On my own 76PA and several other 76/78 family amps Ive worked on there have
been numerous problems on the PC board such as a shorted tantalum cap,
transistors, out of tolerance resistors, etc.
Im not saying thats your problem but it should all be working correct before
possibly spinning wheels.
Ive also replaced bad relays and blown open step start resistors.
Its strange that Dentrons dont have even half the problems which are also well
known and easy to fix.
Carl
KM1H
----- Original Message -----
From: Jeff Blaine
To: Charlie Young ; km1h@jeremy.mv.com ; amps@contesting.com
Sent: Wednesday, May 02, 2012 11:42 AM
Subject: Re: [Amps] A76pa glitch on warm up
Thanks guys for the comment. I measured both and they are still “resistive”
– in that one reads about 10 ohms and the other a bit lower at 7. But not open.
I think I will go ahead and replace that relay while the box is open. Lot of
wires on those terminals and it’s conveniently buried way in the corner. And
with the added junk I have put in there, it’s a bit tough to reach. Not
impossible. Finding a correct sized relay in the junkbox is unlikely so I will
need to order something from Mouser.
I went ahead and checked out the amp with a FB fuse in for F3. It tends to
glitch off when I switch the HV between SSB to CW. But I did not leave it on
for very long.
Today I will pull the anode clips just to make sure I don’t have an issue
with one of these ancient tubes and something strange. I’m sure everyone with
one of these amps lives in fear of the day one of those little 74’s starts to
misbehave.
It’s really a great amp – other than a couple of leaking filter caps, this is
the only problem I’ve had with it in the last 18 months since rescuing it from
a buddy’s garage.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
From: Charlie Young
Sent: Wednesday, May 02, 2012 7:14 AM
To: keepwalking188@yahoo.com ; km1h@jeremy.mv.com ; amps@contesting.com
Subject: RE: [Amps] A76pa glitch on warm up
Jeff, hopefully you won't find the problem I had on a friend's Alpha 91b. I
suspect this problem is rare, but might be worth noting. These amps share a
power transformer design which uses multiple windings on a common core.
A step start resistor was open. After replacement, on startup the Alpha 91b
would immediately blow the step start fuse. I checked all the usual suspects
in the power supply and on the HV side of it, with no problem found.
To make a long story short, the filament winding and HV winding became
connected inside the transformer. With the HV winding connected to the diode
board, the transformer behaved like it had a shorted winding on startup,
immediately blowing the step start fuse. If I lifted the HV winding off the
diode board, the amp would power up fine. The no load voltage across the
disconnected HV winding was good. Only when it was connected to the diode
board did the transformer act shorted.
Again, this is hopefully not your problem with the 76, but I will never
forget this issue, since it took me a while to figure out what the heck was
going on.
I also have a 76PA. It has not been on in two or three years. This reminds
me to power it up slow and blow the cobwebs out of it.
73 Charlie N8RR
> From: keepwalking188@yahoo.com
> To: km1h@jeremy.mv.com; amps@contesting.com
> Date: Tue, 1 May 2012 23:32:01 -0500
> Subject: Re: [Amps] A76pa glitch on warm up
>
> Finally got back to looking into this after some travel and found the
> blasted amp is DOA. Murphy's revenge!
>
> Well, not quite DOA. Power relay K1 is functional. But K2 offers some
> interesting observations... To wit, the red wire side of the step start
> relay K2 seems to be shorted. And fuse F3 - the step start fuse - is blown
> open...
>
> I've not lost F3 before that I can recall. But the amp is 25 years old so
> I'm not sure how to read the short on relay K2... Is the 25 years catching
> up to my sweetheart - or is this a sign of some kind of trouble down the HV
> supply path?
>
> 73/jeff/ac0c
> www.ac0c.com
> alpha-charlie-zero-charlie
>
> -----Original Message-----
> From: Carl
> Sent: Friday, April 06, 2012 9:46 AM
> To: Jeff Blaine ; amps@contesting.com
> Subject: Re: [Amps] A76pa glitch on warm up
>
> Leaky caps and out of tolerance resistors on the PC board would be the first
> place Id look.
>
> Carl
> KM1H
>
>
>
>
> ----- Original Message -----
> From: "Jeff Blaine" <keepwalking188@yahoo.com>
> To: <amps@contesting.com>
> Sent: Thursday, April 05, 2012 11:43 PM
> Subject: [Amps] A76pa glitch on warm up
>
>
> > My beloved A76pa in the last month has started to glitch during the
> > warm-up countdown cycle. And on a couple of occasions it has shown a few
> > hundred mA of Ip when sitting at idle.
> >
> > Wanted to see what the group thought may be at the root of it...
> >
> > 73/jeff/ac0c
> > www.ac0c.com
> > alpha-charlie-zero-charlie
> >
> > _______________________________________________
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> >
> >
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