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NEC PC-9821AP2/U8W repair log

Jan 182015

There are not only arcade boards to repair in the life…but also computers (and japanese ones are really cool)!

I got from Japan this ‘junk’ NEC PC-9821AP2/U8W in the mail some days ago (after a wait of two months), seller said only it was not working without going into details.

Powered it on and I got a solid black screen so was time to disassemble it.Motherboard was clean except for this:

Surpringsly all electrolytic capacitors were in good state for a machine with more than 20 years old but there was sign of corrosion near five 22uF 16V tantalum capacitors ~~which leaked spreading their dielectric on pcb.~~This was most likely caused by the glue used to hold them in place that turned corrosive with age.These capacitors were mounted as decoupling on five 74F245 which are used for DATA communication between DATA buses.In this hardware they were interposed between four NEC 42S4800 DRAM chips and connector of the CPU riser card.So, with these premises, I was quite sure that fault was located in this crucial part of the motherboard.After removing the ~~leaking~~ capacitors and cleaning the circuit, I started to probe continuity between each pin of the five 74F245 and motherboard and , following a scheme that I had prefigured, I found that PIN2 of one of these was not connected to the CPU card connector like all other so one bit was missing and the system was halted.Jumpered the two points and finally system successfully booted:

But immediately something sounded strange to me.According to its specs the PC-9821AP2/U8W model should have 5.6MB of default RAM (first 640KB are conventional memory plus 5MB of extended RAM) while, as you can see from picture above, mine had only 3.6MB so I missed 2MB of them somewhere.Also using an addon RAM card didn’t change the amount of memory.So, something else had to be wrong.Luckily I had another same PC with a good motherboard and tracing it I found that the five 74F245 had PIN1 (direction PIN which select the direction of the DATA transfer) in common while in the faulty motherboard this was only for two of the TTLs.So, due this, DATA could not be transferred from all available RAM to CPU resulting in only 3MB in total.

So I tied PIN1 all toghether with some AWG30 wire:

and the missing 2MB came back as well all extra memory of the addon card:

Another job done.

‘Arcade sync stabilizer’ by Martin Jones

General 2 Responses »

Jan 152015

Sync troubles, i.e. rolling/flickering picture,curved sides, etc.., on your beloved arcade PCBs?

Now the “sync stabilizer”, copyright by ‘MARTINJONESTECHNOLOGY’, beats ’em all !

Today I assembled on a stripboard this little circuit and installed it in my supergun.It really fixed all my sync issues with my troublesome boards (mostly bootlegs) which I never been able to solve in other way (trying, for example, the LM1881 sync stripper/cleaner)

Circuit is simple to do (but very effective),it requires only few common components.Technically speaking, it doesn’t modify the sync signal frequency but only the horizontal pulses lenght bringing them back in the broadcast standards.

For bill of materials and other technical explanations, I refer you to the official page of the creator:

https://martin-jones.com/2014/09/16/that-syncing-feeling-classic-arcade-games-that-wont-stay-still/

I highly recommend you to give it a try, you won’t be disappointed.

Final Starforce PAL dumps added

PAL Updates Comments Off

Jan 142015

Corrado dumped and tested two PALs from Final Starforce.

G.I. Joe repair log #2

PCB Repair Logs, Repair Logs Comments Off

Jan 142015

Yes, yet another G.I. Joe PCB on the bench (this is the third I repair).Let’s start with the log.

PCB was in a clean state:

But, as often happens with this Konami hardware, board had the watchdog active sign that there was a problem in main code execution or in the CPU itself.As usually I started my troubleshooting analyzing the main 68000 CPU and I found that, though the RESET signal was correctly generated by the custom ‘051550’ and reached the CPU, PIN17 (/HALT) and PIN18 (/RESET) of 68000 were stuck LOW so it was not never inizialized (all control lines were obviously silent ,too).So, judging from this,I started to suspect that 68000 was really faulty.I decided to do an “extreme” piggybacking with a good know CPU and sometimes I got it booting showing a bad color RAM:

This was enough to desolder the main 68000 CPU (a 16MHz one manifactured by ST) fitting a proper 64PIN socket:

Trying it in another board I obtained a solid black screen sign that it was realy bad (it’s the first time I came across a faulty 68000 CPU, never happened before).With a new 16MHz 68000 fitted in the socket board succesfully booted always showing the color RAM error above but I quickly fixed this by replacing both 2018 SRAM (also the one @14C was bad)

As I expected in this kind of Konami hardware which use ‘054986A’ hybrid custom sound module there was absolute lack of sound ,only some popping noises sign that the LA4707 amplifier was still alive at least.So was time to replace all the SMT electrolytic capacitors with new tantalum ones as they were leaking and having ESR value out of specs:

and install proper 1.778mm pitch sockets for the module to finish the job:

Son Son repair log

PCB Repair Logs, Repair Logs 1 Response »

Jan 122015

Welcome to my first repair log!

I bought this (very) expensive pcb from ebay in October 2014.

I played the game a couple of times and then after about one week, all the sprites disappeared while I was playing. Only background and text were present:

I couldn’t believe, at first I began to press everywhere hoping it was a loose or oxydized connector but soon I realized something got faulty.

This game hasn’t any schematics available so I knew from the beginning it would have been very tough to fix it.

I connected my trusty logic probe and began to short some pins on the pcb to see what changed on the screen so that I could focus on the right part of the circuit.

After about 30 mins turned on, the sprites started to reapper but missing some lines:

I then remembered that 1942 hardware is very similar to Son Son and the schematics are available.

I was right, the way that TTL chips are connected on 1942 is equal on Son Son. Only the positions of the ICs are different but the logic is really similar.

I probed the 2148 rams @F2 and @F4 on Son Son pcb which on 1942 schematics are described as OBJ rams and I found a stuck /WR signal @F4 on Son Son (J4 on 1942):

I followed back the signal always taking a look on 1942 schematics, and I found finally the source of the problems on the missing signal on pin 11 of an 74LS00 @F11 (on 1942 I circled the equivalent IC @K11).

On 1942 schematics available on internet, someone circled the 74LS00@N8 writing “no plane”, probably he had the same fault. 😉

Replacing the 74LS00@F11 gave me back all the sprites

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