Hello all,
Were those trimmer caps the work of the team of ladies assembling and
testing the PTOs? Could it be that they started with the main resonating
cap and then added those trimmers as needed to get the PTO dead on?
Kris KM2KM
----- Original Message -----
From: "Garey Barrell" <[email protected]>
To: "Eddy Swynar" <[email protected]>
Cc: <[email protected]>
Sent: Thursday, September 22, 2011 2:25 PM
Subject: Re: [Drakelist] Here We Go Again...!!!
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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