Adding to the list of possibilities, I had two similar problems in my Alpha
70/77 amps that were traced to two different causes.
In the first case, the silicon HV wiring failed. I've never been impressed
with the manner in which the HV cabling is attached to the aluminum side walls
in Alpha amps. Nylon ty-wraps are over tightened, compressing the cable to a
distance less than the radius of the silicon jacket to the chassis side wall.
In time, I believe ty-wrap abrasion causes the silicon to break down.
In the second instance, a plastic E.F. Johnson feed-through insulator was
arcing. The feed-through passes HV in a PA-70V amp between the PS and RF
compartments. The fix involved replacement with a glazed-ceramic beehive type.
Both fixes were applied around 1998 and the arcing problems have not reappeared
since then.
Paul, W9AC
Sent from my iPhone6
> On Nov 25, 2015, at 11:22 PM, Jim Thomson <jim.thom@telus.net> wrote:
>
> Date: Tue, 24 Nov 2015 00:41:41 +0000
> From: Manfred Mornhinweg <manfred@ludens.cl>
> To: amps@contesting.com
> Subject: Re: [Amps] Alpha 76 question
>
> Ted,
>
>> My Alpha 76 periodically "snaps" loudly (arcs?) when idling...maybe
>> every 20-30 minutes. Any ideas what this may be?
>
> I don't know anything specific about that Alpha, but I have seen this kind of
> arcing at regular intervals when there is a simple power supply that has no
> bleeder and no load on it. There are very often lots of very narrow but tall
> spikes riding on the power line. When there is no load at all on a power
> supply,
> these will slowly charge the filter capacitor to such an overvoltage that
> something arcs.
>
> So I would suggest to check all bleeder resistors, first of all.
>
> Manfred
>
> ## IF ANY of the EQ resistors across ANY of the electrolytics ever opened
> up, that one electrolytic would blow up, its voltage drop across it, with NO
> eq resistor,
> would rise to the full B+ level. If any of the EQ resistors value, drifted
> way up in value,
> its V drop across that cap would also rise sky high.
>
> ## It would be wise to check all the caps anyway, and perhaps replace the
> entire string
> with modern new variety. I would check and probably replace the EQ
> resistors on
> spec with modern 100K @ 3 watt MOF types, like 1-5% tolerance. You can
> get the
> 100K @ 3 watt resistors from Rich measures, AG6K, who will also hand match
> them for you.
> You can also buy them from mouser, in 1% tolerance. The 1% ones I bought
> from mouser all ended
> up being the exact same value, 99,980 ohms...all 300 of em, in boxes of
> 100..and 3 different lots.
>
> ## Find out where the arcing is coming from 1st. It could be as simple as
> dust bunnys, or other debris in there.
> Or misc junk in there somewhere. You could also pull the tubes, and see if
> the arcing still occurs. It could also be
> HV wiring that has shifted. I have also heard the arcing myself, on a
> drake L4B diode board....and one of the oem diodes
> was arcing on the outside off its casing, with blue sparks snapping away.
> easy fix, just replaced that one diode.
> These days, I would replace the entire mess of diodes with either
> 1N5408s.....or 6A10s. Both use the same gauge wires
> coming out each end. Both are the same body length. Body diameter is
> double on a 6A10. Both are cheap to buy.
>
> Jim VE7RF
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