Sunday, March 11, 2018

Astron Revitalization (Part 3)

So it turns out that there was an outright rebellion of the posters on the Repeater Builders' Yahoo Group. The newcomers ALL said that the problem was the pass transistors. Jeff, WN3A, replied as follows:
> (1) Grief came to the DUT when the PS terminals were
> shorted/reversed polarized by a gel cell I was charging.
> (Long story; yes, I am aware of the caveats arising therefrom.)

Which was it - shorted or reversed? If reversed, then I would think the
first damage to occur would be to the pass transistors since the
base-emitter breakdown voltage is likely to have been exceeded.
> (c) Pass transistor voltages
> E=10.7
> B=23.3
> C=23.0

Sure looks like blown pass transistors to me...
--- Jeff WN3A
Heretofore, I was somewhat squeamish about pulling them and checking them but this sealed the deal and, to my surprise, it was quite simple to do as you just unscrew the little bastages. After pulling them, they were checked with an ohm meter. Now, I was never really clear on romping around a transistor and checking the junctions -- you know, the E-C, C-E, B-E, E-C, X-Y, Z-A and all that crap -- so I went out to YouTube and found a how-to that made it simple:

Following this method showed me that both pass transistors were as dead as last week's meatloaf. Not only that, it showed me that the ones I had in my junk box were not too terribly peachy as well. So I ordered two 2N3771s (the 2N3055's tougher brother) and, until they came, I scrounged a couple of good 2N3055s in my box, greased 'em up, and slammed 'em back in the holes.

Waddya know! The PS came back to life and I am a happy camper! Here are the good voltages:

So what's left to do on this project:

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