Steve, VK6VZ. Are you reading this?
73, Alek, VK6APK
On 27/09/2021 7:34 pm, gudguyham via Amps wrote:
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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