The AM Forum

The modulation meter works and the 866's change with voice changes, but there is no audio on the carrier. The grid control knob is very warm and while it can be set to 8 mils in zero position, it drops to 4 in transmit and can't be adjusted. I plan to replace the pot, but wonder if it is responsible for the audio problem? Thanks for any suggestions. Don, K5WSY

Does the modulation meter show standby modulation current and does it show an upswing on voice? What does your scope show on the rf output connector into a dummy load? Are you monitoring yourself into a dummy load with a receiver nearby?

Have you safely measured the HVB+ on the caps of the 6146 modulator tubes? Have you tested the 6146 modulator tubes?

 
The Grid drive pot is underrated and will always get hot but does fail. RF drive has little to do with modulation.


Phil

Thanks Phil, I didn’t think the grid drive had any effect, but wasn’t sure. I have not checked the hv on the 6146’s, but there is static current on the meter and it does move, but only have a carrier on the scope. I do have some extra tubes to substitute. Thanks again and 73.

I can't explain it but the audio started working. I had the rig on its side and poked around a little. When I hooked it back up, the scope showed modulation and I could hear my voice on a receiver in the other room. After that I put in other 6146's and tweaked the neutralization capacitor. I still suspect problems as the meter readings aren't consistent. I did make a contact on 3.885 with a 10 over report with good modulation. Thanks Phil for your magic. 73, Don K5WSY
 

Is it possible that a switch or relay that shorts the modulation transformer secondary for CW mode could have been stuck? I only ask because there was a bad CW/AM switch here and it was keeping the relay on, and the mod output shorted to bypass B+ to the final. Had plenty! of mod plate current swing and a solid carrier

What OPCOM said above could be a real possibility. Another possibility could be that a solder sliver or a piece of wire could have lodged across the contacts and shorted across the lugs on the Phone/CW switch but was knocked off when you repositioned the chassis.

After you have found your intermittent audio problem, I would encourage you to also adjust R13 (1 Meg Pot. Clamp Tube Circuit) for about 175 volts on the Final screens.

Plate Dips and Loads are required when adjusting this as the screen voltage affects the output tuning. Many people forget about this adjustment but it can prolong the life of the Finals


Phil

Thanks Pat and Phil for your help. I will check the screen voltage as well. I have been piddling with this one for awhile and glad to put it aside as I have a Viking 2 to check. I may be back. 73, Don K5WSY