So here's a nagging problem we see over and over in the older (1978) Uniden-made 40-channel SSB radios like the RatShack TRC457 and 457, the Robyn SB510, Cobra 139XLR and the 1978 (4-pin mike) President (not Uniden) Washington.
The issue is a persistent feedback "squeal" in the AM transmit audio. Turning the mike gain down usualy fixes it, but turning the carrier down makes it worse.
The cause is the current flow from the coil of the transmit relay. All of the relay's coil current flows through the same ground circuit that carries the mike element's audio signal.
The fix is to install a PNP transistor between the mike socket and the relay coil. This reduces the current through the mike cord by a factor of roughly 100. Takes care of the problem.
Cheap.
First step is to take loose the wire that connects the relay coil to the transmit pin of the mike socket. In this TRC-457, the wire is brown with a white stripe. Other models will use a different color wire.
A PNP transistor like the 2N2907/PN2907A or similar gets its emitter lead soldered to the pcb pad where the brown wire was just removed. The collector lead goes to the handy ground foil to the right of the relay, as seen from this angle.
Now the wire leading to the mike socket's transmit pin is soldered to the base lead of the transistor. Yeah, this connection won't win any beauty contests, but it gets the idea across.
In a nutshell, this is why the following year's SSB CB models from Uniden had a five-pin mike socket instead of four. They added a second ground pin to carry JUST the audio current from the mike, with a separate ground wire for the transmit/receive switching currents.
Can't believe I sent a radio home without installing this fix in it last month. Sure enough, it had a feedback problem on AM transmit and had to make a return visit to the shop. Must be old-timer's disease. Maybe now that I've made a written record of this trick I won't have any trouble remembering it every time we see a radio with this circuit board in it.
73
The issue is a persistent feedback "squeal" in the AM transmit audio. Turning the mike gain down usualy fixes it, but turning the carrier down makes it worse.
The cause is the current flow from the coil of the transmit relay. All of the relay's coil current flows through the same ground circuit that carries the mike element's audio signal.
The fix is to install a PNP transistor between the mike socket and the relay coil. This reduces the current through the mike cord by a factor of roughly 100. Takes care of the problem.
Cheap.
First step is to take loose the wire that connects the relay coil to the transmit pin of the mike socket. In this TRC-457, the wire is brown with a white stripe. Other models will use a different color wire.
A PNP transistor like the 2N2907/PN2907A or similar gets its emitter lead soldered to the pcb pad where the brown wire was just removed. The collector lead goes to the handy ground foil to the right of the relay, as seen from this angle.
Now the wire leading to the mike socket's transmit pin is soldered to the base lead of the transistor. Yeah, this connection won't win any beauty contests, but it gets the idea across.
In a nutshell, this is why the following year's SSB CB models from Uniden had a five-pin mike socket instead of four. They added a second ground pin to carry JUST the audio current from the mike, with a separate ground wire for the transmit/receive switching currents.
Can't believe I sent a radio home without installing this fix in it last month. Sure enough, it had a feedback problem on AM transmit and had to make a return visit to the shop. Must be old-timer's disease. Maybe now that I've made a written record of this trick I won't have any trouble remembering it every time we see a radio with this circuit board in it.
73
