Hi Charlie, transformers that fail with HV windings shorting to either the
primary winding or to any other winding is quite typical. I’ve been repairing
amps for 50 years, all sorts except SS. I know typical failure points on many
amps. For example, it’s a common failure on the Heath SB1000 for the HV
winding to short to the 12v control winding. On the TT Titan 425 the power
transformer typically shorts from HV winding to one specific primary winding.
I have a shelf full of shorted 425 transformers. Recently came across a
shorted Acom 1000 transformer. But this 91b appears to have the circuit board
trace wrong. Hard to believe.
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On Monday, September 27, 2021, 8:39 AM, Charlie Young <weeksmgr@hotmail.com>
wrote:
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Hello Lou
Your post reminded me of an issue with a friend’s 91b. Gosh it is has been 10
years ago now. It may be common for those who do amp repairs as a business to
find this but it was a first for me.
The amp HV section would work fine but only if you unplugged the transformer
connector on the LV transformer windings. If you unplugged the HV secondary
connector, all the LV circuits worked fine. All the voltages are supplied by a
common transformer. The transformer had an internal fault between the HV
winding and the filament winding. Only if you had both winding connected to
the amp did the problem (HV short) manifest.
I called Alpha and ordered a transformer, which fortunately they had. The
individual at Alpha did not believe the transformer was bad, so it must not be
a common failure. They expected I would be returning it. Well, the
transformer was bad. I still have the defective one.
These are nice amps and the repaired one is still going strong with original
tubes (WD8CCC).
73 Charlie N8RR
Sent from Mail for Windows
From: gudguyham via Amps
Sent: Monday, September 27, 2021 7:34 AM
To: Amps Group
Subject: [Amps] Alpha 91b anomaly
Hey guys, hope everyone is staying safe these days. We’ll just as you thought
you’ve seen it all something weird comes along. I’m working on a 91b that
belongs to an ARRL director. He bought it recently from a ham that was
liquidating an estate of an SK. The seller doesn’t know anything about the
history of the amp and the guy that does is SK so I guess it ends there. Well
anyway the amp comes to me with the problem “ it won’t turn on”. I won’t bore
you with the step by step troubleshooting process but if you look at the
schematic, the start up process is a major series of logic circuits to get the
damn thing to turn on. I fully checked it all out and suddenly I discovered a
couple wires going off the schematic diagram and over to another diagram of the
screen board. When I followed those wires on the schematic I see a closed loop
going through the contacts on K1 on the screen board. If connects via J4. I
pulled the connector off J4 and I looked on pins 1 and 2 for a 15 ohm reading
which would confirm a closed loop through K1 common and NC contacts. This loop
completes the logic circuit that latches the contractor relay. K1 energizes in
a major Plate current fault and opens the contractor and kicks the amp off
line. As you’ll notice on the schematic the K1 loop is closed when the amp is
de-energized and only opens in a fault. Well when I tested for 15 ohms on pins
1 and 2 I read open. Well ah ha that explains why the amp won’t turn on. It
wasn’t hard to figure there’s something open in that loop. There’s only a 15
ohm resistor and a set of relay contacts. What I discovered is nothing short
of incredible. The 15 ohm resistor was not open and the relay contacts were
not welded closed. The problem is that the 15 ohm resistor which is traced
over to the NC contact of the relay on the schematic is actually traced over to
the NO contact. Subsequently the loop is open and not closed with the relay
not energized. I ohmed out the relay contacts and sure enough the relay is
fine and it ohms out according to the cut sheet. I checked the part number on
the relay with the parts list and it agrees. The trace on the PC board
connects to the wrong terminal on the relay. You can see through the relay
cover and tell that the contacts are not welded. The schematic agrees with the
relay logic, but the PC board simply had the trace misplaced. That said, this
amp could have never worked as is. What’s going on here? Anybody?
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