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Black Tiger repair log #4

Feb 282019

I got the same Black Tiger bootleg board back as in my repair log #3.

The sound had stopped working completely.
I did all the usual visual checks and reseating where I could but no joy.
I probed the Z80 CPU and found following a brief flicker at power on the /HALT pin was always asserted. I pulled the ROM and checked it in my programmer but it came back as good. There is nothing in the way of buffering in between the CPU and ROM.
I decided the quickest way to see what was happening here was to attach the logic analyser to the CPU address bus with the trigger set to the /M1 signal.

From here I could see that it started off good but failed when returning from a call. As the return address is pulled from the stack I decided to remove the RAM and test it. This turned out to be faulty reporting that bit 3 had become disconnected.
Replacing the RAM brought the sound back.

Crude Buster repair log #1

PCB Repair Logs Comments Off

Feb 172019

Got from Portugal this Crude Buster PCB (also known as Two Crude outside Japan), an unusual and underrated beat ’em up developed and published by Data East in 1991:

Board simply booted to a white static screen:

The first thing I did was to check the main CPU (a custom 68000 marked ’59’) circuit and I found that both WORK RAMs (two 8K x 8-bit devices) has stuck data lines whereas address was correctly toggling :

The two RAMs are Toshiba TMM2063, we all know they are very prone to failure:

When I piggybacked them and got some errors (related to code execution) on screen:

Hence I decided to remove the chips:

Although RAMs were both tested fine in my programmer I replaced them and I got the board booting. Game was fully playable but some sound samples were wrong:

Samples are stored in two 1Mbit EPROMs and played by two OKI MSM6295 PCM chips:

The two EPROMs were dumped as good so I fired up my audio probe and started to “listen” to output (pin 36) of the internal DAC of both MSM6295.The samples came out already corrupted from one :

I was about to replace the OKI MSM6295 when I noticed some solder residues on a couple of pins.I magnified the area with a microscope and found a solder bridge that shorted two address lines (pin 20 and 21)

I removed the bridge and sound was fully restored as well as board 100% fixed.

G.I. Joe repair log #3

PCB Repair Logs Comments Off

Feb 162019

Yet another G.I. Joe PCB on the bench from the five faulty ones received from Portugal some time ago :

Board was in good conditions but someone harvested parts from it.In particular the thee 6264 SRAM (part of tilemap circuit) were missing:

As well as the sound ROM @7C and the 62256 SRAM @2D always in audio circuit :

I installed the components and sockets too where needed.When I powered the board up I was greeted by this screen filled with letters :

Usually this kind of issue on Konami boards means there is some trouble in the tilemap generation circuit so I made a visual inspection and found a couple of severed traces on solder side:

Finally the board executed the self-test but two devices were reported as bad :

The one @2D was the 62256 RAM that was missing while @3E lies the custom ASIC ‘054539’ (which is a PCM sound generator ), both ICs share same address/data bus.

At a closer inspection some pins of the custom were lifted so I promptly refowed them.The board successfully booted into game with no further issue.

Yes, yet another G.I. Joe PCB fixed (hopefully not the last one…)!

B&K Precision 560A IC tester repair log

Equipment Repair Logs 1 Response »

Feb 062019

Some days ago I wanted to test some RAM chips from an arcade PCB using my B&K Precision 560A tester :

For the uninitiated this piece of equipment allow you to do out-of-circuit and in-circuit testing of many ICs like TTLs,RAMs and ROMs thanks to an extensive internal IC library.I often use it during my repairs but in that day it suddenly died, no sign of life, nothing came up on display when I powered it up.For first I checked the main fuse and it was good so I decided to open the unit:

Having no schematics I started my troubleshooting from upstream checking the big transformer just after the AC input, voltages were present on its secondary coil.Following the path I figured out the AC voltages are rectified by a circuit on one of the PCBs.In particular the rectified +12V is then regulated by a couple of ‘LAS1605’ fixed +5V voltage regulators (in TO-3 package) mounted on a big heatsink for providing +5V to the circuitries on the other boards:

Two LAS1605 fixed +5V voltage regulators in TO-3 package

I probed the inputs of both and +12V was present:

As well as +5V on the output of one regulator:

But the other one was giving only +1.706V on its output, too few for correct functionality of the logics on PCB:

I looked online for buying a ‘LAS1605’ +5V voltage regulator or a compatible one but found only few at not really cheap price.Then, as a last resort, I asked my local electronics shop, they surpringsly had in stock some TO-3 ‘MC7805CK’ which is a perfect drop-in replacement (although it can deliver up to 1.5A output current against the 2A of the ‘LAS1605’ but this is not really an issue since I don’t think the tester can draw more than 1.5A during normal operation)

I pulled out the suspicious part:

I swapped the spare in and was welcomed by this :

My BK560A rised from its grave! Not too bad for a 2-euro repair! (as much as the replacement part costed…)

Mortal Kombat (original and bootleg PCB) double repair log

PCB Repair Logs Comments Off

Feb 052019

Got from USA an original Mortal Kombat PCB set complete with its sound board:

When I powered the board up the self-test reported two bad devices @U5 and U23:

Then it booted into game but colors were completely wrong:

The two presumed bad devices are the color RAMs (Sony CXK5863 so 8K x 8-bit, pin-to-pin compatible with the more common 6264)

Probing them revealed that pin 27 (Read/Write enable line) of both was stuck high:

This input signal is labeled on schematics as ‘-CRWR’ :

It comes from pin 22 of a 24 pin IC @U8:

The IC was faulty and needed to be replaced.At first glance I thought it was a simple PAL/GAL but actually it turned out to be an Altera E600 EPLD (so still a programmable logic device but something more complex with two clock inputs providing sequential logic too ).I had no chance to dump or reverse it but only to look for a donor board.Luckily I was sent another original Mortal Kombat PCB completely dead which I took the part from.Board was then fixed :

The bootleg I received for repair was the Yawdim one with no Midway logo on title screen :

It was fully playable but some backgrounds graphics were wrong :

As usual I did a visual inspection of PCB and found on solder side a scratch that apparently had severed a trace:

The involved area was the one populated by some 64K x 4-bit dynamic RAM so this was relevant to the kind of fault :

Checking with my multimeter confirmed the trace was really broken so I patched it, this restored background graphics and fixed the board:

Double repair accomplished.

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