Problems with Limiter and Audio Distortion.
While working on an R-390 recently the radio quite suddenly experienced severe audio distortion. I found that by turning on the Limiter Switch the distortion disappeared and the radio sounded normal. I did remember having this exact problem with an R-390A some time ago but had forgotten the solution. A quick query to the net produced several good suggestions. The most obvious one was C-529, the audio coupling capacitor at the output of V-510. Changing this capacitor, an easy task, did nothing to improve the situation.
The next suspect was C-551 and/or C-550, the former being 100pf and the latter 180pf. These two caps are buried just behind the BFO coil and connected to pins 8 and 9 of V507. This puzzled me for a while as C-550 is supposed to be connected to the cathode of V-510. Some testing with an ohmmeter soon revealed that pin 9 of V-507 was connected to the cathode of V-510. Pin 9 is simply an anchor point.
To replace these caps was not an easy task. Right above V-507 socket is an anchor standoff to which is connected a large choke coil.. This needs to be unsoldered and moved out of the way before you can reach these two caps. Even then it is awkward as it is very cramped in there. I made no attempt to unsolder the leads to the tube socket for fear of doing more damage. I just clipped the leads as close to the socket as possible. I installed the two caps (orange drops of course) by wrapping the leads around the socket pins and re-soldering.
I intend to replace all three caps in any future R-390s that I work on and recommend you do likewise