----- Original Message -----
From: "Jim Barber" <audioguy@q.com>
To: <Amps@contesting.com>
Sent: Saturday, October 19, 2013 4:06 AM
Subject: [Amps] Alpha 76A "hums on 160" problem resolved
** You did good Jim and thanks for the report.
The Alpha 76A "hums on 160m" problem has been resolved.
Thanks to all that responded, on an off the list.
The actual problem turned out to be a plate bypass cap that measured good
out of circuit with a 100kHz capacitance meter, but was essentially open
to any RF current on 160. (not a typical failure mode)
** Ive heard of and serviced similar failures on some much newer Command
amps but not on a venerable Alpha using Centralab caps.....weird! Something
to keep in mind. My 76PA was used regularly at a Caribbean contest station
and had no failures in that area; just considerable lack of PM and operator
training that the 8874's and power transformer objected to!
For those who care to know, here's what I ended up doing:
. Replaced the 15uH 1A cathode choke with a 100uH 2A unit
. Replaced the cathode bypass cap (.02uF) with .04uF (four .01 500V in
parallel)
. Replaced the two .002uF caps at the "cold" end of the larger plate choke
with two .01uf 6KV blue Chinese caps in parallel. (It's what I had, don't
tell Carl...)
** Im just going by comments from others who bought those blue firecrackers
on Fleabay and had them fail. Ive no intention of experimenting here (-; At
the relatively low DC voltage and RF current involved they just may survive.
I replaced the plate choke with an Ameritron and the little "VHF" choke with
a WW glitch resistor. Since the original disc bypasses got a bit mangled in
the process I used a pair of .0047 6KV Vishay's for both chokes. I just took
the cover off to remind myself what I actually did maybe 5-6 years ago.
. Replaced the two .002uF caps at the "cold" end of the smaller 15uh plate
choke with two .01uf 6KV blue Chinese caps in parallel, as above.
. and leading up to this, I had already replaced the filter caps and
bleeder/equalizing resistors, as well as the other carbon-comp resistors
on the control board, which were largely out of tolerance.
** I also replaced the small electrolytic plus the tantalum cap in the timer
circuit.
I'm happy to say the amp shows no hum at all now and loads up normally on
160/80/40/20/17/15/10 meters. The two 8874's are 30+ years old, but
still load up to ~ 1100 watts with 100 watts drive from a solid-state rig.
(a little less on 10m, there appears to be some inefficiency in the input
circuit there) The little amp runs quiet and cool at around 800 watts out,
which is exactly what I want it for.
Thanks again es 73,
Jim N7CXI
** With 3 pulls I can push to 1800W on a few bands with 100W an low grid
current during the initial testing after the rebuild but keep it down to
1200W in regular use on SSB/CW and AM PEP.
I dont need any more power to work anything I want.
Carl
KM1H
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 10.0.1432 / Virus Database: 3222/6262 - Release Date: 10/18/13
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|