Eddy -
OK. That's what I call 'Version 4' (of 17) variation of the PTO. The cap I replaced on Steve's,
(same version, just different component designators,) was C88, which is the main resonating cap.
C89 is a DC block, and would have little to no effect on the stability of the oscillator, unless it
was very leaky or perhaps 'mica creep'.
Those 'trim' caps should be about 10 pF total or less, and those ARE the ones on the circuit side of
the board, opposite C88.
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 Garey,
Originally---in my T-4X---I replaced C89 (3000 pfd.) with two 1500 pfd. s.m.
caps in parallel. The white tubular cap that I was referring to at 33 pfd.
is---I think!---either C86, or C87 on the T-4X schematic, both of which are
(unfortunately) simply marked with an asterisk therein...
I wish I had a capacitance tester to confirm the exact value of the thing---but I
suppose I could parallel it with a known value of fixed inductance, connect to the
antenna terminal of my FT-980 in general coverage receive mode,& apply some RF
into the other end with my signal generator, watching for where the S-meter might
dip...
I'd then refer to my trusty ol' ARRL Lightning Calculator to get an approximate
value that way, I s'pose.
I guess Drake used SEVERAL iterations / versions of their PTOs over the years,
dependent upon what rig they were to be used in. That's bad for us trying to
de-bug these things to-day, but as somebody rightfully put it awhile back, who
in their wildest dreams ever thought at the time that these rigs would still
see daily use almost a half century after-the-fact...?!
~73~ de Eddy VE3CUI - VE3XZ
************************************************************************************************************************
On 2011-09-22, at 2:25 PM, Garey Barrell wrote:\.
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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