Steve -
That's NOT an output filter, it's the Collector circuit of the buffer stage. It's drawn in the Pi
arrangement, but it's a 'matching' configuration (note the 'different value' caps on each end). The
output filter, external to the PTO in the receivers ONLY, is a true Pi lowpass filter. If you look
at the schematic for the receiver PTO, you'll see that it has the same 'matching' circuit inside the
PTO, just drawn differently. Obviously, the Common-base buffer in the early receivers has a
different matching network, but ALL the transmitters have Common-emitter buffers.
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:
Yeah and that was the strange thing: I see the filter INSIDE the PTO (i.e.,
inside the dashed lines of the PTO assembly) on the T-4XB schematic. I haven't
checked the T-4X yet. The R-4B has the filter on one of the external boards
under the chassis. It just strikes me as funny that they wouldn't have
standardized the PTO amongst the transmitter and receiver for any given time
period.
I'm stopping by my club room today to borrow the parts cabinets. I'm going to
really need them when I start work on the R-4B...
73,
Steve, W1ES/4
-----Original Message-----
From: Garey Barrell<[email protected]>
Sent: Aug 21, 2011 10:20 PM
To: Steve Wedge<[email protected]>, drakelist<[email protected]>
Subject: Re: [Drakelist] You just know I'm not going to have any hair left...
Steve -
NONE of the T-4X(any) had the low-pass filter on the output of the PTO. Only
the receivers used the
filter, and only in version 2 of the R-4A and up.
It would be OK to just bypass the LP filter in the receiver for testing
purposes. Also, don't
gloss over the possibility of a bad, (cracked?), or just noisy resistor at R49.
Or even poor solder
on those associated connections.
Also, don't overlook the possibility of erratic or intermittent operation of
the BAND oscillator
(listen at 18.1 MHz on 40M).
Possible other resistors associated with V8 are noisy. Since you have no
jitter in TRANSCEIVE (XMTR
control) the problem is in the BAND, PTO or PreMixer, as these are all disabled
in XMTR mode.
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'm going to have to look again. I did swap the VR tube.
Something Garey mentioned made me check the T-4XB schematic. If that schematic
is accurate, Drake
put the filter inside the PTO at some point or just for the T-4XB. I'm
suspecting this was done
for all PTO's at some point, as I couldn't imagine Drake building different
versions of the PTO at
the same time (of course, not everything follows what I or anyone else would
consider
logical...). This could allow me to try temporarily bypassing the filter
board. I did notice
that shielded cable went directly to the PTO when I removed it. Given these
differences, it also
looks like I'll be ripping the receiver apart (again!) to put the original PTO
back in.
Arrrggggghhhhh...
Steve Wedge, W1ES/4
"I can't complain, but sometimes I still do."
- Joe Walsh
If the above message appears, it came from Steve's Son of Laptop!
----- Original Message ----- From:<[email protected]>
To:<[email protected]>
Sent: Sunday, August 21, 2011 6:11 PM
Subject: Re: [Drakelist] You just know I'm not going to have any hair left...
Did you put a scope on the voltage to the PTO and watch it carefully?
As it fails?
73,
Lee, KC9CDT
-----Original Message-----
From: Steve Wedge<[email protected]>
To: Eddy Swynar<[email protected]>
Cc: drakelist<[email protected]>
Sent: Sun, Aug 21, 2011 5:30 pm
Subject: Re: [Drakelist] You just know I'm not going to have any hair
left...
That's the thing: it's a different PTO that was working properly before
I transplanted it.
Looks like I've got to try replacing the components on the filter
board. Something is pulling the frequency...
Steve Wedge, W1ES/4
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