[Swan] VFO drift problem in a 500

[Jeff]

Hi All -

I have an odd problem that I'm trying to track down in a 500.

Everything is working OK now with the 500 I've been rebuilding except 
for one thing.  I'm getting excellent audio reports, full power on all 
bands, and great receiver -- everything EXCEPT:  When I transmit, the 
VFO starts shifting frequency the longer I have PTT engaged...

I got feedback on it on the last Swan net on 20 meters on Sunday, and I 
got the same thing on 40 meters the other night.  So -- here's what I've 
done:

1. replaced electrolytics and out of tolerance resistors throughout the 
radio

2. cleaned switches with QD contact cleaner (bandswitch and VFO).  I 
have a new bottle of Deoxit on the way.

3.  verified that I have sufficient signal amplitude on the scope

4.  checked and cleaned the accessory plug

5.  checked Zener voltage: -9.1 volts consistent with the DMM

6.  checked VR tube: 149 volts on transmit consistent.

7.  Checked bias: -109 steady on transmit.


It doesn't seem to matter whether or not I'm using PTT or tune, whether 
or not there is carrier present or just at resting bias. If I key the 
transmitter for more than about 10 seconds, the VFO frequency starts to 
decrease (12.365.00 and moves down) to almost -100 Hz from where it 
started.  I haven't tried it for much more than 30 seconds or so.  This 
is after the rig has been on for several hours, or several minutes.

I've taken readings of the VFO at the grid of the amp, the accessory 
jack, and right at the board.  No changes.

So, I decided to try my 420 VFO on it, and it's solid as a rock at the 
same test points.  No deviation on final output frequency or VFO output 
whatsoever, so I think that rules out the Zener or the rest of the rig 
downstream.


So... before I tear out the VFO board and start rebuilding it, does 
anyone have any suggestions as to what might be going on with the 
internal VFO drifting ONLY on transmit?  Oh -- and when I release the 
key, the VFO will slowly drift back to the original frequency...


I love Swans, but this VFO design doesn't seem like it was designed for 
the ages.


Thanks!

Jeff

WB3JIH