>Hello all;
>
>I am in the process of repairing an Alpha 76PA for a friend.
>
>From initial inspection, I have found the 2 resistors in the step-start (R1,
>R2) burned and the Plate choke burned in half. The owner states that the amp
>worked fine, took off the top cover to do some dust cleaning inside, and the
>dummy powered it up by holding the AC interlock switch on, failing to notice
>the high-voltage intelock mechanical switch !!!
>
>I would think (by studying the schematic) that the choke would not be affected
>by this act of stupidity,
agreed.
>but i can understand the inrush ckt resistors
>going. My conclusion is that he fried the plate choke befofe hand, then
>opened it up for inspection and then screwed up the step start ckt by failing
>to obvserve the hv safety mechanism.
makes some sense
>Does anyone know if the plate choke is still available from Alpha,
easy to rewind with wire from electric motor shops. Bring ur own empty
spool.
> as well as
>a good source for the 10ohm/12w chassis-mountable reisistors ??
>
>My next thought is to check the 3 8874 tubes for a short. Any suggestions ??
Over half of the kaput 8874s I see have gold sputtering damage. Test
with a high-pot. set to 2x the rated anode/plate v: 2.5kV*2= 5kV. The
leakage should be under 10uA. Perform the Funderberg gold-sputtering
test. Loose gold can damage the hv-rfc. A small amount of loose gold
can be resettled.
[see 'Amplifiers' at < www.vcnet.com/measures >]
cheers, Eric
R. L. Measures, 805-386-3734, AG6K
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