Galaga Power Supply

[smac666]

Ok,

I have an original Galaga machine and I reworked the power supply board with the kit from Bob Roberts. Turned it on and everything seemed to work fine. Came back the next day turned in on and there was no sound. Traced it back to there being no 12v supply to the audio circuit.
Checked with the multimeter and the 12v DC was now giving 20v AC!!
I was concerned at that point that I had fried my Galaga board so made a harness for a switching PS supply and bingo game plays fine with sounds.
I rechecked the orientation of the 4 diodes and the capacitor on the board, reflowed the solder to be sure and measured the out put again 20v AC.

Anyone know whats happening as I seem to be getting 20v AC out through the diodes, which I thought was impossible (although I'm no electrical engineer). I'd like to fix it so I can keep it as orginal as possible.

Regards
Steve

[BobA]

Did you check to see if one of your diodes is shorted?

[smac666]

Yeah I did check for that and there seemed to be no shorts.
They all seem to read the same giving a resistance of 3ohms.

[Ken Layton]

Diodes are polarity sensitive devices. Did you install them in the correct orientation?

[smac666]

Yip I double and triple checked them, they all look good.

Steve

[BobA]

Can you take pictures of the top and bottom of the board?   Someone may be able to notice something you have missed.

[smac666]

Ok I got some pics of the board, I can't see any cut tracers, jumpered solder connection (I even ran a knife down them to be sure). I tested the diodes (in place with a multimeter), small resistance one way, no flow the other (when you reverse the test leads). The only thing I can think of is the capacitor, but I'm not sure how to test those and its brand new.

Regards
Steve

[BobA]

The boards look good an polarity looks ok.   I cannot see the polarity on the single diode near C7.   Can you measure the voltage across the electrolytic?   Even if the cap was open it should read DCV as it has pass thru the bridge rectifier circuit.

[smac666]

The diode next to C7 is still orginal never changed and measures good.

You want me to measure the voltage with the circuit powered up?

I did take a reading last night with the board unplugged and it was reading +9.4v with the value going up slowly. I reversed the meter leads and got 9.6v with the value decreasing. So I guess the capacitor is holding a charge.

The more I think about it the more I think that capacitor is bad somehow. Its the only thing that could let the AC current through as the diodes seem to be working.

[BobA]

The cap is in the circuit after the diodes it serves to filter the rough DC that comes from the 4 diodes that make up the bridge.   Even if the cap was bad you should read DC on the cap side of the diodes.   Can you measure the voltage across the cap with the circuit powered?

[smac666]

I can but it will be later tonight when I get home.

Regards
Steve

[smac666]

Ok I powered it up last night and took a reading, got +1.1v DC
 

[BobA]

Do you still get an AC reading for your 12V supply?

[smac666]

Yip still reading 20v AC on ouptut side of PS board.

[BobA]

The only thing I can think of is a short between the traces of the 4 diodes that make up the bridge.   That looks like the only place AC could be connected to the DC side of the circuit.  You said the diodes checked ok but the only other possibility is a shorted diode.   Sorry it is such a simple circuit there is not much that can go wrong go wrong go wrong 

[smac666]

Thanks,

I'l order some new diodes and replace them all.
Easiest way I think.