GM300 Intermittent Audio Problem

[railtrailbiker]

I'm using a UHF high power GM300 to drive an aftermarket PA in a repeater application. Tx power on the GM300 is set low - about 18 to 20 watts.

An intermittent audio problem developed in one unit - a low rumble in the audio, plus drop out for extended periods. Fortunately, I had a spare unit and was back in service almost immediately.

However, my tech could not duplicate the problem on his bench, this despite cooking the unit, using a microphone, and hooking the 16 pin connector up to a repeater he had in the shop. He returned the unit to me without charging; all he did was to perform a realignment.

I'm hesitant to place the unit in service again, fearing the problem will return. Any ideas as to what could have caused the low rumble and audio drop out? Could the vibration associated with shipping have corrected things, or could it have just been out of alignment?

[jackhackett]

First suspect on a GM300 is usually the feedthrough connector between the logic and RF boards. The early ones marked 64-80535B01 go bad and should be replaced with the newer style 2804637J01.

[kcbooboo]

Definitely the interconnect pins. The realignment confirms that something has changed. It usually shows up as the radio being off-frequency; this is caused by dirty pins.

Remove the logic board and the RF board, then pull the I/C pin assembly and clean the pins with some fine steel wool. Make sure there is no residue or even a thread of steel wool remaining between the pins. If you have some contact cleaner, apply some to the pins and insert and remove them into the boards to clean the socket contacts as well. Reassemble and go through the alignment again; I bet the frequency warp comes right back to the normal 100-103 value.

Bob M.

[jackhackett]

Note that cleaning the pins only works if you already have the newer style connector, if you have the older one it is 100% guaranteed bad and should be replaced.

Also for cleaning connector pins I find that an eraser pencil works quite well, the kind that's like a pencil except it has eraser material instead of lead, and usually a little brush on the end. Use a utility knife to shave the point into a wedge shape so you have a wide flat area to work with, works great, just enough of an abrasive to clean the pins without damaging them.

[railtrailbiker]

Thanks.