[Amps] Alpha 77dx issue

Carcia, Joe, NJ1Q nj1q at arrl.org
Thu Oct 6 06:24:53 PDT 2011

Hi all,

I'm having an issue with an Alpha 77dx.  Before I really dive into it
however, I thought I ask first.

The amp was working fine during a contest.  Shut it down overnight.
When the amp was turned back on in the morn, the step-start popped.  I
replaced it, and it popped again.  I found an arc mark (to the metal
panel where it mounts) from the cover interlock switch (S9) on the fuse
side (F1 - black wire).  I cleared the arc mark, and insulated that part
of the switch.  The step-start fuse no longer blows.   The amp warms up
and the STBY light turns green after about 3-4 minutes.  When placed in
either CW or SSB mode, the plate voltage reads accordingly.  So far, so

But, when I apply RF, the exciter SWR is extremely high and there is no
output power.  The grid current reads maybe 25-30 mills.  According to
the manual, if this condition exists, it's either the RF sensor diodes
(D3-D4) or bias transistors (Q202 or Q203).

I checked the diodes and they appear okay.  Just to be sure, I replaced
the two transistors.  No change.  I actually have a spare 8877 and threw
that in there.  No change.

I checked K4 and K5.  They seem okay.  I could not find any discolored
components or other signs of damage.

So, before I really tear into this amp, I thought I'd first ask the
experts.  I'm guessing it's something simple but that I'm just reading
way too much into it.  Maybe someone had a similar problem??



Joe, NJ1Q


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