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
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