Vic,
I recently had problems working on a solid state final. The transistors
idled fine but the minute I applied drive the collector current went way up
and the output was low. I too, let the smoke out of some expensive
transistors.
What I found, through careful inspection, was a bad chip capacitor in the
collector circuit. This final was also subjected to a mismatched load which,
I believe, caused the cap to fail.
It had me scratching my head though, as I knew where the problem was but
couldn't see the problem until I started pocking around with a bright light,
magnifying glass, and a dental tool.
Good luck in getting that amp up and running!
73,
Jeff - K1LE - CT ><>
----- Original Message -----
From: "Vic Rosenthal" <rakefet@rakefet.com>
To: "Amps reflector" <amps@contesting.com>
Sent: Sunday, November 24, 2002 1:51 PM
Subject: [Amps] Solid state amp advice needed
> I have a Henry SS750HF solid state amp. It has four boards, each with a
pair of
> 2sc2879's. The following happened:
>
> 1) I applied drive on 28 MHz when the filter was set for 14 MHz (he who is
> without sin may cast the first stone). After I finally noticed that there
> wasn't any output, I selected the correct filter and found that the output
was
> much lower than normal. Investigation revealed that the E-B junction of
one of
> the transistors was SHORTED (all of the others were OK).
>
> 2) I had a spare board with transistors from the same lot as the others.
I was
> 99% sure that the board was good. I swapped it in and applied drive (on
the
> same band). Same result -- low output. Investigation revealed that the
E-B
> junctions of BOTH transistors on the board were OPEN. I decided that the
board
> hadn't been any good in the first place, and I ordered a matched pair of
> 2sc2879's from RF Parts.
>
> 3) I put these transistors on the original board. I checked the
transistors and
> adjusted the bias so that each board drew 250 ma in idle. Then I applied
> drive. Result: low output. Investigation revealed that the E-B junctions
of
> the new transistors were now OPEN.
>
> I've looked at the splitter and combiner on the theory that maybe
something was
> funelling all the drive to one board or there was no load, etc. These are
very
> simple units and nothing appears to be wrong. heat is not an issue; it
happens
> right away.
>
> I've converted about $85 (including shipping) of transistors into sand and
I'm
> not prepared to spring for any more unless I can figure out what's going
on.
> Does anyone have any idea? Right now I'm in a mood to entertain offers to
buy
> the unit as is, too.
>
> Vic K2VCO
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