Help fixing +5v on my Midway Space Invaders Power Supply

Check all the tantalum caps for a dead short. I seem to recall there is one cap in the 5volt circuit that is prone to shorting out and causing your symptoms.
 
Check all the tantalum caps for a dead short. I seem to recall there is one cap in the 5volt circuit that is prone to shorting out and causing your symptoms.

So you are saying I should get tone on the cap when I test it, or read near zero resistance?
 
I'm having exact the same problem.

Replaced everything on this board, except resistors and ceramic caps, still no +5v.

Does the sense circuit control the 5 volt line?
 
I'm having exact the same problem.

Replaced everything on this board, except resistors and ceramic caps, still no +5v.

Does the sense circuit control the 5 volt line?

I don't know the answer, but it is connected. That is what I have been thinking.
 
There's only 1 thing I'm an expert at and I don't do it in public... but...

I would start by checking the voltages at the pins on the LM305 and the 2 transistors there (a 2N2905 and the 2N3055) [would have killed them to put reference designators in the bloody schematic?!!!]

cheers
/Tim
 
What is the voltage on the positive side of the 20000uf cap? It should be nearly 9vdc but it should be fairly rough (assuming you have a decent meter the voltage will jump around a bit).

What is the voltage coming out of leg 8 of the LM305A?
 
Check all the tantalum caps for a dead short. I seem to recall there is one cap in the 5volt circuit that is prone to shorting out and causing your symptoms.



I did a full Bob Roberts kit on one and no 5vdc either. I will check the lm305
 
There's only 1 thing I'm an expert at and I don't do it in public... but...

I would start by checking the voltages at the pins on the LM305 and the 2 transistors there (a 2N2905 and the 2N3055) [would have killed them to put reference designators in the bloody schematic?!!!]

cheers
/Tim

I replaced the LM305A and the 2N2905, but I will check the pins on it. Which is pin 8? The 8th clockwise from the tab?




What is the voltage on the positive side of the 20000uf cap? It should be nearly 9vdc but it should be fairly rough (assuming you have a decent meter the voltage will jump around a bit).

What is the voltage coming out of leg 8 of the LM305A?







I did a full Bob Roberts kit on one and no 5vdc either. I will check the lm305
 
Pin 8 will be linked to one side of the .18ohm 5w resistor and it will also go to the +5v output line with no other component in the way. You'll most likely have to flip the board over in order to chase the traces.
 
Just to clarify if anybody's not catching it:

The caps that short out and make the 5v go low are on the GAME board, not on the power supply. So check for your voltages with the gameboard unplugged.

The way you can tell if it's the gameboard, too, is to look where the 5 goes on the gameboard, and measure between 5 and ground. If it's shorted, you've got one of those little skittle looking capacitors shorted on the gameboard... or several. Replace them one at a time until the short is gone, or hell just replace all of them.
 
Just to clarify if anybody's not catching it:

The caps that short out and make the 5v go low are on the GAME board, not on the power supply. So check for your voltages with the gameboard unplugged.

The way you can tell if it's the gameboard, too, is to look where the 5 goes on the gameboard, and measure between 5 and ground. If it's shorted, you've got one of those little skittle looking capacitors shorted on the gameboard... or several. Replace them one at a time until the short is gone, or hell just replace all of them.

I replaced one of them that looked burnt (see other thread) sand but it didn't read shorted. I got enough to replace all the .1 uf but I didn't get the 47uf.
 
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