section should make what's going on pretty clear.

If you haven't got this it can help too:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

[floobydust]

The scope's balance needs to be adjusted, the symptom is the trace shifts up or down depending on the range. Most analog scopes will do this a little bit, depending on temperature.

There is Attenuator Step Balance adjustment R10 and R60 which are probably first to try.

The 2mV/5mV range have their own DC balance adjustment R33, R83.
Two other balance adjustments R25 for 10mV variable (uncal) use, and R75 for Ch.2 invert balance.

The 2235 service manual alignment procedure gives info. Just make sure the scope is warmed up.

If you can 't get it to behave and stay centered, then the front end might be damaged and have to troubleshoot that.

[CarsonReidDavis]

Awesome, thanks for the suggestions.

I'll read through that manual and post an update after making some adjustments.