2101 Pulse gen repair report


Michael Dunn
 

Well, I might be a bit rusty, but it seems I can still repair things :-) In this case, the versatile 2101 pulse generator from 1970.

I had no idea of its condition, so first off, I wiggled all the socketed transistors, then turned it on, saw *some* sort of output, and checked all the supplies (all perfectly fine after 50 years :-) The two shaft couplings are partly broken…I’ll probably just glue them or something :-(

While playing and familiarizing myself with the gear, it suddenly died. Ugh. I thought this would be easy. The core oscillator circuit was still running though, and I finally tracked the culprit to Q182, part of the pulse forming circuitry. The transistor measures largely open now. Weird…suddenly going from working to very dead like that. I stole the corresponding transistor from the delayed pulse generator. Hopefully there is a suitable sub in my junk box.

Next, a more subtle problem. The negative pulse output had a few volts of negative baseline offset. It took me a while to grok the complex output amp circuitry (it’s still somewhat mysterious to me), but after a couple of false trails, I returned to an early hunch: the final, paralleled, output transistors. Sure enough, one of them (Q352) is bad, showing under 1 kΩ from B to C.

Yay…a mostly working unit.

Finding a TO-39 part for the output transistor might be tricky. I do have quite a few in the junk box, but whether any will have good enough AC specs…??? It does kinda hafta be that package, as the heatsink is made to fit it.

As for Q182, it’s an RF transistor—I may have a suitable sub lying around…

So…transistors. Available selection these days seems pretty sad. Am I right, or not? Especially in leaded (no surprise there). The four types of SS xstrs in the 2101 fall into an NPN/PNP-RF/Switching matrix. I sure hope a PNP switch never fails, cuz I couldn’t find a single example at Digikey, regardless of package…

I should know this, but do we have a Tek parts cross reference somewhere in the files? Quite a few parts in the 2101 have no description beyond “Tek Spec”…

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