Space Invaders Power Supply Problem

nanoryan

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Hey guys, I have an SI that runs about 5.25V at the boards when first fired up but then it drifts down to about 4.5v after around an hour and the board starts to freak out.

Any suggestions for what might be wrong with the P/S? Dried out caps?

Thanks, Ryan
 
A falling DC output voltage is a sign of electrolytic capacitors going bad. Also poor connections due to cold solder joints or tarnished connector pins can be a factor.

Remember, this is a 30 year old power supply.
 
>>Question, what about if the +5 volts is reading +7.5 volts? I cant get the pot to adjust it down to the +5, turning the pot either way as no effect. Stays at +7.5 B d caps, or something else? ideas?

Thanks John
 
No suggestions? Trying to avoid paying $70 for a harness and switcher.

Ryan, as Ken has suggested... it sounds like you are having a cap failure issue.

>>Question, what about if the +5 volts is reading +7.5 volts? I cant get the pot to adjust it down to the +5, turning the pot either way as no effect. Stays at +7.5 B d caps, or something else? ideas?

Thanks John

John, nice thread jack. Your problem is more likely a bad diode or BR.
 
Thanks guys, going to get most of the parts from Bob on the power supply and repalce them. on the BR's whats the best way to test them in the board?

Thanks
 
>>Question, what about if the +5 volts is reading +7.5 volts? I cant get the pot to adjust it down to the +5, turning the pot either way as no effect. Stays at +7.5 B d caps, or something else? ideas?

Thanks John

I'm betting a bad 2N3055.

Edward
 
Could be the 7805, but I highly doubt it.

Yeah, I hightly doubt it too...

...especially since there isn't even a 7805 on the board.


Measure the base of the 2n3055 (should be ~.7V above the output voltage) and work your way back to the LM305.

IIRC, The wiper on the pot should be ~1.8V to match the internal diode reference in the LM305... if that's right (and pin 6 of the LM305), the feedback loop is working, but you've got isses with the feedback resistors/pot.
 
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or whatever is supposed to drop the voltage down is not doing its job.(according to the schematic)
 
I was having the 7.5 volt issue and I had some bad zeners, didn't make sense to me but it works now...could be some voodoo too...
 
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