arcade board general diagnosis questions

[doeby]

I have an arcade board, very unique. I can't find the schematics anywhere for it. I cleaned the board, and reseated all the ROM chips, i have everything working but a few sprites. Unfortunately they are needed to play the game right. So everything else on the board works fine.

1. I have it hooked up to a computer power supply that tested 5.05 volts no load. As soon as i hook it up to the board, it drops to 4.75.
should i assume i have a short on the board, and if so, is there anyway to narrow down where on the board, with just a volt meter. i put board in a known working cabinet, and the voltages dropped there also.

2. I tested some ROMS on the very far end of the board, and i am only getting 4.1 volts. I would assume that is not enough for these ROM chips to work. ( read that) If i take out the ROM from sockets, it doesn't change anything.Game still works, and still missing same sprites. So i am assuming that these bank of 4 ROMS are not on at all. because removing them didn't change anything. (and they are probably what is supposed to run my bad sprites.)

3. could i try an easy fix by running a wire from hookup(4.71 volts) to other side of board, and try to boost power like that.

4. if i would try to see if any chip gets hot, like it is shorted out. How long should i have the board running before expecting a shorted chip to get hot.

Thank you for any help
donald

 

[Pommie]

Have you checked the pinball database site as they have some arcade schematics.

Mike.

 

[rjenkinsgb]

Please post some photos of the board? Good detailed ones of both sides?

The PSU should be adjusted to give around 5.1 - 5.2V at the point it connects to the board. Many need a 12V supply as well as the 5V.

If it's an early one it may also need a -5V supply & not having that may possibly cause excess current.

Quite a few game boards use bipolar RAM for the sprite system & those ICs are known to fail at times.

 

[Diver300]

The voltage dropping to 4.75 V might be OK, it is difficult to know. What is the current rating of the 5 V supply?

If there is a fault that is dragging down the power supply, something will be getting hot. You should try to feel what is getting hot as soon as you turn on.

 

[doeby]

I killed it. I took some wire and added 5 volts and a ground to the far end of board. I metered and found a 5 volt spot and ground spot. When i booted it up, Got a garbaged screen, then i unhooked the added wires, and screen was still garbage. so i must have blown something. I did find it strange when i was testing my added wires 5v to 5v got solid tone, good connection,then made sure i didn't short out my soldering, so i tested 5v to ground on my added wire, and i would get one second of beep for connection, and then it would stop. Did this both ways, ground to 5v. and 5v to ground. i assume i should not have had this.

 

[Diver300]

A lot of multimeters will give a solid tone for a short and a short beep for a small voltage drop, such as a diode.

On large board, there can be quite a low impedance between ground and 5 V, so you may get a short beep if you test between them. On many board, if you test the continuity between ground and 5 V the wrong way round, you will get a short beep with a multimeter.

 

[Doomguy42]

Have you had out it in sunlight when you have been working on it at all? UV light will corrupt your Eproms.
I work on fruit machine boards which are very similar to arcade boards. You have video but I have 44-55v rails
I would always start out by by burning a fresh set of Roms. On some Eprom programmers you can check the basic function of alot of ICs like SRAM and 74logic chips. I use the TL366ii it's great. If your board is from the earlier 80's I would check to see if your Eproms are supported.

If there has been work on it before I would test everything on sockets.
Reseating is very basic...old pcbs can really suffer from dirt. Have a look in the sockets and make sure the contacts are clean. Make sure the notches are aligned and ICs go back in the correct way. Finger test to see if anything is getting hot.
I would meticulously clean the board too. Clean it with what you like but always give it a wash in IPA which is extremely flammable so be careful.
I submerge boards in ipa in a plastic container in the garden to finish. The alcohol should displace any moisture. Check for any blue green colour corrosion. With old pcbs from the 80's if the solder mask (green colour) deteriorates from old grease and corrosion it can be slightly conductive which you can check with you meter on the resistance test.
I don't think you would have killed your board with testing the 5v personally. 12v going around would have damaged it.