[time-nuts] Inside of FT1200-100

[Charles Steinmetz]

Christopher wrote:

> And of course it is a discontinued part.
>
> Taking suggestions for a replacement.   COG(NP0)?

I applaud your ingenuity and resourcefulness in tackling this project! 
I just love seeing nice old tech restored to service.

Your three basic choices are mica, C0G/NP0 ceramic, and porcelain (a 
different type of ceramic).  For this application, you want the highest 
Q available (which is probably why the original is a glass capacitor). 
Porcelain wins that contest hands down, and all of its other specs are 
competitive with or superior to the others.  They are also free of 
microphonics.

Digi-Key carries the AVX SQ Series (AVX calls them "Microwave MLCs"):

<http://www.digikey.com/en/product-highlight/a/avx/microwave-multi-layer-capacitors>

<http://www.digikey.com/catalog/en/partgroup/sq-series/3895?mpart=SQCB7M220JAJME&vendor=478>

There are other manufacturers, but I don't believe anyone makes 
porcelain through-hole parts anymore.  You will probably end  up 
soldering leads onto one or a stack of SMD chips.  It appears that the 
original capacitor is installed in sockets.  Measure the lead diameter, 
and use the same size wire.  Or just clip the leads off the original and 
re-use them.

Note that the original glass capacitor has a tempco of about +140 ppm/C, 
and the porcelain and C0G/NP0 parts are close to zero.  This raises the 
question whether the capacitor was intended to provide temperature 
compensation (or whether other circuit elements have tailored tempcos to 
offset the capacitor tempco).  Since the capacitor is in a 
temperature-stabilized environment, this is probably not critical.

Best regards,

Charles