[GreenKeys] Dovetron Mark Hold debugging

[ke6myk@sbcglobal.net]

Hi. I have a Tempest Dovetron RTTY converter with the solid-state LED
display. I don't think that my unit's Mark Hold feature is working
properly. Other than disabling the Mark Hold when turned fully
counter-clockwise, my unit's Threshold knob doesn't seem to do
anything detectable. I would think that with the Threshold knob turned
fully clockwise, a small signal (say, one or two LEDs on the cross
display) would not break the unit out of Mark Hold. On my unit,
signals which are too small to show up on the cross display break it
out of Mark Hold, so my unit spits out a lot of garbage in
weak-signal/no-signal conditions, and I need to flip it into
standby whenever I'm not receiving a fairly strong signal.

I've already had to replace a couple of filter caps, clean out the
pots, replace a missing wire on the ON/STANDBY switch (the ground
connection necessary for the rear-panel PTT jack to work), recalibrate
the VFOs, etc., so it seems quite possible that my unit still isn't
fully operational. I have successfully used it on the air on the west
coast Clatternet (10.137 MHz USB 850 Hz shift, 09:30 Pacific time on
Saturday mornings) with an AN/UGC-74B and a Yaesu ham rig, but the
line noise is a bit annoying.

I don't understand the circuitry which detects signal loss yet, so I'd
appreciate any suggestions that you Dovetron experts out there might
have. I'm just a dumb digital designer who can only count to "1" and
is easily confused by triangular symbols with more than one input. :-)

My unit has the KOS board (yech, I was much happier after I figured
out how to disable that thing!), no regenerator (UART), and has
modifications to select narrow or wide Tx shifts (the mark and space
tone trimpots are removed and wired to a little PCB with four trimpots
and a switch to select one pair or another), and a switch to select
three different receive bandwidths. My manual seems to be for a model
with the regenerator option, so it doesn't exactly match my unit's
configuration. I gathered the impression from the manual that there
were a number of different assembly and tuning options, potentially
making each unit a bit unique, and each unit came from the factory
with a label on top describing how it was configured. Mine has the
label, but the label is no longer correct because I've re-tuned my
unit for the way that I used it. If I recall correctly, my unit has a
dataplate on the top panel which indicates that it was made for the
Coast Guard.

Thanks in advance for any hints!

--
Mark J. Blair, NF6X <ke6myk@sbcglobal.net>
PGP 2.6.2 public key available from http://www.keyserver.net/
Web page: http://www.qsl.net/ke6myk/