Apparently there exists a problem with the HM56 microphone that causes
the radio to key up spontaneously due to capacitor failures on this
microphone.

Can anyone provide some assistance on the details of this procedure.
Supposedly there are 5 surface mount capacitors that need replacement.

In article , Ken Roser wrote:

Apparently there exists a problem with the HM56 microphone that causes
the radio to key up spontaneously due to capacitor failures on this
microphone.

Can anyone provide some assistance on the details of this procedure.
Supposedly there are 5 surface mount capacitors that need replacement.

Ken-

This is the first I've heard that someone had solved the problem, which is
apparently common. Mine keys the transmitter when power is first
applied. After several seconds, the transmitter unkeys and everything is
OK until powered down. Replacing the CMOS battery didn't help.

Somewhere I found a copy of the HM-56A diagram and parts list, possibly at
www.mods.dk. I suspected that a transistor connected to the CPU's reset
line was leaking and slowing down the power-on reset. I had not
considered capacitor leakage as a possible cause.

Please post if you learn more.

73,
Fred
K4DII

Here's the details. ICOM has a service bulletin that describes the
problem and fix. You'll need to replace 5 surface mount capacitors. I
ordered the parts from ICOM and the total cost is about $8. A link to
the bulletin is
http://www.icomamerica.com/support/k...x?Download=180

The text of the bulletin is reproduced below:

"HM56/56A - Electrolytic Capacitor replacement

A common failure of the HM56/56A microphone, causing the radio to remain
in TX mode when the mic is connected, is failure of the electrolytic
capacitors on the microphone circuit board. Generally, replacing all 5
capacitors on the board repairs the problem and allows the microphone to
work properly again. This repair is not a guarantee, simply a common
failure point and reference for repair.

The 5 capacitors on the board are labeled as C3, C4, C7, C13 and C14.
They can be ordered from the Icom parts department using the following
part numbers:"

(see the bulletin for a photograph and parts list)



Fred McKenzie wrote:
In article , Ken Roser wrote:

Apparently there exists a problem with the HM56 microphone that causes
the radio to key up spontaneously due to capacitor failures on this
microphone.

Can anyone provide some assistance on the details of this procedure.
Supposedly there are 5 surface mount capacitors that need replacement.

Ken-

This is the first I've heard that someone had solved the problem, which is
apparently common. Mine keys the transmitter when power is first
applied. After several seconds, the transmitter unkeys and everything is
OK until powered down. Replacing the CMOS battery didn't help.

Somewhere I found a copy of the HM-56A diagram and parts list, possibly at
www.mods.dk. I suspected that a transistor connected to the CPU's reset
line was leaking and slowing down the power-on reset. I had not
considered capacitor leakage as a possible cause.

Please post if you learn more.

73,
Fred
K4DII