Old uniden Grant PC409ac board

[nomadradio]

Wouldn't the regulator do the same in ssb?

No, the AM modulator transistor is switched out of the circuit by the mode selector in sideband modes. That's why the peaks are always a tiny bit larger on sideband than on AM in Uniden SSB CB's of that era. The added voltage drop of that transistor reduces the peak voltage feeding the driver and final in AM mode. The full B+ voltage is what the selector feeds the final and driver in sideband.

This is not a familiar symptom. Would prompt me to ask what it looks like on a 'scope.

I had a 148 decades ago that had a RF-feedback gremlin in the final stage. Would only distort when sideband peaks exceeded 3 Watts. If I turned the mike gain low enough it sounded pretty sweet. But as soon as the voice peaks passed that threshold, it would garble. Not a frequency-pull garble, but a mushy distortion.

Never did find the root of that one. In that case, the 'scope wasn't a lot of help. But it would settle the question of whether the modulator is feeding the 'crackle' into the power stages, or if the RF circuits are the root of it. Just having a peek at the waveform on the bias-test jumper pins might answer that question.

73

 

[MKwrench]

Bump
Last I posted to this thread was nearly a year ago and now here I am again. I ordered a cap kit from Klondike Mike's and got it installed today so I can start using this radio.

I'll rewind just a little bit. So before I did any work to this radio, I noticed kind of a crackle or crunchy audio primarily on the audio peaks BUT only in AM. I thought no big deal, weak caps so I'll recap it. Now it's recapped, aligned and SSB sounds great and right on frequency but AM I'm still getting that nasty crackle at the voice peaks. I tried adjusting the carrier and rechecked bias but no resolve. Again ssb is great and received great reports on the air, AM it's obvious because I can even see the external meter needle flicker with the audio peaks.

I've checked the mic amp section and find nothing. Should I focus in the area of the regulator? Wouldn't the regulator do the same in ssb?

I'm stumped.

I had 2 that did this. On one I noticed that dropping the bias on the driver helped a little, but made sideband crunchy. It was TR41, changing the AM regulator fixed it. On the other it ended up being the driver itself I used an HG branded 2166, use the scope and your ears to set the bias on ssb, sometimes going by the numbers by the book doesnt work out.