Tektronix 7834 instability when vertical amplifier overdriven

[ycui@eml.cc]

I have a 7834 for quite some time, and it works mostly OK. However, recently I found a very subtle problem with it.

To describe it. I need to measure a very small noise on top of a large DC signal. I have 7A13 which is supposed to handle this type situation. If I connect the internal 4V square output into 7A13 and set the vertical scale to 1mV/div, I can observe the display readout moves up and down. The display readout was perfect as long as all the trace stay within the display. They will move crazily as soon as trace go out of display.

So, I am suspecting when the vertical amplifier goes into overdrive condition, the +15V power rail loss regulation and cause the CRT vertical amplifier gain change, which manifest into readout display moves up and down. So, I hope a re-cap of the switching power supply should fix the issue.

I also have a 7613, it does not have this problem when I overdrive 7A13 plugin.

Can anyone confirm my suspicion?

[David Hess]

Usually this is caused by thermal imbalance in the vertical amplifier chain.  When the differential amplifier signal chain is driven into saturation or cutoff, one side dissipates more power than the other increasing the offset voltage.

Sometimes it is caused by leakage through the vertical channel switches.

Some of the 7000 mainframes have a thermal balance adjustment but I do not think the 7834 does.