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/* _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
