OMishra PSU (cnc, pass b1 power supply) - issue

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rbalu72

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Jan 29, 2014
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Thanks for reading.

I am experiencing strange behavior with this very good power supply that I made around 2015.

Just to recap, the PSU can produce a dual-rail and a single rail upto 1A.

I am feeding it with 21-0-21 ac and obtaining +/-12v and 24,0 as outputs.

Since yesterday, the positive rail of +/-12v is drifting towards 6-7v irrespective of trimmer position. Negative rail and 24,0 output is rock solid.

Where to look to fix this fault? I replaced lm317 and removed all caps in this rail thinking something is leaking but the behavior remains.

Any suggestion/view please let me know
 

Hiten

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Try replacing and testing trimmer with one that you are getting correct voltages.
regards
 

spirovious

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If 317 is replaced then trimmer can be an issue.Another important thing to check is chock. Most of the time it restricts current and voltage drops more.I suggest you to take all readings before LM317, Output of 317 ,before both chocks and after them. Take readings without connecting load/Preamp.If readings are perfect ,then something at preamp/load has an issue.I had an issue when l connected a DAC to On PSU.Voltage could drop to 3.7 instead of 5v. After discussing Om, I simply bypassed Chocked by shortening their legs/leads.
Another interesting thing I found that electrolyte caps after 317 and near chocks plays roll to output perfect voltage. I used to get less voltage when I connected different load.So changed 100uf caps to 220uf and issue solved.
Last thing if Pre connected to it has ground issue or some part is short to ground,then also voltage can drop.
So take reading without connecting to PSU.
 
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