UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Superstar 6900N V6

Mike. Watterson · 24th May 2020, 6:46 pm

Repaired the PSU, that's a separate story.

From spending some time trying to make a cheap wideband HF power amp, when I realised CB stuff was using IRF510, about 12+ years ago and looking at my notes, I can't see how you can change the FETs without readjusting the gate bias. It's also a fact that MOSFET PAs with incorrect bias are more unstable with SSB, or CW via drive on/off rather than Supply keying. FM is the most forgiving of bias (because you don't really need much, or even any in Class C). AM bias affects distortion, but the always on carrier means the FETs are less likely to self oscillate.

It's unlikely to be a Mic problem. Since all the switched 8V supplies and TIP36 modulator / PA power regulator via supply volts are OK, then instability due to bias is a suspect, assuming the SSB modulator and final modulators are OK, and it sounds like they are.

The PSU uses 2x 2N3055 parallel as the series regulator. The self oscillating IRF510 with 6V supply and 1A currrent limit destroyed them and I think the dual op-amp. It may have been the op-amp that died first.

24th May 2020, 6:46 pm	#94
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Superstar 6900N V6 - Fault with SSB mode Repaired the PSU, that's a separate story. From spending some time trying to make a cheap wideband HF power amp, when I realised CB stuff was using IRF510, about 12+ years ago and looking at my notes, I can't see how you can change the FETs without readjusting the gate bias. It's also a fact that MOSFET PAs with incorrect bias are more unstable with SSB, or CW via drive on/off rather than Supply keying. FM is the most forgiving of bias (because you don't really need much, or even any in Class C). AM bias affects distortion, but the always on carrier means the FETs are less likely to self oscillate. It's unlikely to be a Mic problem. Since all the switched 8V supplies and TIP36 modulator / PA power regulator via supply volts are OK, then instability due to bias is a suspect, assuming the SSB modulator and final modulators are OK, and it sounds like they are. The PSU uses 2x 2N3055 parallel as the series regulator. The self oscillating IRF510 with 6V supply and 1A currrent limit destroyed them and I think the dual op-amp. It may have been the op-amp that died first.