question about rebuilding midway medium power supply.

[mrbill2084]

question about rebuilding midway medium power supply.

I was messing with a medium PS last night in burgertime. It was 1.2v on the +5v line. As for the voltages, I metered them off the board.

I swapped it with another medium PS and it was +5.58v. The lowest I could turn it down was to +5.15v. Pcb comes up and works, but thats too high..

dug out my bump'n jump put its medium PS in and it adjusts fine.

I was wondering if a bob roberts rebuild kit will fix 1 or both of these? I only need to fix 1.
http://www.therealbobroberts.net/90421b.jpg

Normally I would put in an arcadeshop PS, but I was thinking of keeping the BT original.

 

[RedlandMan]

Bob Roberts sells the kits they are about 20 bucks

 

[mrbill2084]

Bob Roberts sells the kits they are about 20 bucks

I know this. I included the link to his kit in the post.. My question is will it likely fix the problem in 1 or both..

 

[RedlandMan]

If you knew exactly which parts are bad you might be able to fix both but i wouldnt. If you are going to rebuild one i would put the whole kit in one so everything is new and will last you a long time. I purchased a kit for my super pacman but im begining to think i was better of with
a switcher.

 

[ELutz]

I was messing with a medium PS last night in burgertime. It was 1.2v on the +5v line. As for the voltages, I metered them off the board.

I swapped it with another medium PS and it was +5.58v. The lowest I could turn it down was to +5.15v. Pcb comes up and works, but thats too high..

dug out my bump'n jump put its medium PS in and it adjusts fine.
http://

Bob's kit should fix #1.......and for the record....5.15VDC is totally fine. That won't hurt anything, though, it is strange it won't adjust any lower.

Edward

 

[mrbill2084]

Bob's kit should fix #1.......and for the record....5.15VDC is totally fine. That won't hurt anything, though, it is strange it won't adjust any lower.

Edward

thanks for the info.. I will order a kit a recap them. Maybe I will do the 5.15 pcb a few pieces at a time to try to figure out why its high..


I have a 3rd medium power supply thats spot on with +5, -5, but the +12v is at +18 and won't adjust any lower..

 

[ELutz]

thanks for the info.. I will order a kit a recap them. Maybe I will do the 5.15 pcb a few pieces at a time to try to figure out why its high..


I have a 3rd medium power supply thats spot on with +5, -5, but the +12v is at +18 and won't adjust any lower..

Bob's kit should fix this one too. Honestly, caps are (probably) not the problem....though, it's good maintence to replace them.

This third board.....it's probably Q100....maybe Q104 or U5.

The first board.....it's a toss up...Q101, Q102, U2.

Don't forget to check the headers for cracked solder.
Also, these boards output a 12VDC unregulated line for audio. It's not uncommon for this line to read 14 volts to almost 20 volts DC. This line is not adjustable. Don't confuse it for the regulated (and adjustable) 12VDC line.

Edward

 

[WindDrake]

thanks for the info.. I will order a kit a recap them. Maybe I will do the 5.15 pcb a few pieces at a time to try to figure out why its high..


I have a 3rd medium power supply thats spot on with +5, -5, but the +12v is at +18 and won't adjust any lower..

I'd be checking for a bad pot, High ESR/Failed caps, or a failing driver transistor in your regulation circuit. The regulator's output is set via the Pot as a Voltage Divider, so if it won't crank down, I'd check to make sure your pot will swing from Dead Short to full Resistance.

Easy way to do that is to take the pot off and measure from one end leg (pin 1 or 3) to pin 2 (wiper). You should read near a dead short when it's cranked all the way over to the pin you're measuring to, and near the full value of the pot when it's cranked to the opposite end.

Conversely you can just take it out, spray it with contact cleaner, then work it back and forth to break up any oxides inside, and retest when it's dry. But that doesn't always work.

 

[RetroHacker]

Look at the schematic - the 12v and 5v regulator curcuits are pretty separate. If you're having a problem with the 12v circuit, then it's probably going to be the LM317T that regulates that. Likewise, the 5V circuit has a 2N3005 transistor. They each have their own rectifier diodes, and you'll notice that the ones for the 5V circuit often get cooked. Another common problem is the center tap pin from the power transformer burning up.

-Ian

 

[ArcRevival]

I would change the pots first but change them for sure no matter what route you take.

 

[mrbill2084]

thanks for all the ideas.. I will put an order in with bob in the next day or 2 and get busy on them next week..

 

[RetroHacker]

I would change the pots first but change them for sure no matter what route you take.

