Resolved the issue with my 234 Speech Processor not responding to the
passband control. It needed alignment of the BFO circuit (trim capacitor
C-14)-- a procedure which is not described in the manual. I've recorded the
steps here for the benefit of the archive.
The 234 Manual says on page 4, with respect to the SSB Generator board, "An
oscilliscope is required for proper setting of R23-mixer balance, R19-LED
SET, C14-BFO set, and T1-mixer tune. They are factory set and should not
require subsequent alignment". I spoke to Garland Jenkins at Ten-Tec
Service about this problem and he sent me a copy of the alignment
procedures. Since I suspected the passband circuit only, I only applied the
steps 5 and 6 below that pertain to C14. I did not adjust R23, R19 or T1 in
any way (also, I lack an oscilliscope to do so). Here are the full
instructions Garland sent me for all four adjustments. It is assumed that
you have the 234 Manual with schematics and circuit board pictures.
Alignment Procedure - 234 Speech Processor
==========================================
1. Hook up to rig as normal with no input (no mic connected)
2. Remove top cover of 234 (note that two rear screws are different from
two front-- important when you try to replace them! The self-tapping screws
go in the front...)
3. Attach scope to junction of R18 and R19 on Clipper-Detector board (rear
board). Note: R18 is immediately in front of crystal Y-1.
4. Turn R33 BALANCE pot on SSB board fully CW. Peak T-1 for maximum
output. Turn R-33 back CCW for a null in output.
5. Attach counter to TP1 (coax) on SSB Board. This is the center conductor
of the coax that carries the BFO signal from the SSB board to the
Clipper-detector board. Turn front panel passband control fully CCW and
adjust C-14 for 6.100220 MHz (that is, 220 Hz above 6.1 MHz). Turn passband
control back fully CW. Frequency should drop to approximately 6.099780
(that is, 220 Hz below 6.1 MHz). Note: the procedure is to approximately
center the passband around 6.1 MHz. Slight adjustment of C-14 may be
needed.
6. Turn passband control back to center, or position so that your voice
sounds normal to someone who knows your voice. Note: turning the passband
control CCW increases the treble, while turning the control CW increases
bass. This is counter intuitive and not mentioned in the manual.
Conclusion
============
Before alignment, my BFO was somewhere around 6.099560 MHz at center
passband. It had the expected relative shift of ~440Hz through the range of
the passband knob but was not centered on 6.1000 MHz. After centering, I
can hear the difference in my transmitted audio via a second receiver as I
adjust the passband control, however the change is not dramatic. Still, it
is able to shift the emphasis of the audio enough to make it worth the small
effort to complete the re-alignment.
Very 73,
Barrett Thompson / KE4R
Fayetteville, GA
KE4R@arrl.net
-----Original Message-----
From: Barrett M. Thompson [mailto:barrettthompson@mindspring.com]
Sent: Sunday, November 04, 2001 10:01 PM
To: tentec@contesting.com
Subject: Model 234 Speech Proc: Help- Passband problem
Symptom
===========
I have a model 234 Speech Processor in use with my OMNI-D. The processor
seems to work fine (increases average power on the wattmeter) however while
listening to the OMNI-D/Processor combination in a second receiver, I
realized that the Passband control has no effect on the audio
characteristics. Microphone is non-amplified D-104.
According to the 234 manual, the Passband control sets bias to a varactor
diode which acts to shift the BFO frequency slightly. I checked this bias
voltage and it ranges from approx. 0.2 - 7.5v over the full excursion of the
Passband control, so now I suspect the Varactor diode itself. Should I be
looking elsewhere? Any special tests required to confirm a varactor diode
is behaving correctly?
Many thanks,
Barrett Thompson / KE4R
Fayetteville, GA
KE4R@arrl.net
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