[GreenKeys] Dovetron advice? stuck in Mark-Hold

[Nick England]

I picked up a $20 Dovetron MPC-1000CR-II at the Shelby hamfest.
It has the BBP-100 rev A board and a TSR-300D SELCAL/REGEN board.
Unfortunately I can't find a TSR-300D schematic.

I aligned the RY generator for 45 baud and calibrated the VFO and AFSK OUT
for Mark and Space.
It seemed to be working fine on 170 and 850 shift signals - regen was OK,
autostart was OK (I did not test SELCAL), RY was OK, ITTY was OK. Then just
before I went to put the covers back on I turned it on for one last check.
Uh-oh it is stuck in Mark-Hold. The cross-LED shows the M-S detectors are
still working fine. SIGNAL LOSS LED is always lit, output is always MARK.
REGEN ON-OFF makes no difference.
SEND-RCV makes no difference
THRESHOLD makes no difference.

Any suggestions? I tried pulling off the TSR-300D board so I could scope
the BPP board, but then things are stuck in SPACE, even with the REGEN
switch OFF. Next stop is attempting to trace the BPP in/out on the bottom
of the MPC main board I guess, And maybe trying to trace the leads to the
TSR-300D board to figure out what it might be doing.

Has anyone run into a stuck Mark-Hold before?

I confess that I have a hard time figuring out logic made out of
interconnected 741 op amps instead of straightforward TTL or CMOS logic
chips.

And just trying to figure out how the BPP interacts with the MPC board is a
royal pain. FWIW I marked up a schematic with some of the mods made to the
MPC board when installing the BPP.
https://www.navy-radio.com/rtty/dovetron/Dovetron.MPC1000.schematic-gray-bpp.jpg

Does anyone have a better copy of the BPP Rev A schematic 75195?
The one I have is hard to read -
https://www.navy-radio.com/rtty/dovetron/dovetron-bbp100-schem-75195.pdf

Other Dovetron schematics and manuals are at
https://www.navy-radio.com/rtty/dovetron.htm

Cheers,
Nick England K4NYW
www.navy-radio.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20230909/22709a8b/attachment.html>