[Amps] Alpha 77dx issue
Carcia, Joe, NJ1Q
nj1q at arrl.org
Thu Oct 6 06:24:53 PDT 2011
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??
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