[R-390] BFO Drift?

[Tim Shoppa]

As to drifts, I know of at least 4 kinds:

1. BFO that you're postulating
2. PTO that you seem aware of as a possibility

But the kind that I hear most often on 390A's:

3. The band-switched crystal oscillator. There are a lot of crystals and having flaky contacts on the crystal or bandswitch can cause drift symptoms like you're seeing.

4. AGC-related pulling of the 17MHz oscillator.

#4 will appear on only the low bands (below 8MHz) and #3 often on one particular cyrstal (which is likely used on multiple 1-MHz wide bands).

Some of the BFO tuning mechanical couplings can stick in funny ways and then "let go" resulting in shifts.

>>> ANTHONY CASORSO <canthony15 at msn.com> 12/11/2008 11:59:26 AM >>>

I have been listening to lots of CW lately and it seems that there is quite a bit of drifting going on in my 67 EAC R-390a. Even after its been on for more than an hour I can hear the signal slide up in pitch over a several minute period. I am pretty sure (but not absolutely sure) that it is the BFO. In my set I have replaced the 3TF7 with a 12BY7. I was listening to beacons using a waterfall display (Argo) and I could see the carriers slowly angle off over a long period and then, within a minute, come back down. The excursions are over 100Hz. The oven switch is off. Has anybody seen this sort of thing before? I have a counter, so I suppose I should put it on the PTO and confirm that it is OK. Just haven't gotten to the point of opening it up.
 
Thanks,
 
Tony