When was the last time the HV filter cap was replaced? About $450 these
days.
Carl
KM1H
----- Original Message -----
From: <k2cb@comcast.net>
To: <amps@contesting.com>
Sent: Tuesday, January 19, 2016 2:59 PM
Subject: [Amps] Alpha 77 troubleshooting
Good Afternoon,
I am working on an ETO Alpha PA-77. The original model, not a DX or SX.
The previous owner had bypassed the warmup relay in an attempt to
circumvent a failure of the warmup and/or grid overcurrent circuitry,
which work in concert with each other.
I isolated the problem to a bad device at Q2 in the grid overload circuit,
which is a 2N5184. Web searches of the usual places (Mouser, DigiKey,
Allied, etc.) all come up empty. The NTE154 is listed as a replacement,
but I've always avoided the NTE parts if at all possible, for various
reasons. Anyone familiar with or experienced working on this amp know of a
suitable and currently available replacement for the 2N5184 device?
While I am at it, anyone know of a suitable replacement for the 40841 fet
device used on this board? Or the 2N5321 device? While they both check
good, I wouldn't mind have a few on hand just in case, especially if I
need to order the other devices.
The next issue is that it appears the amp has a very soft 8877. No matter
how hard I drive it, the tubes seems to top off at about 600w carrier,
even in the 4KV SSB position. However, after tuning it for maximum carrier
output, in this case 600w, if I switch the exciter to SSB, I can see peaks
of 1000-1100w on a peak reading LP-100A meter. I do notice the on carrier
keydown, the high voltage drops from 4.0kv to almost 3.5kv under load. Is
this amount of plate voltage drop normal for this amp? Even at 3500v under
load, I would still expect to see more out of the tube. Is it normal to
see an 8877 do more PEP than carrier power, as per the description above,
when going soft?
Finally, the owner is asking me to install a fan at the rear of the amp
for added cooling. Has anyone done this? Since the power cord is only
wired for 2-wire 240v plus ground, there is no neutral. A quick look at
the schematic, it appears I could parallel a 120vac fan to one of the two
primary windings of the transformer? Further looking at the schematic, it
looks like the blower motor is 110vac, so I can I just parallel to that
(before the blower's speed adjustment wire-wound resistor, obviously!)?
Thanks for any and all input regarding this classic amp.
Eric
K2CB
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