TAF Swamp Release coil stuck on

elfyhead

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I posted about this about a week ago, but have sleuthed a bit more since then, and thought I'd post in a separate thread (with more precise subject) to see if I could get some help.

My TAF Swamp Release coil is stuck on on power-up.

Here are the things I've done to try to fix it:

1 - Replaced a blown F103.
2 - Replaced the coil with a new AE-27-1200 (coil paper was burned, and it was shorted across the lugs).
3 - Replaced a bad Q20 (also replaced Q19 for good measure, though it measured good).
4 - Replaced U2 (even though all three of the other solenoids on the circuit worked, I thought I'd try it).
5 - Checked D5, R22, R23, R24, R25, R256 - all good.
5 - Reseated J113 on both sides.
6 - Checked J122, and wiring on the drive side, by unsoldering the blue/yellow wire at the coil, and externally connecting a wire directly from the pin in J122 to the drive side of the coil.

None of this made any difference. I have a fuse that is working, a coil that is good (13 ohms across the lugs), power to the coil (obviously since it's stuck on), good passive components, a good 74LS374 (U2), apparently good connectors (J107, J113, J122), good wiring, and yet the coil is stuck on. All three of the other coils/motors on that 74LS374 still work (Thing Eject Hole, Bookcase Motor, and Thing Motor), it's only the Swamp Release that is a problem.

Any help would be greatly appreciated.
 
a good 74LS374 (U2)

how do you know this is good ?

after looking at the circuit and reading what you've done, i would test the 74LS374 by removing it, installing a socket and carefully push in a new one

if that doesnt work, install a new ribbon cable or test it between the power driver board and the CPU board. could be there's a problem on the CPU telling the power driver board to turn on the coil
 
how do you know this is good ?

because i desoldered the old one and soldered in a new one.

after looking at the circuit and reading what you've done, i would test the 74LS374 by removing it, installing a socket and carefully push in a new one

i think i've done the equivalent (though didn't put in a socket - just soldered the new one in directly).

if that doesnt work, install a new ribbon cable or test it between the power driver board and the CPU board. could be there's a problem on the CPU telling the power driver board to turn on the coil

i wondered about this, but had a hard time convincing myself that it could be bad - it's a
very short cable and the pins all look fine on both ends...
 
i found the problem last night. probed around the board, and it turned out Q20 was bad (again). i'm hoping that it's not indicative of some other problem on the board and that it won't blow again, but for now, the problem is fixed, and now i can play my shopped out TAF again! :)

 
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