Coin lights flashing + sound pulsing?

[bkenobi]

One of my MAME cabs has an issue all of a sudden.  The coin lights flash and the speakers pulse (kinda thump) when I turn off the machine.  The volume of the beat and the brightness of the lights dim over a couple minutes and eventually stop.

I initially thought this was a dying relay that I use to turn the thing on.  I use the computer to control the power on for the rest of the machine kinda like a home made Smart Strip (there's a tutorial around here that I used to build it).  I've used this same setup on my other cab and haven't had any issues.

Anyway, I replaced the relay and the issue is a little better, but not fixed (the lights still blink slightly).  I'm wondering if this could be a sign of a dying PSU?  I assume the pulsing is caused by the power being dissipated by the closing of the relay.  Some power builds up, the relay opens, the power dissipates, the relay closes, repeat...  If this continues, it will burn something up (probably the relay again).

I don't happen to have an extra PSU sitting around right now, so I can't just test it like I normally would.  Any thoughts?

[lilshawn]

I'm wondering if this could be a sign of a dying PSU?

my first thought.

but as usual i have no idea what you have in your cabinet, or how you have anything hooked up.

if your running a standard arcade power supply:
 

sometimes if they don't have enough load on the 5V line they can get uppity, especially when they get old and crotchety. adding a 5 or 6 ohm 5 or 10 watt resistor to the 5V line and ground terminals often fixes the issues they have... of course you could always run a bunch of stuff off the supply to load it down properly.

[bkenobi]

Sorry, you are correct that I was not very clear how I have things set up.  I'm running a PC with a standard power supply and a 20" CRT PC monitor as the display.  As for the relay setup, I used something similar to BobA (can't find the link I used, but this is pretty much identical).

http://forum.arcadecontrols.com/index.php?topic=69269.0

The PC power supply drives the relay with the 12v rail.  When the PSU is on, it opens the relay and turns everything else on.  When it's off, the relay closes and nothing should have power.  I'm thinking that the PSU may be generating a small amount of power for some reason.  Maybe I have a PSU somewhere I can test with...

[lilshawn]

you probably need a diode on that relay. 1N4004 or 05 or 06. stripe end of the diode on your + and the other on your -

what i think is happening is a reverse voltage EMP  or inductive kickback or back EMF... it's called all sorts of things..

when you cut power to a coil of wire such as a relay or solenoid, as the magnetic field collapses, it generates a reverse voltage (it can easily reach 200 volts peak). because it's attached to the power supply directly, it dumps into the capacitors of the power supply. charges them up a bit... and they dump back into the relay, clicking it back on for a second (repeats until the field is dissipated.)

the problem with this is the voltage regulators and MOSFETs in the power supply REALLY REALLY REALLY don't like this since they are being driven backwards. eventually they begin to fail.

the diode acts like a resistor when you dump voltage into it backwards and acts like it's not there frontwards. so when the EMP is generated by the coil the diode absorbs it. all that passes through the diode is about .5 volts, low enough for the power supply to dissipate.

if you do indeed have a diode on there, then it's likely blown and need to be replaced.

[bkenobi]

I don't have a diode, but that was one thing I was considering.  Of course, I'm no EE, so I didn't know if that would be effective or a good idea.  Where do I put the diode though?  In BobA's design, he puts it across the 12v to ground legs of the relay.  That seems reasonable since that's the power that needs dissipating.

I just checked my parts box.  I currently have 1N4001 and 1N4148T diodes.  Will either of those work in lieu of the 1N4004?  I can stop by Radio Shack tomorrow if that would be better.

[lilshawn]

Where do I put the diode though?  In BobA's design, he puts it across the 12v to ground legs of the relay.

stripe end of the diode on your + and the other on your -

what he said^^^^

I currently have 1N4001 and 1N4148T diodes.

the 4148 is the better of the 2 you have... and it's better than nothing. the 4004/5/6 can suppress 400/500/600 volts reverse voltage... the 4001 will only do 50 volts and your 4148 will do 100 volts.

your 4148 will probably do okay, but more is better in this case.

[bkenobi]

Ok, so I'll stop by RadioShack tomorrow and pick up a pack of 1N4004/5/6 diodes.  I only want to fix this once and hopefully avoid replacing a power supply.  If this fixes things, I'll have to do the same to the other cab to avoid it running into issues later on as well.

Thanks!

[lilshawn]

be sure to update us for prosperity's sake. it will help future searchers find solutions!!!

[grantspain]

i had a similar issue and to fix it i put led bulbs in

[bkenobi]

Any thoughts to why the LED would resolve this issue?  Perhaps the bulbs you replaced were going out?  Or, maybe the power supply was weak on current?  I'm going to try the diode, but your suggestion is interesting.

[grantspain]

Any thoughts to why the LED would resolve this issue?  Perhaps the bulbs you replaced were going out?  Or, maybe the power supply was weak on current?  I'm going to try the diode, but your suggestion is interesting.

no everything was good,you get the same problems with global vr and raw thrills dedicated cabs if you put normal wedge base bulbs in

[bkenobi]

If that's true, then I should be able to remove the bulbs from the equation and the speakers should stop beating.  I might try that just for giggles.

[bkenobi]

You are correct!  Well then, maybe that would explain why it works...

[grantspain]

when i originally had this problem i added a diode and it made no difference

[lilshawn]

depending on the power supply the 12 volt rail on them isn't much... some of the happ style supplies, they only supply about 2 or 3 amps sometimes even less on the 12 volt rail. it's really easy to overload it when you consider each lightbulb can draw about 200ma (0.2 amps) each. it would only take 10 bulbs to max out the PS.

2 coin door lamps can draw about 1/4 the total current supplied in a happ power supply... and an amplifier for the speakers??? that could draw anything. throw in a harddrive, motherboard, and a cdrom drive... you could be anywhere with your load.

changing to LED's drops your load significantly. not that i'm suggesting that's the answer, but a 12volt coindoor replacement bulb only draws about. 0.03ma... about 1/10th the current.

[bkenobi]

Well, I installed the diode (1N4005) and it seems to have made little if any difference.  I'll try unplugging the lights to see what's up.  I tested my other cabinet (same setup essentially) and it works fine.  I suspect the PC power supply is dead/dying.  I noticed this evening that when the lights blink, the cpu fan twitches.  This is bad, so I'll have to deal with it ASAP!

[bkenobi]

I replaced the power supply and the problem seems to be resolved.  I'm going to leave the diode installed since it may help prevent another failed PSU.  I'll install one in my other cab too just to make sure I don't kill that one too!