[GreenKeys] Dovetron advice? stuck in Mark-Hold

[Nick England]

Update - I finally tracked down a bad 741 op amp in the mark-hold circuit.
I swapped in one and think it is all working OK now. Thanks to all who
responded with help and/or sympathy.

Although I still don't have a schematic for the TSR-300D REGEN/SELCAL
board, I did ohm out the connector and determined it is wired the same as
the TSR-200D REGEN board for which I do have a schematic. That let me
figure out how to bypass and remove that TSR-300D board so I could scope
out the BBP-100 board mounted below it.

And FWIW I found a little better BBP-100 revision A schematic 75195 and
have put it at
https://www.navy-radio.com/rtty/dovetron/dovetron-bbp100-revA-schem-75195.pdf

Yours in radio teletypery,
Nick England K4NYW
www.navy-radio.com


On Sat, Sep 9, 2023 at 2:02 PM Nick England <navy.radio at gmail.com> wrote:

> 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
>
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