Soundcraft CPS 650 PSU regulator replacement?

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syn

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Oct 10, 2005
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Not sure about this. I tested the transistors with my atlas DCA55 and they tested OK. However I had one rail getting as low as ~0.5VDC almost 0VDC and the other staying at 17VDC and the transistors did not do anything, they did not pull healthy 17V rail down. Still, when I take the transistors out they test OK. I'm rebuilding the PSU right now, will post some photos soon. I was thinking of ditching safety circuit all together as It does not work as promised. Think it was +17 rail failing with the negative rail staying at the full potential (-17VDC). All diodes and resistors replaced too.
 

Matt Syson

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A LM317/337/338 if working correctly should have a minimum output voltage of ~1.2 Volts even if the adjust terminal is grounded. This would suggest you have a different issue going on. I think the basic assumption for the 'cross connected' system is that both regulators are behaving 'normally' but it is not infallible.
 

syn

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A LM317/337/338 if working correctly should have a minimum output voltage of ~1.2 Volts even if the adjust terminal is grounded.

The regulator in question was broken, it was current limiting way to early.

I think the basic assumption for the 'cross connected' system is that both regulators are behaving 'normally' but it is not infallible.

Thanks, yes, but if the regulators were behaving 'normally' than PSU would be working OK and I would not need to repair it :) . There is not much to go wrong with this PSU apart from the regulators. 'cross connected' system seems to bring more confusion than benefits...
.
 

Matt Syson

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The point of 'cross connection' is that if one rail gets shorted by some failure anywhere downstream the shutting down, or minimising of both rails prevents the unit sitting with a rail missing which if left for long, risks reverse biassing many capacitors thereby 'killing' them. Thus for gear with many hundreds of capacitors a relatively simple 'fault' can become a nightmare.
 

syn

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Oct 10, 2005
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645
Thank you very much for clearing this out, I'll keep the 'cross connection' in . I already re-cap-ed the whole board, not looking forward to do it again ...
 

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