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President 858 Madison AM mike audio squeal. FIXED!

nomadradio

Analog Retentive
Apr 3, 2005
6,938
11,063
698
Louisville, KY
www.nomadradio.com
Just when you think you've seen everything.

The problem was with an original 1978 President Madison, the radio with a 4-pin mike socket, the uPD858 PLL chip and the hard-to-find MX3P relay.

I installed a carrier control in place of the original AM-carrier regulator circuit, using the front-panel SWR Cal control.

No big deal, but when you turned down the carrier, it would squeal.

Again, no big deal. Not the only kind of decades-old radio that can respond this way. Replaced a dozen or so electrolytic capacitors. No improvement.

Replaced the uPC1156 audio-power chip. No improvement.

Ran out of things to blame, so I removed the wire from the "cold" side of the relay coil. The wire that leads to the mike socket's transmit pin. Grounded the foil pad where the wire had gone to an adjacent ground foil with a pair of hemostats.

BINGO! No more feedback squeal on the AM transmit audio, with the mike gain at full bore, and with the carrier set at any level I liked.

The fix is to insert a PNP transistor between the foil pad for the cold side of the relay coil and ground. The collector goes to that ground foil and the emitter to the cold side of the relay where the wire to the mike socket used to be. The transmit wire leading to the mike socket goes to the base lead of the PNP transistor. Now the mike cord only carries a tiny fraction of the relay's coil current and the transistor carries the bulk of that current directly to the ground foil alongside the relay.

SKeIy4.jpg


Clearly, the relay's coil current flowing "piggyback" in the mike circuit's ground wire was creating a ground loop, and causing the feedback in the mike audio.

But in 40 years of servicing a dozen or more models/ brands with this Uniden circuit board it's the first time I encountered this.

Don't know why I haven't seen it before. But the radio's owner was much relieved to have the thing finally working.

And I was quite relieved I didn't have to reverse the carrier-control installation.

This old dog just keeps learning new tricks with old radios.

73
 


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