[Swan] VFO drift problem in a 500

[wes Bolin]

I would suspect the VFO transistor and temp compensating capacitors.

A fix that might cure the symptom but not the problem is the K4DPK VFO
stabilizer.

Get a can of cool spray and a heat gun.....judiciously use cool and heat to
narrow down the bad component.
Wes

On Wed, Oct 4, 2017 at 7:47 PM, Jeff <jeff at podengo.com> 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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