JAMMArcade.net – Page 257

SEGA Master System repair log #1

Mar 212011

Got a couple of Master Systems for repair.
This one was completely dead on power up.
First thing to check with a dead unit is voltages. The master system has a voltage regulator which in this case was fine. Second thing to check is the clock signal. The Z80 has its clock signal at pin 6. This was quiet. Traced it back to the large custom chip. Looking at the schematics the custom chip takes the clock signal from the crystal and then outputs a clock to the Z80. I checked pin 30 of the custom and this was also quiet.
On this model the crystal is located under a metal housing that is soldered underneath. I removed this cover and checked the output from the 54Mhz crystal. Nothing. Replaced this and the unit boots again.

Double Dragon repair log #4

PCB Repair Logs, Repair Logs Comments Off

Mar 122011

The sound circuits on these DD boards really were bad!
Another one with dodgy sound, music was intermittent and half the SFX were really really low.
Did a full capacitor replace on this board for the sound, half the caps had started to leak. This brought back the music.
The sound effects were a little confusing. Like the last board I could see the data going into the M5205 chips but on IC80 there was nothing coming out of pin 10. I replaced the chip but this time there was no change. The board has had the op amps replaced at some point and there was a little rework done on them. Pin 5 (input) of the op amp at IC97 was shorted to pins 6 and 7 (output) effectively bypassing the op amp altogether, this explained why the sound was really low. Removing the short fixed the sound.
Another one destined for eBay.

Backer 32

General Comments Off

Mar 062011

My friend often regaled me with tales of Backer from his misspent youth. The idea of backing up files and information from your PC to a video tape always struck me as too ridiculous to be true but after many years he has gone and found one and purchased it for nothing but nostalgic purposes.
I asked him to send me some pictures over and that he did.

The idea of being able to store 4 gig of data on a single video must have been quite appealing but I’m not sure I would have wanted to place my trust in it.
We really need to try this beast out.

Rainbow Islands repair log #1

PCB Repair Logs, Repair Logs Comments Off

Mar 052011

Got hold of a very poorly Rainbow Islands Extra board.
This board had some repair work done previously. There are a few burned out tracks and some dodgy looking wire links.

On power up it just showed a black screen, time to break out the Fluke 9010a.
The Fluke showed I had active interrupt lines. The 68000 CPU has 3 interrupts, IPL0, IPL1 and IPL2. IPL1 and 2 are tied to +5v (they are active low therefore unused here). IPL2 was stuck low.
I traced the line to a nearby 74ALS74 chip and then back to a 74LS273 (IC32). Regardless what the inputs were doing, the outputs were all stuck low. I replaced this and tested, still a blank screen. I check the 74ALS74 again and although all the inputs were now present and correct, its output to IPL2 was still stuck low. I piggybacked a new IC onto it and the game came to life. Replaced the IC.

Now the game booted up I could see that the colours of the character sprites were wrong, mainly black.

This caused me a lot of headache as the colours came good when I put the logic probe on different pins of different IC’s. I determined which IC was partially responsible for sprite colours by interrupting lines on various logic chips. I found that if I toggled pin 9 of IC35 (74LS157) the colours came good more regular. I removed this chip and tested it in my logic tester, it passed but if I put it on a loop it would intermittently fail. Replaced this and the colours were now back to normal.

There is also a problem when the sprites are facing left where it shows a broken character but still facing right. The pictures below shows the broken up boss sprite and broken rainbows when fired to the left.

This ended up being a short on pin 15 @ IC38 (74LS153). This pin was shorted very slightly to an exposed track running next to it. Removing this short fixed the graphics completely and the game is now 100%.
Very pleased.