Thursday, 22 June 2017

Thunder Cross II pcb repair

I bought this board originally just to use for parts as I always need konami custom chips. However when it arrived I could see it was in really good condition apart from the missing 051550 custom near the edge connector and a couple of damaged traces. So I decided to see if I could get it going.

I took a 051550 from a Main Event parts board soldered it in and fixed the three damaged traces.


Powering on the board I was greeted with a screen of garbage, the cpu wasn't running. I checked the two main program maskroms with the eprom burner and both returned unknown code. I then burnt two 27C010 eproms with the code from mame which you can see in the picture above and the board would now boot.

It would get as far as the ram / rom check screen and reported 3 failures. Rams 1H, 17H and 18H. 17H and 18H had already been removed from the board and socketed so these were easy to check and both were fine. The soldering on the sockets looked good too, after ruling out the ram at 1H and all other surrounding logic I was left with the custom chips.

On closer inspection two pins on the 051960 custom were bridged, I removed the bridge and reflowed the chip but this made no difference. I decided to replace this custom as it was the most likely candidate from what I had to go on. Luckily I already had a spare desoldered in my parts box marked 'ok'. I used the usual chip quik technique to remove the original chip and soldered on the replacement.


Time for the moment of truth.


All rams now pass, repair complete.

Sunday, 18 June 2017

Mr Do! pcb repair

This Mr Do! pcb wouldn't boot and just displayed a screen of garbage.


This was caused by a bad 74LS138 @A3. Once I replaced the 138 nothing changed, I checked reset pin with my logic probe and it wasn't kicking in. After about 15 seconds it finally kicked in and the game booted.

The capacitor that causes the delay is the cap @ C14 which was a 22uf, interestingly the schematics suggest a 47uf cap which would make the delay even longer.


I checked another Mr Do! pcb and that had a 0.47uf disc cap at C14, so I replaced the 22uf cap with a 0.47uf electrolytic as that's all I had on hand but it worked perfectly and reset kicks in right away now.

This just left one remaining issue, the player and enemy sprites were missing (I forgot to photograph this). I finally tracked it down to a bad output on 74LS32 @ E3.


Repair complete.

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. :)