Sunday, 8 October 2017

Spy Hunter pcb repair

I wasn't planning to log this one but as the fault turned out to be something I haven't come across before I thought it was worth mentioning. That's also why the screen shots are from mame.

This playtronic Spy Hunter pcb would either start with no background graphics or run fine for an hour or two and then the background would vanish.


Intermittent faults are always time consuming and a lot of guess work, it looked like a failing chip that was failing as it heated up but often from cold there was no background and spraying chips in the background circuit with freeze spray made no difference.

The midway schematics came in very handy, the playtronic board doesn't have screened chip locations but the schematics include a picture of the pcb with locations. After ruling out all the single wipe sockets for the ram (2b) and eproms (3a - 6a) used in the background I was left scratching my head.

I had previously replaced all the ribbon cables and also reflowed the connectors with no change to the fault.


I took a short break and when I came back to the board the background had returned, this makes finding the fault impossible so I had to leave it on for around an hour for it to vanish again.

Once the background vanished I checked all the outputs of the TTL connected to eproms (3a - 6a).

The outputs on the 74LS174 @ 13e were all low, the truth table for the 174 and schematics confirmed that pin 1 should be high as it's connected to +5v through a 1K resistor but it was reading low.

Checking the resistor and 5v was going in one side but nothing coming out the other, I changed the resistor and sure enough 5v back at pin 1. The resistor was obviously going open but not just from heat as from cold it was often open too. The resistor has no signs of damage or corrosion.

Best of all out of circuit the multi meter says that the resistor is fine and exactly 1K.

Saturday, 9 September 2017

ZX Interface 2 repair

The Sinclair Inferface 2 adaptor plugs into a ZX Spectrum computer and adds rom cartridge support and two joystick ports. Unfortunately this one had issues with the player one port, left and right were moving very slow and erratically. I opened it up and reflowed the joystick port and connector that goes to the spectrum and also cleaned everything as there was a lot of flux left over from when it was manufactured.


This didn't fix it, meaning the only possible cause could be the custom MT62001 chip. It doesn't look like this chip has a gal or any other type of replacement available or was used in anything else so I thought that was that.

After a little more research though it turns out that Amstrad continued to use this chip in later computers but renamed it to 40057. As luck would have it I have a scrap +2 (grey) motherboard that's been mostly stripped but still had this chip intact. After installing it into the interface 2 it now works perfectly.

Repair complete.

Tuesday, 1 August 2017

Kung Fu Master pcb repair

I've had this Kung Fu Master pcb sat in the to do pile for a year or two now as the custom KN6034201 chip on the top pcb was bad and I wasn't able to find a replacement.

It caused the background graphics to just show up as solid colour blocks.


A big thank you goes out to Caius for not only making a modern day replacement for the custom but also for selling me one of his spare pcbs.

 The KN6034201 reproduction pcb populated and ready to be fitted.



All graphics are now 100%, the only issue remaining is that there is no sound. This turned out to be a bad 6803 cpu, I confirmed this by swapping it into my other Kung Fu Master board. The 6803 cpu seems to be pretty rare at least when it comes to video games, they were more common in pinball machines so I didn't have any on hand.

I ended up ordering some from utsource (who I don't recommend) but they were very cheap so I ordered a few, this was lucky as the first one I tried was DOA. The next one I tried worked and the sound was fully restored.

Repair complete, thanks again Caius for making this repair possible.

Friday, 28 July 2017

Donkey Kong pcb repair

This two stack Donkey Kong pcb had all sounds apart from the music at the how high screen and the background music. A bit of basic research suggested it was most likely caused by bad EPROMS 3H or 3F, the 8035 or LM324 amp. All these were fine so using the schematics I made a note of what else was connected to EPROM 3H (the one that contains the music) the hp comparator flagged all outputs as being bad on the first TTL I checked a 74LS374 @ 4E.


Piggybacking a good 374 brough the music back, I fitted a socket and the new 374 repair complete.

Tuesday, 4 July 2017

Robocop pcb repair

This Robocop pcb booted up but the majority of the graphics were missing with only some text and the life bar showing up. The first thing I did was try the rom board on a known good main board and it worked perfectly so I knew the issue was with the main board.


 

After ruling out all the ram and ttl I started to look at the surface mount customs, pins can often lift on these. I reflowed them all but it made no difference. It went back in the pile after that for a few months until I pulled it out again today and decided to check the only two things I hadn't already which were the two prom chips. Of course it could have been a bad custom but I like to rule everything else out before I even think about replacing those.

Comparing the outputs to the ones on a good board I found that pins 13 and 14 of 7112E prom @E17 on the bad board were stuck low.

I had two scrap data east main boards from a trade many years ago. They were scrap in every sense of the word, the previous owner took a drill to the cpus trying to remove them and cut away parts of the board trying to remove certain chips, but luckily the proms were still there.


I fitted a socket and the 'new' prom.




This restored all the missing graphics, repair complete.

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.