Hi guys...
The by-far-most-likely cause of a toasted plate choke in a
76A/CA/PA-374A-78 or earlier series ALPHA is attempting to operate it on
25-27 MHz without modification. We intentionally placed the first series
resonance of the chokes in these amps so as to discourage (illegal) CB
operation. 12M didn't exist as a ham band at the time and so wasn't a
consideration.
I'd also guess that overheating (melting??) of the bandswitch lead may have
resulted from attempting to tune the 76PA amp in question on 11 or 12M. The
loaded Q of (and circulating current in) the output pi-L is highest on the
highest-frequency bands in virtually all amplifiers. A nearby resonance of
the plate choke, and/or high load VSWR) may make it significantly worse.
The results can be essentially the same as described in the next few
paragraphs for the case of inadequate loading.
Also, when typical power amps with reasonably low loss output tank
components (high UNLOADED tank Q) are much too lightly loaded, assuming the
tank is more or less tuned to resonance, usual consequences are a) abnormal
and excessively high peak rf plate voltage (which can and often does cause
capacitor or bandswitch flashover) AND simultaneously b) abnormal and
excessively high rms circulating rf current in the tank (which can and
often does cause overheating of related tank wiring).
Bandswitch contacts may also overheat to the point of de-tempering
(softening) so that contact pressure is lost, IsquaredR power loss in the
contact increases, and failure occurs.
I'd suggest replacing damaged strapping with copper strip not less than
3/16" wide and soft enough to be installed without significantly stressing
switch lugs. Note that the strapping helps carry heat away from switch
contacts and solder joints. So-called "18 oz." copper is probably the best
compromise. I've sometimes crimped & soldered a heavier copper heat sink
(e.g., 3/4" square of 24 oz. copper) to the highest-frequency bandswitch
lead to help keep it comfortably cool. It also looks a bit mysterious and
prompts lots of questions.)
As Rich suggests, it's best to use solder containing ~5% tin in
high-current rf joints -- makes stronger, more conductive,
higher-melting-point connections.
Good luck & 73,
Dick W0ID (ex-W4ETO)
-----Original Message-----
From: pmarkham [SMTP:pmarkham@newsguy.com]
Sent: Sunday, October 18, 1998 10:29 AM
To: amps@contesting.com
Subject: Re: [AMPS] Alpha 76PA repairs
From: km1h@juno.com
To: amps@contesting.com
Date sent: Sun, 18 Oct 1998 09:58:56 -0400
Subject: Re: [AMPS] Alpha 76PA repairs
About an Apha 76CA, Karl said:
>
> They do not like 17M with the stock choke.
What would you suggest replacing it with, if an out of the box solution
is
available? Rewinding it is an obvious solution ;-)
Are there related WARC band operation related problems with easy fixes
or
general design weaknesses?
If so what are they? URLs and publication pointers will suffice.
TIA,
Pete/wa4hei
HF APRS: 10,149,200/18,107,700/28,293,100 Hz mark (lowest emitted
frequency)
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