Bearcat 980SSB No Transmit problem

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I have a 980SSB that worked fine, one night I think the desk mic may have accidentally got bumped and locked into transmit, blowing the RD16HHF1 fet eventually. The mic is a old Turner Super Sidekick, has that sliding lock switch that can be troublesome if you bump the mic around.

So this thing has sat on the shelf for many months. I pulled the RD16HHF1 Fet out and it tests bad (.02 or so) between the Gate and Source legs (Pins 1 and 2). I don't think it's supposed to read that way, though this is my first time messing with anything with a RD16HHF1.

My attempt to repair it, I found a used IRF520, that tested good, installed it with a insulator and the special bushing for the TO-220 tab so it doesn't ground the Tab to the Chassis. I cross the D/S legs to the circuit board, check everything, nothing shorted, everything connected where it should be, turned it on, same problem. Keys up on the screen, the S/RF meter starts out High, falls back slowly to 0 or 1. No output on the external power meter. Checked the bias, it's working. Used another Radio for RX, I can barely hear it, audio and some signal is coming out. Touched the Tab of the IRF520 with my Scope Probe and it got much much louder and stronger on the receiving radio.

Is there a device or protection diode or something in this radio that handles RX/TX Switching? My next thought would be that this is bad. It's like the radio is telling it to switch RX/TX but it's not happening. The radio pulls current when you key it, alot if you crank the bias up. Maybe it's just the drivers, not sure what they are haven't looked yet. Figured I'd ask here, maybe someone experienced with these radios (I don't even have a schematic for it) could point me in the right direction to get it working again.

Thanks.

 

[443]

Update on this. After closer look with the scope probing around, it appears that I have proper drive signal, Bias voltage, and switching (RX/TX).

The problem appears to be that if you replace the RD16HHF1 with a IRF520, you have to retune the output coils. I did not want to mess with that, will just order a handful of RD16HHF1's and when they get here, try replacing it, if a minor retune is necessary will do that. I think I'll add some sort of heatsink to the back of the radio, using a longer screw, some thermal compound, and a sil-pad insulator (or Mica insulator), since the RD16HHF1 transistors seem to have a reputation for being a little on the fragile side, the extra heat dissipation should take care of most of that issue.

Would have been nice if they would have designed this radio with a little better receiver, and dual RD16HHF1 output transistors for longevity sake. Still cannot believe after all this time, there is not a schematic available for this rig... The only reason I even want to repair it, is because it's in pristine condition, everything works fine, and I kinda like it's layout and functions.