Wow- thanks for solving the mystery, Garey! As it turns out, the PTO from the T-4XB may have the same issue as it has a very similar-looking cap (despite the fact that it's a later version that probably uses FET's).
I put the R-4A aside and started working on my "hot-rodded" B, thinking that tearing through the A was still premature. You must have quite a junkbox to be able to select an exact replacement ;-) Thanks again... Steve Wedge, W1ES/4 Sent from my iPhone On Sep 13, 2011, at 15:07, Garey Barrell <[email protected]> wrote: > > I finally got a chance to take a look at Steve's PTO. As I suspected the > frequency jumping is caused by the large (main) tubular cap. I don't know of > anything special about that cap, it's marked as an NP0, so I just stuck a > Dur-Mica in it's place. I'm sure Drake had a reason for using that type of > cap, but darned if I can figure out what it was. Maybe purchasing got a good > buy on a box of them! :-) I measured the cap I removed as 92.2 pF, the > schematic specifies 82 pF, and the color code reads 93 pF, so who knows.... > I selected the Dur-Mica from a bunch of 90 pF ones, and got a 92.3 to replace > the original, so hopefully it won't destroy the linearity. Considering that > Drake later went to a pair of 45 pF caps and then a 45 and 35 pF pair, the > 'good buy' idea gains credence!! I haven't made any checks of linearity > yet, and I assume the select at test caps in parallel are used to 'fine tune' > for the particular coil. > > A steady 2.2 V p-p output into a 1M counter input. > > Anyway, it's been sitting on the bench in a Pana-Vise all afternoon and has > stayed within 100 Hz, UNTIL the A/C kicks on and blows cool air at it! It's > right in the stream of cold air, and so I consider that as STABLE. Of course > I don't have a Rubidium standard oscillator to calibrate my counter against, > so it could really be "anywhere"!! :-) > > The next step is to fashion some sort of reference pointer so I can do at > least a rough check of linearity > > 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> > > > > Steve Wedge wrote: >> >> I am with Garey, though, in that I'm still finding it hard to believe that >> there are problems with both. That would be like getting struck by >> lightning twice. >> >> I'm still having a hard time thinking it's the supply, as that 0B2 should >> regulate its output at - what is it? - 108V? That gets dropped through the >> big WW resistor and ends up as approximately 10 - 12V, which gets dropped by >> the Zener inside the PTO. To my thinking, even if the 108V were to sag a >> little, as long as the voltage is above the Zener point, it shouldn't matter >> as long as there's sufficient current. >> >> Both PTO assemblies - when in the R-4A - seem to be very touchy. >> >> W1ES >> >> -----Original Message----- >> From: Bill Frost >> Sent: Aug 25, 2011 10:06 AM >> To: Steve Wedge >> Cc: Garey Barrell >> Subject: Re: [Drakelist] Well, it looks like both PTO's DO have issues. >> >> Hello Steve, >> >> OK, Great, it will be interesting to see what Garey finds. >> >> 73, Bill >> >> >> ---------------------------------------------------------------------------------------------------- >> *From:* Steve Wedge <[email protected]> >> *To:* Bill Frost <[email protected]> >> *Cc:* Garey Barrell <[email protected]> >> *Sent:* Thursday, August 25, 2011 9:55 AM >> *Subject:* Re: [Drakelist] Well, it looks like both PTO's DO have issues. >> >> Cover was on each during the tests. Tried running it with cover off to >> try and poke at >> components to provoke them ;-) Of course, I'd have to rig up some way of >> holding the yoke down. >> I'm actually going to send the older one to Garey tomorrow to get a fresh >> set of eyes (and a >> better-stocked parts bin) on it. >> 73 >> W1ES >> >> -----Original Message----- >> From: Bill Frost >> Sent: Aug 25, 2011 8:21 AM >> To: Steve Wedge >> Cc: Garey Barrell >> Subject: Re: [Drakelist] Well, it looks like both PTO's DO have >> issues. >> >> Hello Steve, >> >> If you preformed the tests with the PTO cover removed, not a good >> test, you must have the >> PTO yoke spring pulled and clipped to the back of the coil form or >> ??? Allowing the >> yoke/slider bar to ride free is not a good test as the PTO will >> always chirp or drift >> around without tension on the yoke/slider bar. It's best to leave >> the cover on and spring >> engaged during any test of the PTO assembly. >> >> 73, Bill >> >> >> ---------------------------------------------------------------------------------------------------- >> *From:* Steve Wedge <[email protected]> >> *To:* [email protected] >> *Sent:* Wednesday, August 24, 2011 9:16 PM >> *Subject:* [Drakelist] Well, it looks like both PTO's DO have issues. >> >> UUT #1: PTO from T-4XB, mounted in R-4A. R-4A when using its own PTO >> gets chirpy/unstable >> signals after warm-up. >> UUT #2: Original PTO from R-4A, out of the rig. >> Power: >> Internal receiver power >> Station power supply (30A linear), turned down to 11.1V >> monitor: Elecraft K3 tuned to 4917 kHz (approx) with series 10 dB pad >> and x10 scope probe. >> Test setup #1: >> UUT #1 connected to power buss from 0B2 and normal chassis ground. >> Signal output open >> except for test equipment. Connected to monitor. Warmed up >> approximately 25 minutes, >> signal became unstable. Turned R-4A OFF and immediately connected >> station supply. Instability would not immediately resume. >> Long-term, frequency would jump and >> occasionally chirp. >> Disconnected remainder of the 11V buss in the receiver from the PTO >> so that station supply >> was powering only PTO. After a period of time, instability returned. >> receiver was off. >> Conclusion: UUT#1 (the T-4XB FET PTO) is the source of the unstable >> signals. >> Test setup #2: >> UUT #2 connected to station power supply and monitor same as #1. >> After about 30 minutes, >> signal was shifting but not as bad as #1. >> Conclusion: UUT #2 also is unstable. >> I need to find a counter now so that I can determine the frequency >> with the cover >> removed. From that point, I can try stressing individual parts. >> Garey, I may still send the older PTO to you and you can document it. >> This receiver is worth spending the time on as it came out my >> refurbishment looking and >> otherwise sounding very good. Cosmetically, it's about an 8 / 10 >> with some brown spots on >> the chassis and small scratches on the cabinet. >> Thanks to all for feeding me ideas. This has been fun but I want to >> finish ;-) >> Steve Wedge, W1ES/4 >> >> _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
