Data bus line problem on OB-XA. schem help?

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rjd2
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Data bus line problem on OB-XA. schem help?

Post by rjd2 » Wed Jan 18, 2012 5:18 pm

hey folks, hope you may be able to help me. here's the service manual, the pot board is on page 53:

http://www.synthfool.com/docs/Oberheim/ ... nual_3.pdf

and here's the problem:

a number of panel switches on my OBXA are inoperable, or intermittent. they ALL lie along outputs 4 and 5 of of the 74LS42(U4) of the pot board. the input pins of U4(12,13,14,15) are not all showing the same voltage. 14 and 15 show 1.79V(about normal for a data bus line, right?), but 12=3.1V, and 13=.6V. also, all the output pins read about 4V EXCEPT pin 4, which reads 3V.

when a switch is normally depressed, it will push the correlating output pin of U4 up by about .3V momentarily. the offending switches will NOT push their voltage up. interestingly, there are a number of switches that run along output pin 4 of U4; the working ones will push U4 pin 4 up to 3.3V from 3V. the non-working ones which lie on the SAME output will NOT push it up.

i've verified that the problem is not the 74LS42 or any of the IC's on the pot board(i swapped with new IC's, and also tested with all other IC's removed. same results).

any ideas? it seems that input pins 12 and 13 are my most likely culprits, but i'm having a hard time tracing where they go on the schematics., fyi, the ribbon connector of the pot board connects to the lower processor board; i tried new IC's at U20-U26, and this did not solve the problem. thanks for any help.

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madtheory
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Re: Data bus line problem on OB-XA. schem help?

Post by madtheory » Wed Jan 18, 2012 9:47 pm

Probably obvious, but have you tried hooking a continuity tester into each switch to test them, then do the same with end of ribbon connector, and lastly with each trace on the board out of the connector?

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