Topic: Best way to debug a stuck bus line?!  (Read 1292 times)

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Offline Elgen

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Best way to debug a stuck bus line?!
« on: January 26, 2013, 10:40:02 AM »
Okay, here's the situation:
I'm currently working on a bootleg 1942 where the only initial problem was that some of the controller lines was stuck low causing the plane to go left and use all rolls at start-up. That fix was easy: Changed a couple of fried 367's in the input section.
However while working on it, I suddenly lost all sound?! I don't think it's related to the input problem at all, just a coincidence.
The sound CPU is a Z80 controlling 2 AY-3-8910 via the Z80's databus. On the same bus is also an SRAM and the sound ROM. Now one of the 8 lines is stuck low (one right in the middle). That is it fluctuates between GND and 0.5V, so it looks to me like something is trying to drive it. It also have direct contact to GND (used continuity tester). But I can't seem to find the component that's pulling it down. Tried to cut the relevant pins on both the AYs, the ROM (just pulled it from the socket), the RAM, the CPU, and also a 374 I found to be on the bus. I would rather avoid desoldering the CPU and the AYs.
As bonus info I have a working SolomonsKey with 3 of the same sound chips, and here ALL datalines are active.
By the way, the way I tried to find ICs and components that connects to the stuck, is by starting at the AYs and beeping the healthy bus line just besides the stuck one around the PCB and then looking if data pins 'close to' it on the ICs I found were stuck low.

Where would go next? Is it desoldering the AYs and CPU to see how the circuit is under them, or is there more to check before that?

Any help could be useful };-P

Offline Elgen

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Offline andy_rea

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Re: Best way to debug a stuck bus line?!
« Reply #2 on: February 04, 2013, 05:29:01 PM »
Bit late now, since as you found that pesky short  ;D  However perhaps something like this http://dangerousprototypes.com/2012/06/25/half-ohm-mohm-addapter-for-multimeters/

Would be useful for future shorts, able to measure down to 1 Milli-ohm, as your probes get closer to the short the resistance will drop.

Just a thought, BTW i do enjoy reading you repair logs, keep posting em  8)

Regards Andy
ZX81 nutter....