Have a Cobra 138XLR on the bench right now with a bit of weird problem, as the thread title implies. It's got what appears to be an audio self oscillation, but it's worst in LSB. It pops up in USB, but it isn't there constantly like in LSB. Haven't noticed it in AM, but it's probably there. I also noticed that no matter how I adjust VR4 (carrier balance), I've still got a tiny bit of carrier on LSB that doesn't seem to be there in USB.
For the record, it has been recapped. I have removed any of the "sonybond" glue I've found. I've tightened the screws holding the board to the frame. I had to rebuild the power input as both the coils on the positive and negative sides were burned and fused to their respective plastic sleeves, and the reverse polarity diode was gone. I suspect it had been hooked up in reverse polarity in the past, and just removing the diode was easier than fixing it the right way. I also had to reattach D23 where it had been clipped.
Also, this radio has had a few modifications done prior to it coming to me. It looks like someone has added a 33 uFd cap near the final/driver area (I replaced it as part of the recap) and an extra resistor or two. They also added a TIP 120 transistor to the radio for carrier control, with the Tone pot as the controller. Clarifier is unlocked, of course. Finally, and I have no idea if this was part of the clarifier mod or not, the adjustable trimmer caps for the three loop crystals have all been removed. Strangely enough, it's still able to get and stay on frequency.
This is not to say that any of these are the root cause, because I just don't know at this point. I've never run across an 858 chassis with the trimmers removed, so this one has already got me going "WTF?" before even really getting into the problem. I'm just laying this all out there in case someone has seen a similar issue before and any of these items help kickstart those brain cells.
So, any troubleshooting tips, suggestions, sarcasm, etc. welcomed on this one. Sometimes writing the problem out like this helps me figure it out before hitting "POST THREAD". Not this time.
For the record, it has been recapped. I have removed any of the "sonybond" glue I've found. I've tightened the screws holding the board to the frame. I had to rebuild the power input as both the coils on the positive and negative sides were burned and fused to their respective plastic sleeves, and the reverse polarity diode was gone. I suspect it had been hooked up in reverse polarity in the past, and just removing the diode was easier than fixing it the right way. I also had to reattach D23 where it had been clipped.
Also, this radio has had a few modifications done prior to it coming to me. It looks like someone has added a 33 uFd cap near the final/driver area (I replaced it as part of the recap) and an extra resistor or two. They also added a TIP 120 transistor to the radio for carrier control, with the Tone pot as the controller. Clarifier is unlocked, of course. Finally, and I have no idea if this was part of the clarifier mod or not, the adjustable trimmer caps for the three loop crystals have all been removed. Strangely enough, it's still able to get and stay on frequency.
This is not to say that any of these are the root cause, because I just don't know at this point. I've never run across an 858 chassis with the trimmers removed, so this one has already got me going "WTF?" before even really getting into the problem. I'm just laying this all out there in case someone has seen a similar issue before and any of these items help kickstart those brain cells.
So, any troubleshooting tips, suggestions, sarcasm, etc. welcomed on this one. Sometimes writing the problem out like this helps me figure it out before hitting "POST THREAD". Not this time.
