[R-390] 60 Hz Hum Output from R-390A IF Module

[2002tii]

Jon wrote:

>When I get the IF module behaving again (bursting
>oscillation) I'll try DC filaments straight away.

I have not been following this thread, but if you are still operating 
the IF module on an umbilical, note that the power supply RF/IF 
bypass caps may not be working to full effect because the umbilical 
has some unwanted inductance (as well as resistance) in the return 
("ground") lead.  This could conceivably lead to the bursting 
oscillation you are seeing.  I'm not saying that's what's happening, 
just that it's not implausible.  Alternatively (or additionally), you 
could, just coincidentally, have one or more bad bypass caps in the 
module.  If you have an oscilloscope, I'd suggest looking at the 
power supply distribution points in the IF module as well as the IF 
module ground, referenced to chassis ground.

Best regards,

Don