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Q*bert Repair Log


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I thought I post this up for future reference, just in case it helps in the future. I don't normally document my repairs, as I usually can't be bothered, and even with this repair I omitted to take a few pictures explaining what's going on (CRO shots).

 

This is an original dedicated Gottlieb Q*bert that also happens to have a multi-kit fitted.

 

Although the game was running, the foreground graphics were either missing or made up of lines across the screen.

 

IMG_2158a.jpg

 

First job was to remove the multi-kit and return the board to its original condition. This would allow access to the board for probing, and also rule out the multi-kit as part of the problem. I also didn't want to risk damaging the kit during any repair.

 

With a new set of Roms burnt and fitted, the game still displayed the same fault, so the kit was assumed to be ok.

 

I didn't want to go to the effort of building a test harness for this game, so I positioned the cab as close to my test bench as possible.

 

IMG_2156a.jpg

 

I still needed to hook up another monitor so I could see what was happening on the screen. I also had to make an extension cable from the cab to the extra monitor, but it was still quicker than making a complete test harness.

 

The fault was with the foreground graphics, so I grabbed the board schematics and started probing that section of the PCB. I checked all the usuals suspects with my logic probe, but everything seemed to look ok. Normally with a board on the bench I use my CRO over a logic probe, but as I was working in the back of a cab, trying to see what was happening on the CRO was near impossible with my crappy eyes now, so the logic probe was easier to use.

 

After everything else seemed ok, I then concentrated of the line position Ram and line object Ram where I suspected the problem lay. In total, there are 10 x 74s189 Ram chips all sharing the same address bus. The major issue with any IC on a shared address or data bus, is that a fault on any IC is very hard to see, and basically impossible with a logic probe.

 

All these Ram are addressed via a 74157 (Quad 2-Line to 1-Line Data Selectors/Multiplexers). As mentioned, I suspected the fault lay in this part of the board, but the outputs on the 74157 looked to be pulsing nicely on my logic probe. I decided to test this anyway by temporarily shorting the address lines to each other. Doing this brought up some of the missing foreground on the screen so I suspected the 74157 wasn't driving the Ram as expected. I then hooked up the CRO which showed that the output on pin 7 wasn't quite right. This is a prime example of where a logic probe has its limitations with these kind of faults.

 

I removed the IC and decided to test it on my Wellon programmer. Even under test, it passed.

 

IMG_2160a.jpg

 

I also decided to try it on my cheap crappy Chinese tester. Again the IC passed.

 

IMG_2161a.jpg

 

Even though the IC passed both test, I wasn't happy with the output on the CRO, so a new IC was fitted to the board. This fixed the fault, and the game displayed all graphics correctly. The Q*bert multi-kit was reinstalled and is working great.

 

IMG_2159a.jpg

 

So why did the IC pass testing? Basically the output is unable to drive all the Ram in circuit, but on its own no problem, so it passes test. To be honest, I never rely on any IC tester for this very reason. If I suspect an IC is faulty and I remove it from the board, I always fit a new one.

 

Regards,

 

Johns-Arcade.

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I was thinking about getting a multi kit for my original Q-Bert but have heard they can be problematic?

 

Brilliant game, easily the one I play the most in my collection.

 

I also own a Q*bert, but I'm not really interested in the other games available on the kit. I've also heard they can be problematic and easily damaged, hence why I removed it during this repair (and to rule it out as a possible problem)

 

I think this board would be a better buy.

 

http://www.arcadeshop.com/i/1060/mylstar-jamma-pcb.htm

 

Regards,

 

Johns-Arcade.

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