"Automatic Side Band" circuit in Palomar Elite 250

[BayouRadioAmplifier]

I have a few of these 250's here. This RF key circuit has been messed with
in both amps. One is keyed up all the time. I traced the diagram out
and it makes little sense. So did this circuit ever work? AM and SSB?

These amps seem to be OK. The transistors are biased, unlike many
of these amps.

 

[nomadradio]

Pretty sure this is the keying circuit it used. Four parts, a 15pf disc cap from the center pin of the radio jack to the base lead of a PN2907A transistor. A 470 ohm resistor from base to emitter, a 0.1uf cap from collector to emitter.

Not a lot to go wrong. Then again, I sell a keying circuit on a small pc board at fleabay.

https://www.ebay.com/itm/1160941166...pid=5336136228&customid=&toolid=10001&mkevt=1

Sure, it's a shameless plug, but this board got developed as a labor-saving fix for amplifiers with lame keying circuits.

The attached schematic isn't for this model, but I'm fairly sure it came from the same source as the 250. Odds are the keying circuit at the lower-left corner is the same as yours.

The "automatic" sideband is the fat 1000uf cap wired in parallel with the two coil pins on the relay. If the factory keying circuit is original, removing it and replacing it with a 4.7 or 10uf cap will prevent it from chattering or damaging the keying transistor. Just taking it out altogether runs that risk.

More than one way to skin this cat.

73

 

[BayouRadioAmplifier]

yes, these amplifiers have "lame keying circuits."

most of the amps I get, the RF key circuit is blown out,
and has an attempt at repairing it.
I put in a more robust circuit, that will not blow out.