Bob,

can you tell me what part of the circuit needs the negative cof cap
and what value you think would be best?

Thank you for the help!!

Steve


On Wed, 16 Nov 2005 01:08:20 GMT, Bob wrote:

Steven Fritts wrote:
Hello All,
I picked up a really nice working Swan 350 at a hamfest this past
weekend. I bought it for nostalgic reasons as it was the kind of rig
my dad bought for me in 1965. I need info on how to fix the infamous
"Swan drift problem". Anyone have any websites I can go to get mods
for this rig including the drift fix?
Any help will be appreciated!

Steve W4SEF
The problem is, for the most part, lack of proper temperature
compensation. Sure, the voltage regulation isn't that great, but it's
not the main problem.

Looking at the diagram for the VFO you can see the tuned circuit
components. You can add a negative temperature coefficient capacitor to
the circuit to compensate for the (usually) positive coefficients of the
parts.

I did this successfully in a Viking Valiant. I reduced the drift from
about 10kHz or more on 10 meters to about 1kHz. One thing I did was
move a hot resistor out of the VFO compartment to reduce the amount of
temperature rise of the critical parts.

You can also leave the unit on, at least the VFO, at all times so the
temperature will be more stable.

73, Bob K6DDX

Steven Fritts wrote:
Bob,

can you tell me what part of the circuit needs the negative cof cap
and what value you think would be best?

Thank you for the help!!

Steve


On Wed, 16 Nov 2005 01:08:20 GMT, Bob wrote:


Steven Fritts wrote:

Hello All,
I picked up a really nice working Swan 350 at a hamfest this past
weekend. I bought it for nostalgic reasons as it was the kind of rig
my dad bought for me in 1965. I need info on how to fix the infamous
"Swan drift problem". Anyone have any websites I can go to get mods
for this rig including the drift fix?
Any help will be appreciated!

Steve W4SEF

The problem is, for the most part, lack of proper temperature
compensation. Sure, the voltage regulation isn't that great, but it's
not the main problem.

Looking at the diagram for the VFO you can see the tuned circuit
components. You can add a negative temperature coefficient capacitor to
the circuit to compensate for the (usually) positive coefficients of the
parts.

I did this successfully in a Viking Valiant. I reduced the drift from
about 10kHz or more on 10 meters to about 1kHz. One thing I did was
move a hot resistor out of the VFO compartment to reduce the amount of
temperature rise of the critical parts.

You can also leave the unit on, at least the VFO, at all times so the
temperature will be more stable.

73, Bob K6DDX


Steve,

I don't have the circuit diagram so I can't advise. But across the
inductor is the first place I would try. Of course you will have to
reduce any other capacitors by that amount so that the circuit will
resonate at the same frequency as before.

As someone suggested, a differential arrangement will work but it's a
bit tricky. I'd prefer to substitute various values until I hit one
that is close enough.

Bob

On Wed, 16 Nov 2005 17:00:57 GMT, Bob wrote:

As someone suggested, a differential arrangement will work but it's a
bit tricky. I'd prefer to substitute various values until I hit one
that is close enough.

It can be hard to get a differential capacitor these days.

One alternative could be to add a varactor (and a series capacitor)
into the resonant circuit and vary the varactor control voltage
according to the temperature. A suitable NTC or PTC in the voltage
divider feeding the varactor could be used.

Paul OH3LWR