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Default Autohelm ST50 display jams seatalk. Schematic wanted or 9401RLA80chip pinout or parts

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Hi guys,

*** Background ***

I'm trying to fix an Autohelm ST50 Compass display Z136 (Black, red
trim, round Seatalk connectors, but serial sticker inside says "Raytheon
Marine) that continuously pulls down the Seatalk bus. It otherwise
appears to be working though I haven't tested it with its fluxgate

I need it to add a remote sensor for heading reference to a ST2000+
tiller pilot + other Seatalk instruments due to deviation at the tiller
that cannot be economically eliminated. There is no budget for new
replacement, its fix it or do without as this will be a 'shoestring'
upgrade if I succeed. For the curious, this is for a 26' yacht (long
keel Bermudan sloop).

*** Model ***
Autohelm ST50 Compass display Z136
(Black, red trim, round Seatalk connectors, but serial sticker inside
says "Raytheon Marine. May actually be from the ST50+ range)
PCB marked "3015-076 ISS E".

**** Symptoms ***
Does not respond to Seatalk commands (Tested with L0 - L3 illumination
commands), does not transmit Seatalk data, Jams bus preventing
communications between other working instruments.

Yellow wire (Seatalk data I/O) is permanently low when display is
powered but is Hi-Z when un-powered so not a direct short.

No signs of any data at all (on a scope) while cycling illumination level.

No evidence of water intrusion, but possibility of mechanical shock
damage. (cracked mounting pillars in housing successfully repaired)

*** Troubleshooting ***

As above + data I/O wire traced to pin 21 of 24 pin SOIC IC:

RT C 9737

*** Help Wanted ***
Any or all of:

Schematic for this specific display.

Schematic for any instrument in the same general series with a round
analog dial.

Schematic for any instrument using the same I/O chip.

Source of 9410RLA80 chips and/or pinout info (though it may well be the
MCU driving it).

Similar displays for spares or same for use*.

Any other troubleshooting help/support you can think of!

* Free is best - can pay P&P. Cheap is next best. If you want typical
dealer markup I would need fair to good condition fully working with a
warranty. PLEASE CONTACT BY EMAIL - Address + de-munging instructions
in Sig below.

*** Experience ***

20 years of analog and digital component level repair work +
microcontroller experience. I have hardware and software to monitor and
decode Seatalk messages and have the usual equipment and tools of a
small repair shop.

Thank you for your attention.


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