[Amps] Alpha 91b anomaly

Mon Sep 27 09:30:39 EDT 2021

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 at 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

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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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