Ground bus pushed to +8.5v: best culprits?

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rjd2
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Ground bus pushed to +8.5v: best culprits?

Post by rjd2 » Mon Oct 03, 2016 9:20 pm

Hi all, so I am in my LAST phase of a CS80 resto, and I have a problem that is truly stumping me....

there are a bunch of ground bus lines to the poly AT boards. These should all read 0v, obviously, but they all read +8.5v. there are a bunch of them, on multiple boards, so it's very very labor intensive to lift each one.

i did, however, lift the +8.5v rail for each of the lower boards, and the ground buses still stay stuck at +8.5v. the PCB that runs all the gate/cv lines has at least one +8.5v rail; if i lift it, the ground buses drop to about +6v. there may be more +8.5v lines to this pcb; it's impossible to access it without a major deconstruction of the whole keying mechanism.

so, folks who are smarter than i; what would be your plan of attack? other than bridging, i'm not sure what to look for. thanks, rj

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Re: Ground bus pushed to +8.5v: best culprits?

Post by Stab Frenzy » Tue Oct 04, 2016 12:22 am

I don't know the internals of the CS-80, but my first thought would be checking for tantalum decoupling caps that have failed to a short.

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Re: Ground bus pushed to +8.5v: best culprits?

Post by rjd2 » Tue Oct 04, 2016 1:07 am

stab-thanks. all caps have been replaced; i don't think it can be local to the lower 5 boards, as best i can tell.

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Re: Ground bus pushed to +8.5v: best culprits?

Post by Werecow » Sat Oct 08, 2016 3:48 am

What point are you using for your reference when reading +8.5 on the grounds? From your description of the problem, I'd be looking for an open ground connection to those boards, rather than something shorted.

-B.

Edit: Just to clarify, circuit ground is supposed to be "absolute", electrically the same point throughout the unit. If it's not, it's disconnected somewhere.

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Re: Ground bus pushed to +8.5v: best culprits?

Post by rjd2 » Tue Oct 18, 2016 10:13 pm

werecow-aha! thanks for the reply, sorry for my late response.

i hadn't even thought to check the ground continuity on those boards, as i was so focused on the 8.5v rail, assuming it was "bleeding" somewhere. i'm not measuring against chassis ground, i'm measuring against the "internal", or circuit ground. often times these seem to be one and the same, but it does not appear to be so on the CS80.

i'll sniff around on those boards, and see if impedance looks nice and low across all circuit grounds of the lower boards. thanks.

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Re: Ground bus pushed to +8.5v: best culprits?

Post by rjd2 » Sun Oct 23, 2016 3:51 am

WERECOW!!! you nailed it. wow. hats off, my friend.

the ground wiring on the 5 underside boards is wired in series, not parallel. there was a ground break that i couldn't see visually, which caused the ground to go "silent" all the way downstream. fixed it! poly AT is responding! if i can calibrate the AT, i think i'm done here...

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Re: Ground bus pushed to +8.5v: best culprits?

Post by Werecow » Tue Oct 25, 2016 1:11 am

Good job! Glad you got it fixed. :)

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