Repair Logs – Page 117 – JAMMArcade.net

SEGA Master System repair log #2

Mar 102012

Stiggy dropped round today for a cuppa and brought his sick Master System with him. I have 2 of the mark 1 MS units so we did a swap.
Basically this system was always seeing the UP direction pressed on port 1 even with no controller connected.
Looking at the schematics it would appear that the custom IC 315-5237 has an internal pull-up resistance on each pin and the controller grounds this to register a button press. As I found the pin giving out a permanent low signal chances are something has gone wrong internally.
What I did was fit a resistor between this pin and VCC which changes the default state to a logic HIGH but will still allow the controller to drive it LOW for the button press. Almost certainly not the ideal way of doing it but I don’t know the MS very well at all so this will have to do.
The system now works perfectly. This MS also has a couple of built in games which my old one didn’t.

Jumping repair log #2

Mar 042012

As mentioned in a previous post, when I converted my Jumping board to Rainbow Islands I noticed that all the text in game was black. Going back through some pictures I took when I first got the board I see its present there too.

So where do I start?
I know from making my conversion that the palette values are stored in the program ROMs and they are pushed out to the palette RAM at address 200000.
I looked at the RAM with a scope and all seemed fine plus all the other colours seemed fine.
I also know that all the values required for this text colour is when the 2 least significant bits (A0 and A1) are active, so I traced these out down to the video board.

After a long session of following various signals I came across a 74LS374 chip.

This had all its inputs stuck low but according the the scope had a fair bit of noise on them, enough noise in fact to make my logic probe see it as pulsing HIGH/LOW. Following these inputs over to the chip next door I see a 74LS273 with its outputs giving off this crazy signal.
I replaced the 74273 and booted the game

If you look carefully you can see the lower part of the text is missing and a bit scrambled. Taking another look at that 74374 chip and its outputs are still crazy despite having good inputs. After a quick desolder and replacment I got this

All fixed and now my conversion is totally complete.
I have left one bit of graphics the same as the Jumping. If you play the game, in the lower left corner, you number of current lives is displayed as blue stars. On the original these are small rainbows. Just a note in case I ever release my modified files and someone tries to sell it as an original or something.

Super Chase repair log

PCB Repair Logs, Repair Logs Comments Off

Mar 022012

So I saw this Taito board on eBay. The seller says its broken and unknown but was most likely a Super Chase and supplied a System 16/24 adapter with it, which in its self is very nicely made and they can cost quite a bit.

Only thing is, Super Chase is not a System 16 or 24 pinout.

*pinout supplied by Stefan Lindberg

I made a quick and dirty adapter up with and tested the board, dead.
Now I made a very stupid mistake at about this point. I moved the board down from the test bench and into one of my cabs just to rule out any voltage issues that ive seen before BUT for some unknown reason I left the pinout adapter in the test rig.

After the smoke dispersed I checked the damage.

Power supply is fried. Turns out, without the adapter I tied the +12v line to the +5v line and destroyed the PSU.
At this point I didn’t hold out much hope for the board anymore but tested it anyway.

The board was unsurprisingly still dead.

I check the RESET and HALT lines on the CPU, the watchdog was doing its thing.
Pulled and checked the program ROM’s and 2 didn’t match.
A strange thing here. One has only 1 byte difference right at the end and the other was clearly trashed with random read outs at every read.
I copied the first dump over to MAME and ran it. Turns out this is probably a different region to the one in MAME and that byte difference either displays the country warning screen or not. Mine does not.

I got nothing but a bit of garbage on screen

I fired up MAME and checked what the game should be doing and it does perform RAM checks. I also saw in the code that it has RAM fail messages on screen should it encounter a problem.
Probing around the board in this state didn’t help at all as the watchdog reset everything before I could really see what was going on.
After a quick look at the CPU code I see I can bypass the shared and screen RAM checks very by changing only 1 byte. I did this and replaced the relevant EPROM and watched the game boot

So the screen RAM is at fault. The screen RAMs are the 2 Sony 58257 chips in the center of the board.
I pulled both of these and both failed. Replaced them and the game boots once more.

A narrow escape I think. This will hopefully teach me to slow down a bit, but I doubt it

Commodore 128 repair log #3

Computer Repair Logs, Repair Logs 1 Response »

Feb 252012

Motherboard clearly had a dodgy ON/OFF switch.
Removed it and replaced it with one from a C64.

On power up I had a blank screen.
Did a few quick checks with the scope and all data/address lines were pulsing properly.
Checked the voltages with a multimeter, the 5v was present but the 9VAC was not. Traced this right the way back to the switch I had fitted.
When I soldered the new switch in, one of the legs musnt have made a connection as they were a little shorter than the original. I resoldered the legs on top of the motherboard as well as the underside and it now boots up.

One of the CIA chips was missing, fitted a new one and C64 mode now works too.

Virtual Boy repair log #1

Console Repair Logs, Repair Logs Comments Off

Jan 212012

Got a Virtual Boy the other day with the Flash Boy Plus cartridge.
The unit worked fine with the Red Alarm game that was already on the cart but when I changed the game to Bound High I was greeted with a solid red screen. The music played and I could run the game blind but that was no use.

I reflashed with Red Alarm and after a couple of screen artifacts in the left screen the game played fine.
After a quick Google around it turns out that this can and will happen to every Virtual Boy made and occurs when the glue holding the FCC ribbon deteriorates so that connections become unreliable.
There are a couple of different fixes for this. One is to strip the plastic back and solder the very fine connections directly onto the board which is a permanent fix, the other is to but the little module into the oven at about 80 degrees C for a couple of minutes then apply pressure to the cable afterwards in the hope that the glue will reset making a nicer connection. This is not a permanent fix but will get you away. The later method is what i opted to do as I’m pretty lazy and dont have the chemicals needed to dissolve the plastic.

The first hurdle is actually getting into the unit as Nintendo kept with the same gamebit screws they used for other devices but they also stuck them down a deep hole so standard gamebit drivers cannot normally reach as they are too fat.
The solution is to make your own from a flat blade driver. I used my milling machine to cut it down and cut a groove in the end. Its certainly not pretty but worked perfectly.

On opening the ribbon cables are on either side of the unit and is held in at the board by 2 little crosshead screws. DO NOT try removing the 2 bigger screws near by, they control alignment.

The top of the cable just pulls out (carefully!).

Get a bit of tin foil, sit the offending article onto said foil, preheat the oven to around 80 degrees C and bake for around 1.5 – 2 minutes.
After its done remove it (its not hot) and rub or apply pressure onto the connection side until its sufficiently cooled and the glue will have reset.
After this, take some sticky tape and apply it to the area wrapping it round so that it applies a little pressure onto the connection.
Refit and test.
This worked perfect for me and many other people have said the same thing.

Time to waste my weekend playing some headache inducing goodness

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