Eddy -
The cap that was the problem in Steve's PTO was C145, which is the main resonating cap for the
variable inductor. This is the long, brown or silver tubular cap on the bottom of the board. The
schematic is marked 85 pF, but Steve's was coded as 90 pF and measured 92.5 pF. C143 and C144 are
the caps 'on top' that 'trim' that cap, usually just a pF or so, presumably to fine tune the desired
portion of the inductor for linearity. ALL are NP0 caps. I don't know how well a small trimmer cap
would work. Guess it depends upon the quality of the cap, but would be an interesting experiment.
All I did on Steve's PTO was to measure the removed cap (92.5 pF), and select one as close as I had,
which turned out to be 92.3 pF I believe. I used a Dur-Mica, because that was the best that I
have, which is about -50 ppm. Not quite an NP0, but stayed within +/- 100 Hz for days at a time
just sitting out on the bench. It would move a bit more when the AC vent above it blew cold air on
it, but safely ensconced in, and bolted to, a nice warm receiver chassis I believe it will meet
original specs.
Keep us posted.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Eddy Swynar wrote:
*/Hi Guys,/*
Am I /cursed/here, or what...?!
After literally /weeks/of flawless, dependable service, the PTO in my /Drake T-4X/transmitter has
returned to its good ol' wobbly self again (/SIGH!/). It started last night---thinking that maybe
the PTO signal was being affected by something external to it, to-day I meticulously cleaned each
& every wafer / contact on each & every switch, cleaned the tube sockets / pins of all of the
tubes & crystals, and visually inspected everything that I could under the chassis...
My reward for all this work...? /Continuous/wobbliness / warbling of my
signal...!
I think I'll do what Garey did with Steve's PTO, & simply wholesale replace that white tubular
capacitor that's inside the PTO (in the T-4X that I have, it's the cap that has one lead soldered
to the main horizontal PTO circuit board, with the other lead connected up top the front /
vertical "sub" board). If memory serves me well, Garey, you deduced, from my description of the
colour dots on my cap, that it is a 33 pfd. unit.
I think I have some silver mica caps on hand here that I can sub it with---may have to parallel
whatever I replace in there with a small trimmer of some description to preserve linearity...
/WHAT A PAIN...!!!/
*/~73~ de Eddy VE3CUI - VE3XZ/*
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