Arcade Game Repair: Bad TTL VIL

[Phil Bennett]

I'm diagnosing a fault with an old arcade game PCB. It is comprised of two PCBs: a 'CPU' PCB and a 'Video' PCB. I don't have complete schematics for either (this was the late 1980s, when companies began to only provide partial schematics for games).

One bit of a bus (referred to as MDDB7) is not reaching a valid TTL logic low level:



The bus is the lower 8 bits of a V30 CPU's data bus buffered through a 74ALS245. Both components are on the CPU PCB, and with only that PCB powered, I see reasonable logic levels on MDDB7:



The ALS245 was replaced with a new, tested-good part. I believe that a component on the Video PCB is pulling up MDDB7 to a dodgy VIL.

On the Video PCB, the resistance between MDDB7 and 5V is 81Ω - considerably lower than the rest of the bus. MDDB7 is connected to at least 18 devices, most of which are LS245s but include an HD6445P CRTC and a couple of gate arrays. I don't see any visible shorts between MDDB7 and other signals.

Normally when I suspect a bad device on a bus, I'll go around each IC and use my desoldering gun to remove enough solder to disconnect the suspect pin from the bus until the fault subsides. Unfortunately the through holes on this PCB are tiny and this technique isn't working. Short of cutting every suspect pin until I find the culprit, does anybody have any ideas for narrowing down the faulty IC?

[edavid]

Do you have a 5 or 6 digit DMM?  Measure the DC voltage at all the connected device pins.  The highest voltage should indicate the culprit.

Stopping the CPU and/or adding a pulldown resistor might make this easier, but are not strictly needed.

[Phil Bennett]

Do you have a 5 or 6 digit DMM?  Measure the DC voltage at all the connected device pins.  The highest voltage should indicate the culprit.

Stopping the CPU and/or adding a pulldown resistor might make this easier, but are not strictly needed.

I only have a 4.5 digit meter alas. I’ll give it a try nonetheless.