[Collins] 51J4 Crystal Filter

[Dr. Gerald N. Johnson]

On 9/25/2011 7:25 AM, Carl wrote:
> True in that specific case only.
> My concern is that someone is going to assume those Murata trimmers are
> a cure all in any radio and that he will be in for a nasty surprise.

I wish we had better trimmers than are on the market today. And that is 
a valid concern that these parts won't handle RF voltage, DC voltage, or 
RF current like the Erie 8-50 pf ceramic caps. Sometime in the future 
when all the old stock 1/2" diameter trimmers have been used up and 
rebuilt beyond reason we may have to resort to the low voltage trimmers 
even in the transmitter circuits. One technique might be to use a fixed 
capacitor just a few pf lower than the necessary value, with a small (2x 
those few pf) trimmer for adjustment but with the fixed capacitor 
carrying most of the RF current of the circuit.

In the meantime I look for new small trimmers that will handle PA grid 
and neutralization duties.
>
> In most cases the value of the deposited cap can be measured when the
> coil wires are removed as they often are unshorted when no DC is
> applied. A next closest standard value dipped mica or 1000V NPO disc is
> sufficient as the coil core has sufficient range and 10-20pf has little
> effect at such a low frequency. Ive made many swept measurements over
> the years.

My test works only if the cap is open, not if its shorted, but if its 
shorted by the coil for DC its not going to see DC applied and so the 
silver migration that causes them to short has to be from something else 
other than applied DC. With two caps on the same insulator, there is 
going to be DC between the caps.

Changing the tuning cap wildly can change the coupling of the slugs and 
so change the transformer bandwidth. For that matter in some its 
critical whether the slugs are close together or far apart when inserted 
partly in each coil to have the desired coupling.

While the NPO disk is decently stable, the GMV or Z5H are not. They can 
loose 80% of their capacitance from room temperature up to soldering 
temperature. Messed up my construction of a very compact HF IF for a 
college receiver project long ago. And the dipped silver mica still 
should have better stability and isn't piezoelectric.
>
> There are several construction methods used but other than time required
> the repair is straightforward.
>
> Carl
> KM1H
>
>
73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.