Introduction and issue with a 1A1 plugin



I've read this group off and on for several years. Finally joined.

I am a BSEE of ~30yrs and have used Tek gear since technician school in the mid 80s. My first scope I owned is a 1956 type 531. Actually have several other newer 500 series scopes acquired over the years but the 531 is the only one of them in working order.... I also have a TDS210 and THS720 rescued from the scrap and repaired. Also a few other things... Labs at work have a lot of Tek gear amongst other brands.


The other day in my home lab, thought I needed to use this 1A1 plugin I have in the 531 as the normal CA I have in there not sensitive enough for what I wanted to do. Note I seldom use this old stuff when I have some relatively modern things that are not room heaters! For some reason I was thinking that the TDS210 and THS720 were not sensitive enough either (my error)...

The 1A1 I got from a coworker at a former employer about 25yrs ago and I've never really used it as it had scratchy pots and switches. Also some other issue I could not recall.... I think it spent some time in a damp garage. So, I sprayed the pots and switches with deoxit and put it in the 531 and fired it up.

The cleaner spray helped a lot and its working fairly well. BUT the reason for posting here...

The channel 2 AC/DC coupling has a issue. Displaying the calibrator waveform from the 531, things are fine. But, switch to AC coupling and instead of the trace sinking to straddle the 0V reference as expected, it actually rises up and has a net DC offset putting the whole waveform positive. Channel 1 does not no this.

What could be the fault? Bad AC coupling capacitor at the input? The gain of the channel looks right (or close enough).



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