PCB Voltages Very Low - Please Help!

[Grish]

Hi All,

I'm a compete newbie, but am learning as I go, I've just aquired my first arcade PCB along with a bunch of other useful stuff (erprom programmer, eproms, multimeter, jamma harness and cga2vga on it's way).

I just plugged an ATX power supply into an arcade pcb for voltage testing purposes and found that whilst the PSU gives a steady +5vdc the board is only running on about +4vdc...!!

It's not an issue with the Jamma harness or the Sega to Jamma adapter (I've tested these in isolation) - I've also tested with two top range PC power supplies (Modular Corsair and a Seasonic) same issue.

My first suspicion is the edge connector, it looks pretty worn.. is there anything else I can look for? Or anything I can do about a crappy worn edge connector?

By the way, I don't have a screen hooked up, waiting for cga2vga adapter, so I have no idea if the game is even booting or not

[Asure]

1. Show pics of your Jamma harness. Cheap chinese? -> Crappy awg wires cause voltage drop
2. Which pcb did you say you were testing this with again?
3. Where did you measure the +5v? Inside the psu? on a plug? On the jamma edge?

Details schmeetails you might say. but in this case we are talking about electronics which requires precise attention & care.

[Grish]

No problem:

Jamma harness is chinese, the power wires are significantly thicker and have 20AWG printed on them.

I'm now starting to think the problem might be the PSU dropping when it gets load (only a single red wire hooked into the molex plug...)

I have no idea how much current this board takes, it's a Wonder Boy in Monsterland bootleg:

WBML BL:

Before plugging into PCB:

Plugged into PCB:

[Asure]

Cut the red wire and you will see: There is almost no copper inside, just 3-4mm insulation and perhaps 0.5mm copper. They are not 20AWG.. Same for the yellow wire.
These chinese crap things are all over the place.. Copper is the most expensive thing in that harness, and they'll shave off whatever possible.

To fix the problem you need to add better wires for the +5V pads. (A medium-to-large size pcb like that wbml you have there will never run with such low voltages.)

[Grish]

Thanks Asure, you were right, it is just a scraping of copper. I ripped it out and soldered 4 red wires from the ATX directly to the Jamma.

Cranked it up and to my frustration only a slight improvement, I'm now getting around 4.5-4.6 volts at the Jamma end, and between 4.4-4.5 on the board components.

Should I keep going and bundle up even more red wires from the ATX?

Also, I didn't swap out the black ground wire, does or could this matter?

[Asure]

There's four of those black mini wires going to the grounds, so it should be a bit better than the +5V, but still a bit sparse..
I would try to add another big awg wire for +5V and if that does not improve, add at least two fatter awg ground wire as well.

Could also be the ATX power supply is just crap under load?

[zumbito]

How did this end up? I have the xact same problem. 5v with no pcb, connecting the PCB goes down to 4... I assume its not the cables as they are able to deliver 5v when the pcb is not plugged...

[grantspain]

this is why switch mode psu with +5 adjust were made,install the correct item for the task

[zumbito]

Well, I'd love to, but this is not my permanent residence and I had to go with what I have. In my case it is a CPS1, not sure why it drops.

[skate323k137]

You should always measure your voltages under load. Measurements without one are useless.

[zumbito]

I see. But all ATX output 5v, do they all fall to 4v when plugged?

[skate323k137]

I see. But all ATX output 5v, do they all fall to 4v when plugged?

Like I said it's a matter of measuring under load; the voltage should measure around 5v at the device being powered when the PSU is under load. Some PSU's will measure higher or lower without load; I have one that will show 4v with no load but outputs a proper 5v once it has a board hooked up. It can also affect other outputs like the 12v

You may have a faulty PSU or it may not be capable of enough amps on the 5v rail. Adjustable PSUs like the happ power pro are very cheap and allow you to dial in the 5v with a board hooked up. I generally run 5.1 (never over 5.19) as measured at the JAMMA edge so that chips are getting ~5v on the board itself. Depending on the length of the wiring harness you may measure something like 5.3 or 5.4 at the PSU itself to achieve proper power to your game.

[zumbito]

Then I'm screwed, cause I don't want another PSU, I wonder how is everybody using ATZ psu, and they don't have this problem.

[grantspain]

its all down to the thickness of the +5 and ground cable,the power draw of the game board and the power of the power supply
stick a atx on a system 246 and see it drop over 1 volt due to the power required by the game board

copy board are notorious for this sort of issue btw

when i first started working on vids we needed to double up +5 and ground cables for some games

[zumbito]

But if I measure 5v in the harness I can rule out the cables, can't i?

The PSU specs are 130w, 5V21A, 12V 10A.

[grantspain]

most psu are 16a +5v

[zumbito]

most psu are 16a +5v

This one is 21A +5V should be more than enough to power the board. Are CPS1 that demanding?

[invzim]

Sorry if I'm being captain obvious, but there is a logic to all this, and the theory is not terribly complicated - but often overlooked.
At the base of it all is Ohm's law, V=RI, where V=Volt, I=Current(or load, in amps) and R=Resistance, or

Vdrop = IR
where:
I : the current through the object, measured in amperes
R : the resistance of the wires, measured in ohms

So the higher the load, the more voltage drop you will get. Also, the higher the resistance of the cable, the more voltage drop you will get.

With 0 load, you will get 0 voltage drop - which is basically what happens when you measure without a PCB.

The danger is is having crappy harnesses with thin wires(high resistance). If you you have a high load PCB, tune it to 5V and then later swap the PCB for a low current, the voltage will be too high and you may fry it.

With a high quality harness, you will generally not have to muck about with adjusting the voltage as the resistance is low enough so it doesn't matter..

[Asayuki]

But if I measure 5v in the harness I can rule out the cables, can't i?

Grantspain is telling you that the wires in your harness are behaving more like resistors rather than cables. And he is most likely right. It is nothing to do with your PSU. Jamma has 4 pins for +5V power supply wires, for a reason, and your photo shows that you have just one lousy chinese wire going there, probably all rubber and no copper... Why don't you just try following his suggestion and replace that with 4 proper wires like real AWG18?

invzim has also just given you a technical explanation of this.

[invzim]

I've updated the wiki with a voltage drop calculator:
http://wiki.arcadeotaku.com/w/Wire_Gauge

As I'm working on something that will power jamma PCB's in a JVS cab - anyone have any measurements on how much current different PCBs draw? I'd measure it myself, but currently working abroad and don't have access to either games or my bench.