I finally sat down with my RTL-SDR in hand to do some starting measurements before tearing into it. I fired up my trusty old Singer CSM-1 service monitor only to find that it was not putting out a clean tone on frequency, and was generating very strong signals every 4 MHz or so for roughly 20 MHz on either side of the center frequency. Also, the center frequency signal, in addition to being dirty, was also very weak at roughly 10dB above the noise floor (much of that noise was coming from the CSM-1 itself).
Since the attenuator is passive and the last device in series before the output port, I thought I could at least use those strong signals to do my testing. Then I realized that I have no way of accurately knowing what the baseline amplitude of those signals was, so I can't use them to measure receive sensitivity in the SDR.
I've spent the last two days tearing into the CSM-1 with a screwdriver in one hand and the service manual in the other, but I have been unable to determine what the problem actually is.
If anyone has any specific knowledge about this issue, I welcome your feedback. I really would like to have my service monitor working again, as I don't have any other equipment that can stand in for it.
The rebuild of the RTL-SDR may be on hold for a bit while I try to figure this out.