[Swan] VFO drift problem in a 500

[Jeff]

Thought I might post a follow up...

I replaced the two transistors on the VFO board -- things seem to be 
better now.  Frequency stability seems very good now, with transmitter 
keyed up to 30 seconds.

I'm not going to declare victory just yet, but it definitely shows promise.


A second question -

Can anyone suggest a source for the ferrite slug that fits into the 10 
meter VFO coil form?  A prior owner shattered the one in the rig -- I 
was able to extract the pieces cleanly, but now, obviously, no VFO out 
on 10...

I see surplus sales of Nebraska has something that looks close, but 
would like to know if anyone has used one (which one?) from them.

Jeff

WB3JIH




On 10/4/2017 8:47 PM, Jeff wrote:
> 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
>
>
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