Installing switching power supply in Midway Space invaders

XScArY1X

Member
Joined
Apr 16, 2011
Messages
140
Reaction score
0
Location
Eaton County, Michigan
Installing switching power supply in Midway Space invaders

Alright so i have a dedicated Midway space invaders machine and last night the +12 on the power board went bad. I want to install a switching power supply but im not exactly sure which wires do what that go into the original power board. does anyone have any experience with this?
 
If its easier to just repair my midway power board does anyone know how to fix the +12 it is only giving off .25 on my voltmeter if its an easy fix with a capacitor replacement let me know.
 
I've hooked up a switcher to test SI boards on the bench. I just followed the wiring diagram.

In your case, I'd personally fix the linear PS. There's not much to the 12V supply circuit. Assuming the AC input is good, there's a pair of diodes (check them for shorts), a 3-pin regulator IC (looks like a transistor, but will be labeled 7812 or LM-something), a few resistors and a couple caps. A good schematic is in the SI Deluxe manual, available online a variety of places.
 
An older PC supply without 3.3v will work fine. The only catch is the reset line. Its opposite what the SI board wants to see. You can invert it with a 7404 section and then it will be active low.
 
Im having a bit of trouble locating the specific kit on his site maybe im missing it somewhere?

If you cant find one there I grabbed a kit from alex at elektron forge,great peice of kit and came with comprehensive instructions(with photos) that even somebody with no-experience could follow easily http://www.elektronforge.com/products.htm
 
http://www.therealbobroberts.net/parts.html#pacstuff

About 3/4 the way down the page.....Midway OEM PS (90400) Cap Kit

This is only the capacitors......you've probably got a bad 7812 regulator.

Edward

replaced the 7812 regulator today didnt fix my problem still only getting .30 on the +12

I checked the input voltage on the 7812 regulator and thats about +20 but the output is only +.30 so maybe thatll help eliminate things that could be wrong.
 
replaced the 7812 regulator today didnt fix my problem still only getting .30 on the +12

I checked the input voltage on the 7812 regulator and thats about +20 but the output is only +.30 so maybe thatll help eliminate things that could be wrong.

Something is shorted on the +12 power side. Look at every part the output of that regulator goes to. It will probably be the audio amp or filter cap.
 
Something is shorted on the +12 power side. Look at every part the output of that regulator goes to. It will probably be the audio amp or filter cap.

alright so i traced the path of the output and it went to two things one was one of those brown discs the other is the capacitor I circled in the picture below it has some browning around it on the board and the color bands are different on this one compared to the other ones its between the -5 and +12 adjusters what do you guys think?

also if thats the one thats bad what is it so i can just pic one up at my local radio shack.
 
Last edited:
Thats a 270 ohm 5% ½ watt resistor.


To test it pull one leg from the board and then use a DMM set to 2k ohm and meassure the resistence, 5% of 270 ohm is 13.5 so if the resistor reads anywhere from 256.5 ohm to 288.5 ohm the resistor is good, anything below 256.5 ohm or above 288.5 ohm the resistor has failed and its time to replace.


The "Brown Disc" is a 0.1uF 50 volt Ceramin Disc capacitor.
 
Last edited:
Thats a 270 ohm 5% ½ watt resistor.


To test it pull one leg from the board and then use a DMM set to 2k ohm and meassure the resistence, 5% of 270 ohm is 13.5 so if the resistor reads anywhere from 256.5 ohm to 288.5 ohm the resistor is good, anything below 256.5 ohm or above 288.5 ohm the resistor has failed and its time to replace.


The "Brown Disc" is a 0.1uF 50 volt Ceramin Disc capacitor.

alright the resistor gave me a reading of 321 so im guessing this one is bad
 
Did you pull one leg of the resistor from the board, if you didnt it would probably read much higher since its in circuit and you would be getting a reading from the resistor as well as the other parts along that trace. If you did pull a leg then yes that resistor is deffinately bad and would account for why you have such a low reading on the 12v line.


After replacing the resistor you can try putting your old regulator back in and see if it works then keep it or the new one as a spare for future use.
 
Last edited:
Did you pull one leg of the resistor from the board, if you didnt it would probably read much higher since its in circuit and you would be getting a reading from the resistor as well as the other parts along that trace. If you did pull a leg then yes that resistor is deffinately bad and would account for why you have such a low reading on the 12v line.


After replacing the resistor you can try putting your old regulator back in and see if it works then keep it or the new one as a spare for future use.

yeah i did pull one leg off and i replaced that resistor but still the same results except now my +12 is giving me .40 :/

anyone know what else it could be?
 
Check the 12 v pot

alright im getting nothing at the +12 pot and i traced it all the way back to the +12 wire and i got nothing coming in. I traced the thick yellow wire all the way back to the transformer. i am 80% sure that all the yellow wires that are coming out of the transformer on the top where im pointing in the picture are supposed to be giving off +12 but the are giving off 0.
 
Last edited:
Did you pull one leg of the resistor from the board, if you didnt it would probably read much higher since its in circuit and you would be getting a reading from the resistor as well as the other parts along that trace.

Sorry for the tangent, but I have to correct this. A resistor will never read higher in circuit.
 
I checked the input voltage on the 7812 regulator and thats about +20

alright im getting nothing at the +12 pot and i traced it all the way back to the +12 wire and i got nothing coming in. I traced the thick yellow wire all the way back to the transformer. i am 80% sure that all the yellow wires that are coming out of the transformer on the top where im pointing in the picture are supposed to be giving off +12 but the are giving off 0.

These two finding are inconsistent. If you have 20VDC on the input leg of the regulator (relative to GND), then there is nothing wrong with your transformer.
 
Back
Top Bottom