Satan Of Saturn repair log

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Jun 282017
 

Got this game cheap for my collection because the seller stated that it has very noisy sound

When powered, the game had a constant white noise while music and the other sound effects were working correctly.

The sound problem was not due to the amplifier or bad caps but it was produced by the logic of the game.

To make a long story short, this game has a similar circuit to what Space Invaders use to produce analog sounds.

It uses a SN76477, which is a programmable sound chip which has a white noise function.

On Space Invaders and in this game, the chip continuosly output the white noise which is switched OFF and ON using operational amplifiers and it is modulated with capacitors and resistors to produce different sounds.

I downloaded the schematics from the US version ( called Zarzon ) where it is shown how the chip is connected to the LM3900 OP AMPS

 

 

Instead of trying to understand which of the two chips are broken, I piggybacked a good one on both chips until

the white noise disappeared.

Problem was fixed by changing the LM3900 Nr.47 on the schematics

Burnin’ Rubber repair log

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Jun 282017
 

I bought this game for my collection some months ago.

 

 

I tested briefly and it was OK. After a few weeks I decided to make a good play but the game developed a strange problem:

 

 

 

 

As you can see from the pics above, the screen was very dark and the brightness was not steady.

Also all the objects left trails of different brightness.

The problem was somewhat similar to when you don’t have video GND connected but as you can see

the text HI SCORE was repeated on top and bottom.

Adjusting the horizontal size of the image with the monitor pots ( vertical on the game) I noticed the screen didn’t shrink, it was like it had no borders.

I had immediately remembered that on the CPS1 hardware , on certain screens of some games ( 3 wonders selection screen for example)

the image is very dark and this is due to a flaw in the video circuit of the CPS1 hardware which doesn’t blank properly the video at the edges

leading to some monitor electronics to loose the black reference and behave strangely.

Charles McDonald on his website  https://www.techno-junk.org/ has a fix but right now the link is missing because he is restructuring the page.

 

Anyway back to the topic, I was convinced that my problem was due to video blanking not working properly, so I downloaded the Bump and Jump

schematics available from Bally (  the japanese ones are impossible to find ) and searched for a video blank signal and tested all the TTLs involved until I found this:

 

Output pin 5 of 74ls74@1B was floating and didn’t clear the 74LS273 connected to the video DAC

 

Therefore the video was not blanked properly on the edges.

Changing the TTL@1B  fixed the problem

 

Unfortunately after a couple of plays,  the pcb developed two other problems:

  1. Car didn’t go left
  2. car crash sound was broken

 

The first problem was fixed by replacing the 74LS367@4E on the sound board ( directly connected to the left direction pin)

Second problem was fixed by replacing one AY8910.

The game, for the moment, is working nicely 😉

Data I/O 29A & Unipak2 repair log

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Jun 072017
 

I bought this setup from a good friend quite a few months ago now. I knew it needed some attention when I bought it.
On power up I got this most of the time
https://youtu.be/tzGoiafa2Wk

No response from any inputs from the keypad.
The keyboard generates an interrupt for the 6802 CPU. Using the scope I could see the /IRQ signal was being asserted.
I checked all of the ROM’s and found no issue and also checked to surrounding logic and found no issue.
Looking around the main PCB I found a couple of problem areas.

As you can see, both of these chips had some corrosion. I replaced them but they made no difference to the problem I had so after a while I came to the conclusion the CPU must be bad.
Searching eBay turned up nothing UK based so I fired off an email to my friend, Purity to see if he had a spare I could buy. He had one and said I could have it.

And now I get this
https://youtu.be/BgGFMV-0hL4

Problem solved.
Next issue was with the Unipak 2 itself.
I could successfully select and read chips but the data being read back was a little wrong.
Reading a few carefully selected addresses of an EPROM I found that bits 2, 3, 4 & 5 were stuck high.
Looking at the schematics I quickly came to a potential problem area.

You can see from the schematic above that the LM339 comparator is responsible for those exact bits.
I removed the chip and tested it out of circuit. The chip failed and I ordered some new ones.

With a new one fitted everything was back to working status.

I’m really happy to finally have this in my collection and working.
Massive thank you to Purity for his generosity. He has been very kind to me recently and also a great help. I hope one day to be able to return the favor.

 Posted by at 5:17 pm

Defender repair log #1

 General, PCB Repair Logs  1 Response »
May 252017
 

I got for my collection an untested boardset of Defender which was really in great conditions with no battery acid leak.

The boardset is the newer revision of Defender with the red label roms.

The game is very difficult to find it in working conditions nowadays because it has several weak points, expecially the 24 drams 4116 which are very unreliable to due the fact the run very hot and they require +5V, -5V and +12V to be applied at the same time, otherwise they will be damaged.

The old Williams PSU become defective and often they ruin the drams.

After converting it to Jamma and triple checked all the power supply lines I booted it up but the game appeared to be dead.

Fortunately Defender has a lot of bibliography and a very good manual which is also a troubleshooting guide.

I check the clock and it was working correctly, but after checking the reset line I saw it was pulled low all the time.

To make it short, Defender has two +12V power supplies, one regulated and one not.

You have to supply +12V also to the not regulated one because it is needed for the power on reset.

After adding that +12V t, the game booted but as soon as the message ALL UNIT OK was displayed it reset in a never ending loop.

I found out that the problem was caused by the ribbon cable , after reseating it a couple of times, the game booted correctly.

I started the game but the ship kept always going down.

With the test menu , it reported that the down direction was always pushed.

I checked the interface board which has the circuit for the inputs. the hex inverters 4049 were all toggling correctly, the pull up resistors were good therefore there was only one chip to check: the Peripheral Interface adapter 6821P.

I desoldered it and installed another one I had on stock taken from another arcade games.

Since I had no way to test it, I decided to put a 40 pin socket just to be sure I could easily swap it with another one in case it was defective.

Luckily the inputs were now correct, thefore I could declare the board fixed.

In the end I was very lucky because the game had only the input problem and no other issues.

 

Rastan repair log #5

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May 252017
 

The game had some missing background tiles

 

 

At first I thought it was a maskrom problem but once I checked all of them only to find they were good, I started to probe the ram responsible for the background.

A sram @IC31 had some weak data signals so I decided to change it and the guess was right because the game was fixed.

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