Space Invaders PS problem

[DP Louie]

My SI was having problems (randomly resetting) so I ordered a PS rebuild kit from Bob Roberts. Installed it today, checked the voltages per the info on electron forge, everything looked good. Connected the board and the game played for about 5 minutes, then started randomly resetting, followed by nothing but garbage on the screen. I checked the PS voltages again, all fine except for the -5, it is at -10.2 and will not adjust via the pot. Any ideas? Bad pot, or should I look elsewhere?

 

[ELutz]

My SI was having problems (randomly resetting) so I ordered a PS rebuild kit from Bob Roberts. Installed it today, checked the voltages per the info on electron forge, everything looked good. Connected the board and the game played for about 5 minutes, then started randomly resetting, followed by nothing but garbage on the screen. I checked the PS voltages again, all fine except for the -5, it is at -10.2 and will not adjust via the pot. Any ideas? Bad pot, or should I look elsewhere?

Was this -10.2V with the motherboard connected or disconnected? A bad pot would not cause a reading this far off.

Edward

 

[DP Louie]

Both - I checked with the motherboard connected and disconnected, no change in the voltage.

 

[ELutz]

Both - I checked with the motherboard connected and disconnected, no change in the voltage.

I'd look at the LM7905 voltage regulator. This takes voltage in....outputs negative 5VDC.

Edward

 

[DP Louie]

Thanks Edward. Bob has them for $1, so I'll just replace it. I'm assuming that this voltage problem is the root of the resetting and garbage on the screen. I'll update the post when I get the replacement.

 

[ELutz]

Thanks Edward. Bob has them for $1, so I'll just replace it. I'm assuming that this voltage problem is the root of the resetting and garbage on the screen. I'll update the post when I get the replacement.

When you rebuild the power supply, was the old regulator bad? Maybe just toss the old one back in.

Edward

 

[DP Louie]

When you rebuild the power supply, was the old regulator bad? Maybe just toss the old one back in.

Edward

Well, I didn't replace it as it wasn't included in Bob's rebuild kit. As long as I'm going to order, I might replace the 12v regulator as well.

 

[DP Louie]

Actually, I just had a thought... I have several old, working switching power supplies, I'll have to check them to see if I could rob a -5v regulator from one (if they even have them)

 

[orion3311]

Don't think they do - they work a little different. Did Bob's kit just have all caps? Anything else?

 

[DP Louie]

Okay, I was able to scavenge a 7905 from some old Stern ps boards I had... all voltages check out okay now at the ps and at the edge connector, BUT - I'm still having a problem. I have the Braze multikit installed, and when you turn on the game, the screen alternates between garbage and the test screen from the multikit. I can get quick flashes of the message "bank 0 checksum failed" with different hex numbers after. I'm guessing this is indicating a bad RAM, but which one?

 

[ELutz]

Actually, the Braze kit will give a series of beeps/tones to indicate a bad RAM. You might try cleaning the legs of the ROM chips. Be careful as these sometimes get brittle. I would also reseat the big connector where the two boards mate.

Edward

 

[DP Louie]

Actually, the Braze kit will give a series of beeps/tones to indicate a bad RAM. You might try cleaning the legs of the ROM chips. Be careful as these sometimes get brittle. I would also reseat the big connector where the two boards mate.

Edward

Okay, I decided to remove the Braze kit since most of the troubleshooting info is based on just the regular board. So, I removed it, and removed all the ROMs, cleaned and reseated the boards, and now upon powerup, I get the 16 lines on the screen, but they alternate between okay and corrupted looking. There's also random sounds, the sounds stop when I hit the slam switch, but there's no change in the video. The video is also kind of "jerky" at times, which I read somewhere is an indication of a poor conection between the boards... I'm going to try to reflow the solder on the big edge connector between the boards and see if that helps. I wish I had a spare 8080 I could throw in too, but all I have is Z80's.

 

[ELutz]

Weird....one picture looks good.....the other two have the look of bad RAM. I also forgot, the Braze kit eliminates the need for ROMs....so, I doubt they were the issue.

