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redwire

0-30V Stabilized Power Supply

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hi guys, i have the original kit from greece , i made it in 1995 and some day it was burned.

i did the audioguru rev7 update with mc34072 and all the changes. only thing i didnt change is the 12000 cap , i am using 2x3300/63v cap and 28vac transformer.

VDC is 39volts , ref voltage is 11,8 , negative voltage ok .

i have checked all components twice.then the following happened:

when i increase the voltage , it goes from 0.0 to 30.0 SHARP, BUT at 6.7 volts there is a "jump" from my panel ANALOG voltmeter to 8,7 volts ! i checked with the oscilloscope and it appears that at 6,7 volts there is a huge noise that the analog voltmeter averages and thats the cause of the "jump".

if i use a load that requires less than 6,7 volts everything works fine , lets say 100 ohm load the psu works fine cause it uses less amps (30/100=300ma) and current regulation works fine.

if i switch to 10 ohms load (30/10=3A ) that can use the full spectrum of volts and amps ,the power supply goes INSANE , it RINGS SO LOUD , and it even BURNED the C7 cap alive with flames...

if i use a 2ohm load , the psu increases the voltage BUT after the 2.2 amp it rings again (till 2.2 amps current regulation works fine).

if i short the outputs the psu goes over 3amps no matter what i do with the volts or amps (exception if i use 30volts and i short the output , the current regulation works , under 30volts if i short the output the needle goes over 3 amps the the ringing happens (the needle bounces a lot too).

probing at U2 shows insane ringing above 3-4 volts but after 20v it smoothens somehow.R12,C6,C9,C5,C7 seems ok but still lots of ringing.. most ringing is at pin6 of U2 and at the base of BD139.

 

i have checked everything after the blow of C7 in order to see what might be wrong or burned and i really cant find anything misplaced,wrong value,cold joint etc.

does this behaviour ring to any of you any bells for the cause of the problem? i am going nuts with this psu...

 

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