[Kenwood] TS-850 PLL Lock instability

Sun, 11 Aug 2002 09:48:57 -0500

The plastic trimmer is there because of bean counters. Replace it with
ceramic and don't worry about re-inventing the wheel.

Clif Holland, KA5IPF
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----- Original Message -----
From: "Dave" <[email protected]>
To: <[email protected]>
Sent: Sunday, August 11, 2002 7:58 AM
Subject: [Kenwood] TS-850 PLL Lock instability

> I am currently "resurrecting" a 10+ year old TS-850SAT.
>
> One of the well documented problems of some older TS-850's is the unlock
of
> the PLL because of the apparent instability of the small trimmer cap TC1
in
> VCO-2. I have reset TC1 a number of times ( for the required 5Volts at
> TP2). the radio will play fine for a few hours or even a few days but
> ultimately it goes out of lock again resetting TC-1 brings it right back.
> Sometimes just tapping on VCO-2 case will result in relocking of the VCO
> and getting something close to 5V at TP2. While monitoring TP2 and tapping
> on the case for VCO2, TP2 jumps around from 4.6 to 5.4 volts, other times
> rather sharp tapping causes no change at all.
>
> So two questions.
>
> 1) Is what I'm seeing here the symptom of some other problem like to
little
> loop gain as the result of some other degraded component and the TC1 is so
> critical as a result to appear to be the problem, but more loop gain would
> result in a less critical setting of TC1?
>
> OR
>
> 2) If TC1 really is the culprit and needs to be replaced, has anyone ever
> experimented with using a better cap? In this case a ceramic piston
trimmer
> looks like an ideal substitute, inherently more stable and easier to
> adjust. How about putting a fixed cap in place of TC1 and adding a slug to
> the coil next to it for tuning in  the VCO  instead?
>
>
>
> Dave
> NR1DX
> [email protected]
>
> "A man who picks a cat up by the tail learns a lesson he can learn no
other
> way" .... Mark Twain
>
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>
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>
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