Edward

 

[modessitt]

BTW - resetting issues can sometimes be fixed by bumping up the +5. Get it up to about 5.20...

 

[DP Louie]

BTW - resetting issues can sometimes be fixed by bumping up the +5. Get it up to about 5.20...

If I max out the pot on the PS, I measure it at 5.38 on the ps and only 5.14 at the edge connector on the board. I reflowed all the solder on the big connector between the boards and made sure the boards seated tightly, and with the Braze kit out and the original roms in, the game comes up, but continually resets, sometimes with random graphics on the screen, sometimes it looks normal. You can coin up the game, but it will reset before it starts. I've monitored the voltage on the reset line of the ps, and it varies from .07 to .18 - this should be low enough, correct? Or should it be a steady reading, and not vary? Maybe I need to get the +5 bumped up somehow at the board? Thanks guys

 

[modessitt]

Yeah, I had one that did that (Midway upright) and ended up putting a switcher in just for the +5 and some of the grounds. Bumped up the +5 and it went away....

 

[DP Louie]

Well, I rigged up a switcher to supply the +5 to the board - it measured (adjustment maxed) 5.45 at the switcher, but 5.21 at the board. It seemed to help, the game is a little more "stable", but still resets. I don't understand why I'm getting the drop in voltage at the board - could there be an issue with the board itself making it overly power hungry? I'll see if I can try a different switcher that can maybe supply 5.3 or better at the board, since there's been a few threads recommending 5.3 as the magic number for a SI to boot.

 

[ELutz]

Well, I rigged up a switcher to supply the +5 to the board - it measured (adjustment maxed) 5.45 at the switcher, but 5.21 at the board. It seemed to help, the game is a little more "stable", but still resets. I don't understand why I'm getting the drop in voltage at the board - could there be an issue with the board itself making it overly power hungry? I'll see if I can try a different switcher that can maybe supply 5.3 or better at the board, since there's been a few threads recommending 5.3 as the magic number for a SI to boot.

If you've got 5.45 at the power supply, and 5.21 at the motherboard.....it's not the motherboard sucking it down. If that was the case (and can happen), the voltage would be low at the motherboard AND the power supply. You have voltage drop. It happens from old wiring, shitty connectors, and cold solder joints at headers. Cold solder at headers, and weak connectors is very common on all games....the older the game, the more likely to be a problem.

Edward

 

[chaneman]

On the motherboard pcb to the left of the edge connector (if it's facing you) there are three 10uf 16v or 25v capacitors. Replace them with 470uf 16v caps observing polarity. These are on the supply rails for +5 +12 -5. Doing so will take a little bit of the load off of the power supply. They are used for "stiffening" not filtering and the originals are grossly under rated for the job they do. The one to the right side of the connector should be replaced at this time as well if it hasn't been yet. Replace it with the same value it is: 10uf 16v. This may brighten up the video output a bit if it was failing. And yes there will be some loss in the wire loom from the power supply to the game pcb, so be sure the pins in the connectors are clean and make good contact or they will need to be replaced. There might be a failing ic on your pcb but doing the above mentioned replacements might just rule that out.

 

[DP Louie]

If you've got 5.45 at the power supply, and 5.21 at the motherboard.....it's not the motherboard sucking it down. If that was the case (and can happen), the voltage would be low at the motherboard AND the power supply. You have voltage drop. It happens from old wiring, shitty connectors, and cold solder joints at headers. Cold solder at headers, and weak connectors is very common on all games....the older the game, the more likely to be a problem.

Edward

Thanks Edward for all your help. The thing I don't understand is that I can measure the the voltages at the edge connector with the pcb disconnected and they match the readings taken directly at the power supply. If I then connect the edge connector, and measure the voltages again from the same point on the edge connector that I took the previous readings, they are ALL (+5, -5, +12) down about .20v. The edge connector is clean and making good contact with the pcb. Could this be a ground issue?