Modulation Bias Adj Pot value on Valiant

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WZ8J:
I think it's time to wrap up this long thread on the Valiant's latest issues with both hum and RF feedback on 40M.
The RF feedback was resolved by completely removing the the amplification components from the D104 and finally getting the mic properly wired.
The hum returned after I thought I had it licked. It turns out the source of the hum after trying many different tests, was that I had installed the aluminum shield on around the mic jac wrong side up! Duh. I only discovered this when I looked at pictures in the manual.
Very happy that the rig now gets good audio reports, no hum and works fine now on 40M. It also appears to have resolved the chirp I was getting on 40M CW.

Thanks to all who helped, especially N1BCG who has spent hours helping me tame this little beast. I hope others will find this thread helpful in solving some of their own Valiant issues.

N1BCG:
Sounded great on the air!

One thing to check for lingering chirp is that the modulator is disabled in the CW mode. The circuit is designed so that the mod screen grids are brought to 0 Volts and the xformer secondary is shorted. See if you can hear your voice when in CW and that the modulator current is reduced below the 50-70 resting range.

Glad you hung in there and remained patient throughout the long process. This thread will certainly show others that troubleshooting classic gear can be challenging, but in the end, rewarding and educational. Think of how familiar you are with the Valiant now!

Unfortunately, discontinued microprocessors and surface mount technology in newer rigs make such hands-on repair nearly impossible.

WA1HZK:
Killed some of these myself. The important thing is to find a high wattage pot. Adjust the value of the fixed resistor if you can't get it within range. Some mods have put a regulator tube in that circuit where the rectifier used to live. That assumes you have solid stated the PS. Those screens need a decent stable voltage for the best audio.
Keith
Wa1hzk

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