Recently, I installed a Harbach RM-100 metering/diode board in my SB-220.
It fixed a low HV meter reading and has performed admirably for a couple of
months.
This evening, I went to tune up on 80 meters and noticed below-normal
output. While tuning, key-down, in high-voltage (SSB) position, I noticed
declining RF output
(measured on an outboard meter), coupled with climbing plate current over
the key-down period, followed by smoke from the area of the metering board.
Trying to figure out what was happening, I tried the amplifier key-down on
20 and 40 meter, and all meter readings seemed normal. I then went back to
80, and verified that the same combination of symptoms was still happening,
both in HV and LV (CW) position. At this point, there was a popping sound,
the plate current meter pinned (with the amplifier unkeyed), something went
white-hot on the metering board, and I shut things off.
Inspecting the metering board, it appears that the lower of the two large
resistors on the right side of the circuit board (viewed from the front)
went white-hot and then failed open. There's a burned spot under the
resistor. The one above it also appears to have an overheated band around
its center, which suggests that I'd better think about replacing it too. I
don't have a schematic of the replacement metering board, but it appears
these are the multiplier and shunt resistors associated with the plate meter.
I'm presuming that replacing the resistors will restore the plate metering
function, and have contacted Harbach about those, but am concerned that
there's some underlying cause. Does anyone have any idea what happened?
Except for the plate current and power output changes on 80m., there were
no signs of trouble. I was operating the amp into an antenna SWR of less
than 1.5 to 1. Is there some underlying cause I need to chase down?
Any advice greatly appreciated.
73, Pete Smith N4ZR
n4zr@contesting.com
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