Re: 2101 Pulse gen repair report

Tom Lee

Hi Michael,

I expect the same transistors to be good subs for the actual output amp, too, so if the thing blows again, I'd give those a try. I don't know what Tek sorted for, but a (very) crude proxy for ft is 1/BVCBO. Lower breakdown voltage is correlated with higher ft. So if you find the risetime to be marginal, that's one sort you can perform to identify the most promising candidates (assuming you have, or have access to, a curve tracer).

And the 2N918 family includes a great many different part numbers (the 2N918 is sort of the 2N3904 of ~1GHz transistors). Even though through-hole parts are obsolete (or at least obsolescent), I've never had difficulty finding hobbyist-level quantities of devices in that family. Most recently, I bought a handful of 2N5770s from our own Brad Thompson. Those transistors are part of the 918 family. That part, as well as the PNP complement (2N5771), are good to keep in the junk box.


Sent from an iThing, so please forgive the typos and brevity

On Apr 16, 2021, at 4:39, "Michael Dunn" <> wrote:

Thanks Tom, tho, a few notes:

On Fri, Apr 16, 2021 at 05:17 AM, Tom Lee wrote:
For the output amp, the NPNs are 2N3053 and the PNPs are 2N4036, replaceable
here by the venerable 2N2219A and 2N2905, respectively. Those parts are still
available cheaply.
Those parts are for baseline offset, not output amps per se. No AC requirements, so, yes, fairly easily subbed.

To "fix" the output amp, I just removed the offending transistor of the paralleled pair. The remaining part will be fine, as long as I don't push the output too hard.

Q182 is a 2N3563, replaceable by several devices in the 2N918/PN918/2N2857
Which seem to be obsolete, or close enough to :-( At least here, a modern/available SMD could be kluged in if need be. I did find a suitable old part in my junk box (2N5179), though its DC specs may be marginal. I hope it lasts.

Be thankful that it's a 25MHz gen, rather than, say, a 100MHz one. You should
be able to find replacement transistors to keep it going pretty much forever.

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