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2980 WX Replacement Finals

Really doesn't sound as if the SSB bias was adjusted successfully. The component values visible in the pics are valid for a MOSFET final and driver, not a bipolars. AM mode is not so finicky about this, but listening to the sideband transmit audio on another radio should reveal an issue. Turn down the mike gain so the wattmeter won't kick above 1 Watt when you talk. If I'm right there will be a "gritty" quality to the audio that goes away bit by bit as the mike gain is increased.

Popping the "mirror" board and connecting a current meter to check SSB collector current with the mike gain at zero should show zero idle current.

And if AM is all you care about, it ain't broke for that mode.

73
 
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Been
Eldorado828 said:
The only thing I see that isn't factory are those oversized ceramic insulators,
Click to expand...

It's been a while, but we saw the bigger insulators in RCI-made mobile radios. Had to cock the one under the driver transistor just right to one side so the outboard heatsink screw shaft wouldn't interfere with it.

The smaller ones have been in use for at least 10 years.

73
 
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i worked on these before.
the fets are factory as are the ceramic insulators.

trying to convert it to bipolar operation could be a pretty daunting task.

remember that not all details are shown on the schematics. for example, the jumpers aren't shown.
also there are ferrite beads that need to be mounted on certain resistors when using a bipolar that aren't required when using mosfets.
also, the diodes in front of the driver and finals usually goes the other way, and its a different type of diode.
the silkscreen on the board could show that the diodes in there now look backwards but obviously they're not.

im just saying there's a lot that can be left open to interpretation.

also, how do you know this will fix your problem?

you could have an issue in the output tank circuit that runs after the final and before the antenna jack.

is it the radio's internal meter that is showing the high SWR?
confirmed with an external meter?

there is a pot on the back side of that vertical SWR board in that radio and if someone messed with it you could be getting false readings.
LC
 
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The original problem is that the SWR goes over 3:1 on my Autek meter. No other radios in my shack do this. Everything seems to work fine, it even sounds good on the air. I will hook it back up in the shack to see if I can get more details. When I opened the lid, I saw the FETs and the heat sink compound was a mess. I cleaned it up a little before I took the pictures. So I was thinking it was a botched FET conversion, and wanted to put it "back" to bipolar.

It looks like the fix is not as simple as I hoped. So for the high SWR, maybe the output is oscillating? Maybe the tx cans are not adjusted correctly? I need to figure what could cause this and how to check it out?

As you guys can tell, I am not a radio tech. Thanks to everyone for their responses so far.
 
BC Coyote said:
Has the LPF been messed with ? It sounds like it is radiating higher order harmonics and that is what is throwing your SWR way high.....
Click to expand...
The coils appear to be normal ( not spread out) but maybe a capacitor failed. One of the first things to check. I have an antenna match box that I can try, might tell me the output needs more capacitance. Something to try I guess. I will update my progress as time comes available. I will probably start a new thread and cross link this one to it.
 
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The bias circuit that sets gate voltage on the FET finals can't supply nearly enough current to the base terminal of a bipolar final.

The AM carrier will switch on the base circuit of the driver and final, just like it does in a Cobra 29 with a bipolar driver and final.

Sounds like that's what makes this one transmit on AM.

But hooking up a current meter to the 'mirror'-board pins will reveal zero current draw when keyed on sideband with zero mike gain.

This will severely affect SSB audio quality.

Restoring the input circuits to the driver and finals to match the original bipolar-final design is the only way to make bipolar transistors behave themselves properly on sideband transmit. Just copy the "old" circuit.

Someone with a really helpful attitude could come up with a bill of needed materials, a graphic showing which parts to remove, and what new part goes in place of the factory-originals.

Won't be me any time soon.

73
 
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