Hi Garey & Co., Well, for what it's worth, here's a chronology of my fun day to-day, most of it spent at the work bench before the dis-assembled T-4X, my Heathkit SB-301 receiver, & a digital frequency meter...I'm STILL not sure as to what, if anything(!), I've learned from all this! Gonna take a day, or two, for me to digest it all.
Guys, If you've never ever had issues with your Drake PTO, feel free to DELETE this message right now---otherwise, read on (it might make sense to you! Hi). In any event, here goes: -I pre-set both the SB-301 & the T-4X on 7000-KHz, as confirmed by the crystal calibrator of the receiver; -I then removed the PTO shield can. The frequency of the transmitter rose to 7114-KHz, as expected; -I then snipped out that white tubular capacitor, which we all assumed was a 33-pfd. NPO unit. With the can still off of the PTO, the transmitter frequency dropped to 7038-KHz; -I did not replace the cap, & noted that the frequency rose to 7055-KHz with the can replaced over the PTO; -I couldn't find a 33-pfd. replacement in my junque box for the tubular white capacitor---instead, I barely managed to wedge a ceramic NPO trimmer capacitor (2.5 - 25-pfd.) where the fixed cap was; -I then adjusted the trimmer, looking for the original 7114-KHz with the PTO out of the can; -Once I set it to that frequency, I replaced the can---the frequency was some 7060-KHz, rather than the 7000-KHz that I expected (I guess the close proximity of the trimmer to the can was the key factor here); -I re-set the capacitor several times, removing & replacing the can: I could NOT get the transmitter to 7000-KHz (the lowest I could attain was 7050-KHz); -I figured that at 25-pfd., the trimmer needed more "C" to replicate the original cap's 33-pfd. value, & to get the frequency down to 7000-KHz---so I paralleled the trimmer with a fixed silver mica cap of 22-pfd. (I figured I could compensate down to 33-pfd. by virtue of the trimmer's adjustment); -The LOWEST frequency I could achieve with the two caps now was some 7500-KHz! That certainly made no sense to me, unless the tubular cap is itself hooked-up into the oscillator tank in some sort of final SERIES mode; -I removed BOTH caps, and installed a "gimmick" capacitor of just a few pfd.---the lowest frequency was some 7020 KHz, and I had to "slip" the dial manually for proper calibration, and finally, -I re-installed the original tubular cap! Funny thing is, the signal was warbley-sounding right up to the time that I commenced my "surgery"---after I removed the tubular cap, the warbling stopped...and the signal is still clean now, even with the original cap re-installed. Does any of this make sense to anyone...? This thing is driving me to drink...! ~73~ de Eddy VE3CUI - VE3XZ ************************************************************************************************************************ On 2011-09-23, at 9:46 AM, Garey Barrell wrote: > Eddy - > > I 'think' you're barking up the wrong tree unless you have already replaced > the two 45 pF cap. If the frequency of the PTO shifts several kHz or tens > of kHz, that small, (probably less than 10% of the total resonating > capacitance,) isn't likely to correct it. > > By the way, many small ceramic trimmers are temperature compensated, usually > coded by color. Many ceramic compression trimmers you see are N750, so be > careful what you use. > > 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, >> >> Well, to-day I plan to "snip" that white tubular capacitor out of my PTO,& >> see what might happen... >> >> I'll note the frequency on my receiver AND the PTO before doing this, as the >> frequency from the PTO climbs some 100-KHz in the absence of its "can": I'll >> then, probably, temporarily install a small air trimmer in place of the >> tubular cap,& play with the settings until the frequency might return to >> where it originally was, in the interest of linearity. >> >> I can then sub the trimmer with a fixed s.m. capacitor, perhaps in parallel >> with a small ceramic trimmer. >> >> Stay tuned---and wish me luck! >> >> ~73~ de Eddy VE3CUI - VE3XZ >> >> >> ************************************************************************************************************************* >> >> >> >> On 2011-09-22, at 3:36 PM, Garey Barrell wrote: >> >>> Yes, that is a 'trim' cap and should be less than 10 pF or so. It's not >>> likely to have much effect on the stability of the oscillator. More likely >>> are the caps on the 'other' side of the board, shown further down, the two >>> brown tubular caps to the left of the Diode D13 are 45 pF each. That's >>> probably a 'C-Line' Version 7 PTO. Probably a result of either a better >>> buy on two 45 pF caps over a single 90 pF, or a mechanical rearrangement of >>> the board layout. Those two caps are the most critical as far as stability >>> of the oscillator is concerned. >>> >>> 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, >>>> >>>> For a look-see at the suspect tubular capacitor that I am referring to in >>>> my T-4X PTO, go to this website: >>>> >>>> http://www.wb4hfn.com/DRAKE/DrakeArticles/TechTips/PTO-Repair.htm >>>> >>>> Scroll down half the page to where you see the heading "ELECTRICAL >>>> FAILURES": the photograph with the arrowed inscription marked "solder >>>> point" is immediately left of the white tubular capacitor that I am >>>> referring to... >>>> >>>> ~73~ de Eddy VE3CUI - VE3XZ >> > > _______________________________________________ > Drakelist mailing list > Drakelist@zerobeat.net > http://mailman.zerobeat.net/mailman/listinfo/drakelist _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
