click fix and squeal on transmit #ubitx


Hi -- I have had my v3 ubitx working fine, and thought I would install the T/R click fix version 2 from I'm fairly confident in the circuit I assembled, slightly less confident in the jumpers, trace-cutting, and attachment to the board. The click is still audible in the relay but not in headphones on either transition, and I get a loud squeal on transmit into a dummy load when the volume pot is more than slightly turned up. What should I be looking at to diagnose this?

I noticed continuity between the 12v line and the point marked Rx connection in this section after cutting the trace, which seemed odd:

Don, ND6T

When the rig is not powered or in the receive mode there should be continuity from the Rx connection point to 12V. through relay K1 contacts 3 and 5. That is normal. The squeal is not!
First step in diagnosis is to check the voltage on the gate of Q3 when in transmit mode. It should be below 1 volt referenced to the ground connection on that board. If not, check to see that the ground connection there has the same potential as the ground plane of the BITX. Then track down the voltage discrepancy. The operation depends on Q3, acting as a gate, being turned off, thus opening that gate. If that gate is allowing the I/O (when in transmit, it is the microphone pre-amp output) to be leaking into the audio output then you will get enough feedback to oscillate. If the gate of Q3 is below 1 volt referenced to the source during transmit then Q3 may be bad. If there is voltage on the source, check your solder connections of R4.
Keep us informed, please. 73, -Don


Thanks, Don! In a surprise move, the problem turned out to be uncarefulness on my part. My confidence was misplaced, since I'd used wrong capacitors in a total of four places: both caps in the circuit were 10μf, not 10 nf, and my replacements for C50 and C63 were not polarized. Now that I've replaced all four with the correct caps, the squeal is gone. The click is still audible in the headphones when the volume pot is turned way up, but it's much less than before. I think I'm going to declare this a qualified success and put the radio back in its enclosure.