[Amps] Alpha 76A "hums on 160" problem resolved

[Carl]

----- Original Message ----- 
From: "Jim Barber" <audioguy at q.com>
To: <Amps at 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


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