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Michael Jackson’s MoonWalker repair log #2

PCB Repair Logs Comments Off

Sep 072017

Got this Michael Jackson’s MoonWalker PCB (Sega System 18) for repair:

Board booted but sound was missing at all and game behaved strangely, you could no longer move your character whenever you pressed the buttons or being hit by enemies :

At first glance I thought the FD1094 custom CPU module was slightly suicided so I swapped it with a good one along with the i8751 MCU but with no luck.I began to think of a correlation between the lack of sound and freeze.Probing the Z80 sound CPU revealed /HALT line was asserted while the /INT was floating.The latter comes from one of the two YM3438 FM sound generator @IC88 :

I removed the chip and tested it on a good motherboard, it was bad.But replacing it made no difference, the shared data bus of the two YM3438 was still inactive.So I pulled the other one @IC87:

It was bad too.Replacing it fixed the board.

Cabal (bootleg) PAL dumps added

PAL Updates Comments Off

Aug 292017

Today I dumped the two PALs from a working bootleg of Cabal.Original devices were two secured PAL16L8 which I reversed and successfully tested onto GAL16V8.

ROMCMP Frontend

General Comments Off

Aug 272017

In an attempt to help more people start using MAME’s fantastic ROMCMP utility I have made a simple GUI frontend for it.

Program can be found in the Downloads section under “PC Software”

Rainbow Islands repair log #11

PCB Repair Logs Comments Off

Aug 262017

This is the eleventh repair log for Rainbow Islands, almost a record for a board…

PCB was in fair conditions :

But the TC0070RGB DAC module was missing and its solderside was brutally burnt during removal :

Hence, for first I soldered back a spare module and patched a burnt trace:

On power up I was greeted by a blank static screen, /HALT and /RESET lines of 68000 were continuously going high/low sign that the watchdog circuit was active:

Usually this means that main code is not properly executed due some problems in CPU/RAM/ROM circuitry.For first I focused on WORK RAMs, two Sony CXK6864 (6264 pin to pin compatible).When I piggyback the one @IC26 I got this screen:

The watchdog shut down too, this lead me to remove the chip, it failed when tested out of circuit :

Now I got always that garbage screen.Probing the two 62256 RAMs @IC-IC4 in tilemap generation circuit revealed unhealthy signals on some data lines (good signal on the left for comparison)

Piggkybacking both RAMs at same time allowed the game to boot although with scrambled backgrounds:

I desoldered the RAMs having confirm they were both bad:

Graphics was perfect now but sound was faint and scratchy:

I used my audio probe to troubleshoot the sound circuit.This revealed that sound came out from PIN12 (which is the analog output to the TL074 OP-AMP) of the YM3012 DAC already corrupted.Indeed I could measure on this pin a weak voltage :

But a bad output means also a bad input and PIN11 (the analog input) was not healthy too:

This came from pin 8 of the near TL074 OP-AMP (through a 560 Ohm resistor)

I pulled it and it resulted faulty when tested :

Fitted a good OP-AMP restored the sound and fixed completely the board.

Commodore 1084-D repair log

Monitor Repair Log, Repair Logs 2 Responses »

Aug 262017

This fantastic monitor has been on my test bench for as long as i’ve been in this hobby and its been a great little work horse.
I powered it up the other day to test something and noticed the upper half of the screen had what I would describe as thick scanlines whereas to lower half of the screen looked a little compressed.
I hooked up my NES to it and the problem was a lot more apparent

Keep you eye on Luigi when he jumps to the top of the screen. As he moves up he gets longer

Asking a couple of people quickly, they all recommended changing the capacitors. The ‘shotgun’ approach isn’t really something I like doing as I like to understand what would cause issues before I randomly poke around but monitors are not my strong point so I started ESR testing in-circuit.
All the capacitors I checked were perfect with really low ESR values and all the capacitance readings were within spec. All except for one little capacitor, C306. This is a 47uF 25v capacitor and is connected to pin 12 of the vertical deflection circuit (TDA1670A)

I tested the capacitor out of circuit and confirmed for sure it was actually bad.

With the monitor taken apart I hooked up the Muter BMR95 to the tube to check the condition of the guns
A tube can usually be identified by the label that is on the back of the tube

E2940B22 is the one we want. Next we cross reference this to the supplied manual to find which adapter we need to use.

Adapter 808 is the one we need for this tube.
All hooked up

The guns aren’t too bad and certainly no need to attempt any cleaning or rejuvenating. Sometimes this can cause more harm than good. I have a spreadsheet prepared too as outlined in the BMR 95 manual to give a better indication of the life expectancy of a tube but i’ve not included it here.

With all that done it was time to test the monitor again

All seems to be fixed again and im happy with that.
You can see the stretching better with a picture comparisson

Before

After

Never underestimate the importance of a single capacitor!

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