[Amps] Alpha 76 question

[Jim Thomson]

Date: Tue, 24 Nov 2015 00:41:41 +0000
From: Manfred Mornhinweg <manfred at ludens.cl>
To: amps at 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