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Author Topic: johnson j150 millennium pico fuses blowing  (Read 2059 times)

EDWARDEFFECT1

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johnson j150 millennium pico fuses blowing
« on: September 21, 2011, 09:53:13 AM »
i have a johnson 150 millenniium 2x12 combo amp and it keeps blowing the 2 pico fuses(f1 and f2).i have no 32v supply and no +12 or -12 supply voltage coming off the power board. everthing checks out good on the supply board and there appears to be no shorted output chips,or shorted capacitors. i also checked the main board too and found nothing.any help appreciated. i have the + / - 5v supply and the b+ supply.thanks...ed
« Last Edit: September 21, 2011, 10:06:28 AM by EDWARDEFFECT1 »

Enzo

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Re: johnson j150 millennium pico fuses blowing
« Reply #1 on: September 21, 2011, 10:38:53 PM »
If they are blowing, excess current flows through them.  If you have checked all four sides of the bridge for diode drops, and no shorts across the filter caps, then the problem probably is elsewhere. 

If you have ICs running on +/-12v, you may not find either rail shorted to ground, but an IC could have the +12 shorted to the -12 internally, and that would do it.  ANy IC look discolored or cracked?  Or the board under one a little browned?  ANd while you can check with an ophm meter from +12 to -12, due to semiconductors, and internal shorted IC may not measure that way on a meter.  Even if no visual on it, such a bad IC would get hot.

EDWARDEFFECT1

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Re: johnson j150 millennium pico fuses blowing
« Reply #2 on: September 21, 2011, 11:24:41 PM »
the bridge rectifier tests good and has been replaced.it was good,but replaced it as a precaution.when testing it gets a barrier voltage on the ends only.you have to check diagnally for the other 2 voltages.all the ic's look normal no burn marks anywhere on the board while viewing it with good light and a head magnifier. the ic's don't appear to be shorted between the 12v+/- terminals.the fuses blow before the ic's get hot.if i check from ground to the pins on the ic i get no short or low ohm readings.i have no +12v or -12v to the ist ic in the preamp right after the input jack due to the pico fuses blowing.i also don't have any 32 volt supply which is not layed out on the print where it goes other than connector diagrams.i looked everwhere for a low ohms or dead short figuring that a short is causing both fuses to blow.was hoping c11 was shorted as it is used by both the 32v and +/-12v supplies.no luck though. i traced the 32v supply from the power board with a continuity test and it goes from the connector on the power board to the main board, then it goes along a trace to another connector(ribbon connector this time) to the jack board which has a connector for a pedal control board. i am guessing the pedal board is powered by this 32v supply.
« Last Edit: September 22, 2011, 12:52:30 AM by EDWARDEFFECT1 »