[TenTec] 238 Tuner Mod

[Dr. Gerald N. Johnson]

On Tue, 2008-03-04 at 09:55 -0500, Jack Mandelman wrote:
> I made a similar mod to my 229B tuner.  However, instead of using 
> doorknob caps I replaced the originals with 6KV 470pF Y5P ceramics. 
> These capacitors are physically larger than the originals, having a 
> thicker dielectric for increased breakdown voltage and larger plate 
> surface area for increased current carrying capacity.  The drift problem 
> was nearly completely, but not entirely, mitigated.

Y5P ceramics are still fairly drifty. The large physical size of the 6
KV caps is a benefit. It would be better to look for high voltage NPO
temperature coefficient disk ceramics. Perhaps 3 of the Sprague
561R10TCCT15 from Mouser in parallel for each 470 pf. Just 1KV but NPO
which also makes them large and most temperature stable.

>   Complete 
> disassembly of the tuner was unnecessary, since I was able to work in 
> the tight space around the switch with small forceps.
> 
> Now, I'm considering using the doorknob caps.  Would appreciate any 
> suggestions about disassembly of the tuner for easier installation of 
> the doorknob caps.
> 
> 73,
> 
> Jack K1VT
> 
73, Jerry, K0CQ