[Amps] Solid state amp advice needed

[mark]

Vic,

It has  happend to us as well (MRF154) It boiled down that the Power supply
lost it's regulation
and when it recovered from a step current request it overshoot the MAX
Colector Break Down
Voltage by many times. Given the fact that your transistors keep blowing
regardles of what board you
are inserting points to a problem OUTSIDE of the transistor area...

I would check the power supply with an active load that can impose a1ms Load
Pulse form 0 to MAX
current available and whatch the recovery voltage on the outputs...

My 2 Euro Cents :)

Mark
www.sandlabs.com


----- 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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