On Thu, 18 Sep 1997 10:39:52 Pete Smith <n4zr@contesting.com> writes:
>
>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.
Probably a grid choke frying and trying to tell you a problem was
happening. It could also be a grid bypass capacitor, particularly if it
was one of the old style rectangular ones. At that point you should have
shut off and investigated.
>
>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.
About time! You probably lost a tube...which was damaged from the prior
big bang; and maybe the meter also unless there were protection diodes
across it.
Other things probably blown are one or more of the bias diodes on the
Harbach board.
Private E mail me and I'll give you a list of other things to check
>
>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.
They are the same values and in the same position as the original Heath.
>From top to bottom:
.82 Ohms, 3600 Ohms, 1 Ohm
> 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?
Probably time to think about replacing the tubes!
>Any advice greatly appreciated.
Here is some free advice. When something STARTS to happen, shut down
immediately to investigate. The SB-220 is a very forgiving amp and often
gives warning signs....just heed them.
73 Carl KM1H
>73, Pete Smith N4ZR
>n4zr@contesting.com
>
>
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>
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