I've never actually found a bad pot on one of these power supplies. I always seem to find bad diodes and caps, occasionally transistors, but not the pots themselves. There's very little voltage on them in circuit, they don't seem to deteriorate. And, they're much better quality than, say, the pots on a G07 chassis.

-Ian

 

[joeycuda]

I used the Bob kit on my Gorf p/s board and I would definitely do it again over using a switcher. On these Midway games, it's a real good idea to repin the harness side and replace the Molex headers on the p/s side.

Here are my before and after pics:

 

[Ghostnuke]

I need to do this to my Galaga. Did those new header pins come with the kit?

 

[ArcRevival]

I've never actually found a bad pot on one of these power supplies. I always seem to find bad diodes and caps, occasionally transistors, but not the pots themselves. There's very little voltage on them in circuit, they don't seem to deteriorate. And, they're much better quality than, say, the pots on a G07 chassis.

-Ian

I have seen bad ones. I had a Tron that i fixed up and sold, went to check the +5 and it was fluctuating all over the place. I had my meter clipped on and was just watching it. I changed out the pot and it was totally stable. I had another one that would not adjust down. The pot fixed it. Bob puts those in the kit for a reason.

 

[omegaman]

Like winddrake mentioned, check the small blue ceramic caps, they go short circuit often.

 

[mrbill2084]

just a little update.. I assume q100 is closest to U1

PS#1- +5.15v lowest +5 could adjust to before. I put the full bob roberts kit in and now the lowest it can go is 5.85! (good thing I had a bad pcb in the cab) Any ideas? I guess I should check the small blue ceramic caps?

PS#2 - +1.5 was the highest +5 could go. replaced q101 and +5v adjust fine now..

PS#3- +18v want the lowest the +12v would go. I replaced q104, u5 vr102 and q104(because it looked heated up on pcb beneath it) and no change. Changed q100 and +12v works fine.

BUT when adjusting the voltage on PS #3, my hand slipped when metering the -5v and I killed the -5v line. I was metering off the the power supply pin #2. I either touched pin#1((audio circuit) or pin #3(+12v to coin contols lights). I could smell something burnt, but I can't find anyting burnt.. The +5v and the +12v is fine.. Any ideas what I might have killed? (cab is fine, so problem is in the linear pcb).

 

[ELutz]

just a little update.. I assume q100 is closest to U1

PS#1- +5.15v lowest +5 could adjust to before. I put the full bob roberts kit in and now the lowest it can go is 5.85! (good thing I had a bad pcb in the cab) Any ideas? I guess I should check the small blue ceramic caps?

PS#2 - +1.5 was the highest +5 could go. replaced q101 and +5v adjust fine now..

PS#3- +18v want the lowest the +12v would go. I replaced q104, u5 vr102 and q104(because it looked heated up on pcb beneath it) and no change. Changed q100 and +12v works fine.

BUT when adjusting the voltage on PS #3, my hand slipped when metering the -5v and I killed the -5v line. I was metering off the the power supply pin #2. I either touched pin#1((audio circuit) or pin #3(+12v to coin contols lights). I could smell something burnt, but I can't find anyting burnt.. The +5v and the +12v is fine.. Any ideas what I might have killed? (cab is fine, so problem is in the linear pcb).

PS#1 is probably a broken trace somewhere. The likeliest suspect is probably one of the transistors on the heat sink. Some of those have traces that go to the bottom side, up through the thru-hole, and then continue on via the top side. It's real easy to break that thru-hole plating....cousing the connection to be broken from top to bottom.

PS#3 is more than likely U1....the negative 5 volt regulator....the LM7905.

Edward

 

[mrbill2084]

PS#1 is probably a broken trace somewhere. The likeliest suspect is probably one of the transistors on the heat sink. Some of those have traces that go to the bottom side, up through the thru-hole, and then continue on via the top side. It's real easy to break that thru-hole plating....cousing the connection to be broken from top to bottom.

PS#3 is more than likely U1....the negative 5 volt regulator....the LM7905.

Edward

thanks ED! I will try that..

update.
PS#3 I tried changing u1 and no change..

 

[ELutz]

thanks ED! I will try that..

update.
PS#3 I tried changing u1 and no change..

Damn, sorry Bill. There's not a lot to the -5VDC circuit. AC voltage enters and goes through diodes D109 and D108. These trun AC voltage to DC voltage and go across capacitor C115. You should have 8-12VDC (give or take) across this cap. Besides the negative 5 volt regulator (LM7905)...there's only a couple resistors (R100 and R101)....and a couple more capacitors (C100 and C101). Maybe one of those caps shorted, or you took out one of those rectifier diodes. Check the diodes and caps for shorts.

Edward