Kris -
There is no trimmer cap in the original PTOs. There are small, selected NP0 caps used to trim the
main resonating cap. I suspect a tubular NP0 was less expensive than a 'decent' quality trimmer,
eliminated the possibility of a 'golden screwdriver' attack, and long term stability of fixed caps
is likely better.
I just re-read your message.... Yes, it appears that the selected 'few pF' caps were used to 'trim'
the portion of the coil used.
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>
Kris Merschrod wrote:
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" <k4...@mindspring.com>
To: "Eddy Swynar" <deswy...@xplornet.ca>
Cc: <drakelist@zerobeat.net>
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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