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Arcade Collecting > Restorations & repair

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bperkins01:
Well.. You seen *some* of my shop.. Am I afraid to buy tools? You know I'm not.. I LOVE tools. I've already spent more that I'll make back (well.. maybe.. who knows)

Now... 101 work? cmon... at minimum 102... and really .. 201! Finding a bad clock in a sync circuit with a logic probe (and not Google?).. that just good stuff!
I'm in the matrix already.. I see a scope in my future... But I'm a bit away from a Fluke... Not sure I'll need to bite into that...

jennifer:
Jennifer laughs all amused...I ain't shaming you, what you done there with a probe is not to be taken lightly, but for others reading this there is far more than that involved long term.

bperkins01:
Back to fixing board#2 - The last item on this was the trackball response....

I replaced the 74LS157 - but no change. The trackball has the green boards which have been known to have this backspin issue.
I *thought* I ordered a set of red boards - turns out they are $20 each, not per set... Had I known that I would have ordered an entire trackball for $45..
That said - the backspin issue is a problem when the shooter is going east west.. So I replaced that green board with the red one and problem solved. (Thanks MikeA)

The solution was well known. Not sure why 2 out of 3 boards worked correctly.. But this PCB is now 100%

bperkins01:
Because I can't help myself:

Board #3
Purchased off eBay for a good price and training board

Triage:
Board dead - no picture or test mode

* All voltages on board good (+5v, -5v,+15v, -15v, +22v, -22v, -30v)
* Missing ROM#210
Work done:
Removed and cleaned legs of all chips that were socketed.
Verified/replaced missing and bad ROMS - 210 missing, 208, 209, 212 were bad
Checked for any hot chips (none)

Board powers up and shows this screen:

This tells me:
- Sync circuit is working
- Video memory is working
- CPU is running

The watchdog circuit is triggering and when you put the machine in test mode it beeps at you. Testing pin 40 with the logic probe on the CPU confirms it.. It's pulsing.

(New to me) trick I just learned:

The ROM, ROM0, ROM1, ROM2, ROM3 test points make it very easy to determine if the ROMs are getting accessed. They should all be pulsing with the logic probe.
Turns out ROM0 is stuck!

From the schematic, the 74LS139 at J2 chip is the one that cycles through the ROMx lines.. ROM, ROM1,2,3 are all pulsing and ROM0 is stuck high.. Pins 13,14,15 all seem to be doing their work. Next up is to replace J2 and see if the machine boots past the watchdog.

******
Replaced the chip at J2 - no change.
Learned yet another new thing.

On the Centipede board there is are test points to disable the watchdog circuit.
If you jump the disable watchdog test point to ground, the CPU will get to run the RAM check without the watchdog interrupting.

Once I did this it signaled RAM chip#2 as bad.. (A 2114 chip)
Replaced it and ran it again.

Then signaled RAM chip#7 as bad (A 2101 chip)
Waiting for that to be delivered....

Moral of the story: Make sure the RAM self test is actually running.. I thought it was.. turns out.. it wasn't..

One more item for my triage checklist..

bperkins01:
Got back to this board today.

Replaced RAM chip #7 at M7
Board now passes RAM test and plays.

Background graphics are messed up however. I was able to play enough to test high score retention circuit (works). The on board testing says (per the manual) that color RAM C8 has failed and would not have been flagged in the standard RAM check. Need to probe to determine if its getting all its inputs, etc. before replacing.

I'm glad I waited before just ordering a chip, swapping it out and determining it was not the issue. In self test mode - you hit different buttons to do different tests.

The Background Color Test and the Object Color Test were behaving strangely..

Player1 Start is the background color test - it showed its bit toggling on the screen, but did nothing.

Player2 Start is the object color test - it showed its bit toggling, changed the colors of the letters, etc. But ALSO changed the background colors.

At first I thought the chip both of these connected to was the issue. (74LS257 @ L9) The trackball feeds that same chip and it had done a couple weird things is test mode. The troubling part was the bits were flipping and displaying correctly on the diagnostic screen in all cases.

These bits were obviously being picked up correctly on the data bus and being interpreted - or they would not display correctly on the screen. So the input circuitry was good and the code that displayed the switch positions was running.

The data lines the Player1 and Player2 switches used were DB0-DB3. So I decided to follow the data. Looking into the video output circuits - Maybe they were not getting clean information from the part of the system that controlled the background? The color ram at C8 (where the self test said the failure was) seemed to be working with the data lines pulsing in with the logic probe. I swapped in my known good board and tested C8 and both boards seemed to be behaving the same. Using just a logic probe - that is about the best you can do.

Moving backward though the circuit - it pointed toward the Picture Data ROM circuit. Reading though the description:

This had a lot of promise. I read this as: display some objects and then display some colors. Something to that effect...

Which caused me to focused in this area.

I started checking lines into J8/K8/F8/H8.. The lower set F8/H8 seemed to have some dead lines coming in (neither high or low) Compared to J8/F8 - they were behaving different.

F7 (ROM 211) was original and H/J7 (ROM 212) was a newly burned ROM. I had cleaned 211, Deoxit and reinserted. The board had been passing RAM and ROM checks at this point. Using the logic probe I worked my way around F7 just to see data was going in/out.. When I do this I touch all the pins just so I can compare with a component that mirrors it. Except - pin24 was not showing anything - wait - that's power! I pulled the ROM and gave the pins a light sanding with 600 grit paper. Put it back, the game started playing correctly!

It seems F7 was sorta partially powered (pin 21) - that is why it didn't flag in the ROM check and why some items seemed to work. I suspect the socket is flaky. I've ordered a replacement socket to make sure this board doesn't end back up in this spot again.

After testing (playing) the machine for about 10 minutes - the screen had a burst of crazy graphics, the shooter changed to a line and 0's popped onto the screen.. WFT?

I put it in test mode - it was watch-dogging again. So I hit the reset switch and it counted to RAM chip #3 (K7). Back to the bench, swapped out K7 and the board plays again. I'll let it burn in over night to see if any other RAM is going to give out. Not sure if that is common or not. Having a chip die WHILE playing it..

Once the socket is replaced and its burned in a day or two - this board will be considered done.

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