Ms Pac problems

DP Louie

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Last week I picked up a Ms Pac from bigdaddynsir, and I think the bumpy ride home may have done some damage - it was playing blind before I got it home, but when I got it unloaded, the pcb had worked it's way out of the plastic holders that it slides into, and was hanging by the edge connector. I checked everything out, reseated the connector and mounted the board correctly, plugged it in and all I get is a loud feedback noise from the speaker, and it blows both 7A fuses. The sound is continuous even after the fuses blow. Any ideas on where I should start?
 
Shouldnt be any 7A fuses in it. You should have two 1A and two 5A in the 4 position fuse block.
Did you unplug the edge connector at any point in time? If so you may have plugged it in backwards. If you plug in backwards it will blow at least two of the four fuses.
 
Shouldnt be any 7A fuses in it. You should have two 1A and two 5A in the 4 position fuse block.
Did you unplug the edge connector at any point in time? If so you may have plugged it in backwards. If you plug in backwards it will blow at least two of the four fuses.

Thanks, Riptor. You're right, there are two 5A fuses, I was thinking of the voltage (7v). I never unplugged the edge connector, and I did a search on the proper orientation, and it's plugged in the right way, with the power wires (red,orange) toward the rear of the cab. It blows both 5A fuses, although not immediately, it takes a few seconds. As soon as you turn it on, you get this loud, deep sound out of the speaker. Sounds like someone is trying to play a bass note on a tuba. It does this until you shut it off. It does it even after the 5A fuses blow. I'm thinking it may be an issue with the audio amp portion of the board?
 
Flip the board over and look for shorted pins(bent over to another trace) or scratched/smeared traces. The positve leads of the small caps on the edge of the board are really close to the ground rail and can get shorted pretty easily. Check those out.

Most of the sound section is at the bottom left of the board, check out that area. The amp is top right.
 
Okay, I'll pull the board and check it out. I checked the power output at the lugs of the MT-80 transformer, and the 12 volt output is fine (12.5 on both), but the 7 volt outputs measure too low (2.7 on the green, .7 on the red and .7 on the orange). Could that be the problem? Don't know why the fuses would blow with low voltage, though. Multiple problems I suppose? So where can I get a replacement for the MT-80?
 
I highly doubt the transformer is bad, your probably measuring at the wrong points.
I will see if I can get a pic to show you where to test at.
 
Here's a pic of the transformer, and the lugs I tested. I grounded my meter on the ground lug, and the other lead was used to read the voltages on the other lugs. These readings were taken directly at the lugs, not at the fuses. I just got home from work, I'll pull the pcb and check it out later tonight...
 

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Set your meter to AC. Negative lead on the red/black lug and positive on either one to the right and left of it. Do the same thing on the other side, negative on the yellow/green lug.
You cant use the same neutral lug for both sides, you will get incorrect readings.
The lug you have marked as ground is correct except I guess its technically a neutral wire since this is AC. The red w/ black stripe lug on the other side is the neutral for the other two.

While your working on the machine, replace those crappy brass fuse holders. If you dont have any, the radio shack ones are decent quality. These fuse holders cause alot of problems in the pacs.
 
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Thanks Riptor - you were right, I'm a dumbass. But I'm learning... The transformer is putting out 12.5 and 7.5 volts. I'll get the pcb pulled and check it now. Thanks again for your help and patience.
 
.....I never said such a thing.

Doh! Sorry about that. What I meant was you were right about how unlikely it was that the transformer was bad. The "I'm a dumbass" was directed from me at myself, a bad attempt at self-depreciating humor. Seriously, though, I do appreciate all of your help, and I appologize if you took that the wrong way.
 
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