Sunday, 21 May 2017

TMNT pcb sound repair

This Teenage Mutant Ninja Turtles (TMNT) pcb was working but without sound, a common cause of no sound on a TMNT is a missing or bad 640khz resonator. The first one checked fine on the scope but the second one labeled X2 was dead. I replaced it and could see it working on the scope but this made no difference.


The Z80 and 6116 ram @ F16 both looked healthy and piggybacking the ram made no difference. I then decided to make use of the very nice high quality schematics available at jammarcade.net. The next most likely cause was the 74LS374 @ F21. This was confirmed bad with the chip tester and a new 374 brought all sound back except for the theme song.


The theme song is stored on a maskrom @ D5, I desoldered it at read it in my programmer as a 27C400 and it read as blank. I took another D5 maskrom off a scrap board and the theme tune returned.

Turtle power!

Wednesday, 17 May 2017

1084ST Commodore monitor repair

These Commodore monitors are great for the test bench (when they work) as they accept RGB video and have lots of nice adjust pots. This ST version of the 1084 seems to be the most obscure version with little information online and one of the only versions that doesn't have schematics available (typical).

When I got it, it was completely dead the led didn't even light up. With no schematics available I just poked around looking for any obvious problems and hoped for the best but found nothing. After testing all the capacitors, diodes, bridge rectifier and anything else I could test I couldn't find any issues. I even tried a replacement HR Diemen flyback but that made no difference, still dead.

I took a break from looking at it for a while but when I came back to it I spotted an 8 pin ic labeled TDA4605, a google for the data sheet reviled that it's control circuit ic for switch mode power supplies. I am sure I could have tested it in circuit with the scope but as I was running out of parts to check and a new one was only £2 including postage I decided just to order a new one and try it.

I fired it up and.....

 

Composite works too. :)

Friday, 14 April 2017

Donkey Kong Jr pcb (Mario Bros conversion) repair

I got this original Donkey Kong Jr pcb fairly cheap as it had been converted to Mario Bros and was in quite a state, it also didn't work. Since I already had a working original Mario Bros and my only DKJr was a bootleg board the plan was to convert it back to DKJr.



This is what it looked like when I got it, first thing I did was strip all the socketed chips and mod wires and clean both pcbs. Once it was clean(er) I was able to see all the cut traces and lifted pins much easier and start restoring it to stock.

After replacing all the eproms with the correct DKJr code and three colour proms I powered it up and was greeted with a screen of garbage. The cpu wasn't running, checking the Z80 and it was dead. I replaced it but no change, the fluke confirmed that I had bad ram (2114s). I replaced the rams at 3A and 3B this brought the game back to life.


It was mostly working but there were black lines and scrambled sprites. I found a 74LS283 @8R with floating output on the video (bottom) board, replacing this helped slightly but the image stayed mostly the same.

Another 74LS283 @8J failed in circuit tests, replacing this helped a lot but didn't quite fix it.


Almost fixed but there's a line through junior, and even more lines when going left or right.

I then found yet another bad 74LS283 @8P.

 All Fujitsu of course.


 Three sockets and three new 74LS283s.

 Colour changes in the screen shots is because of the camera not the board.

This fixed all the graphics issues, all that was left to do was remove all the components that had been added to add on board audio amplification since my jamma adapter does that and I didn't want it amplified twice.



Repair complete.

Wednesday, 15 March 2017

Karate Champ (bootleg) pcb repair

I've had this one stored away for years and now finally got around to making a jamma adapter for it. It just displayed a screen of static garbage, it looked like the cpu (Z80) wasn't running so started there. The clock and reset were working so next I removed the cpu as it was socketed and checked it in my tester, it was fine.


I then moved on to the TTL chips surrounding the cpu, I found a 74LS245 @C12 with two output pins (11 and 13) stuck low and all the inputs were pulsing away nicely. I cut these two pins and checked them with a logic probe at the board and the lines were now floating instead of low, this confirmed that the 245 was bad and that it wasn't something else on the lines forcing them low.

 Socket and new 74LS245 fitted.


After replacing a few damaged capacitors repair complete.

Tuesday, 7 March 2017

Sidearms pcb repair

The next pcb from that faulty lot of four to go on the bench is a original capcom Sidearms. This one had sprite issues much like the 1943 pcb. The player and enemy sprites were missing with just the odd glitch showing up.



Again like the 1943 pcb I checked the bottom pcb near the graphics roms but this time didn't find any problems. This just leaves the custom chip which I did suspect from the start but it's best to rule out the easy stuff first just incase. Luckily I had a non working Block Block pcb with the same custom chip (86S105) in the scrap pile.

Even with the desuicide mod / roms this Block Block board was not booting so I had no way of knowing if I was swapping one bad 86S105 for another.

Replacement 86S105 fitted. 

Luckily the custom was fine. :)

Thursday, 2 March 2017

1943 pcb repair

I bought this 1943 pcb in a lot of four faulty boards, it was shown as having sprite glitches in the ebay auction. I used dip switch 2 to pause the board on the explosion that happens at the very start of the game as this is very easy to see unlike some of the more minor issues.

Using mame I found that the explosion sprite is stored in eproms 6 and 10 on the bottom pcb so I focused on this area.


After pocking around a bit with the logic probe I found that probing the 74LS74 @4C caused the image to get worse. I stuck the logic comparitor on it but it didn't report any issue. I wasn't conviced though and piggybacked a known good 74LS74 on the one at 4C.



This completely fixed the issue.

I soldered in the replacement, repair complete.

Taito F3 Region Free mod

Taito F3 motherboards have plastic pegs / keys in the connectors stopping you from plugging in cartridges from different regions. Capcom did a similar thing with CPS2 motherboards, having certain regions use different connectors. Anyway these pegs are the only thing stopping you from plugging in and playing a cartridge from another region, there are no other security measures. So an hour with a soldering iron and sharp knife you've got yourself a region free motherboard.


I forgot to do a before picture but this is one of the four connectors with the two plastic pegs removed, I suspect the 6 pins on each connector aren't even used and it would be easier to remove the pins and just snap the plastic off with a pair of needle nose pliers. As I wasn't sure if this is the case as I only have a euro cart I did it this way, which I guess is considered the 'proper' way.