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Help with bizarre problem

6/17/2003 12:05 PM
drewl Help with bizarre problem
hi, I have a Peavey Ultra 100w (similiar to 5150) with 120hz coming through the output tranny.amp had a blown screen res. you'd think power supply filter caps right?  
well, I replaced them, here's what's weird....the ac is on the output tranny center tap, or right after diode bridge, well if you disconnect the tranny, the ac goes away. the diodes seem good because they are rectifying the ac.  
i was going to jumper in a new output tranny to see if that's the cause.  
I disconnected all ribbon cables to output tube board (yuch!) and jumpered the B+ and the noise is still there. but only with output tranny connected.  
Tranny ohms out okay, but under voltage....who knows.
6/17/2003 1:03 PM

If there is AC after the rectifiers why do yu think they are good?  
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6/17/2003 3:21 PM
They're fine without the load (output tranny center tap), plus i jumpered in 2 new ones.  
unless they're crapping out under load or the other 2 in the bridge are bad.  
it's probably something stupid (like me) but I'm just trying to rule out the tranny.
6/17/2003 5:30 PM
M Cook

I would check each diode in the rectifier. It sounds like one or more is open.  
Michael :)
6/17/2003 5:41 PM
Blown screen resistor? Did you replace the tubes?
Usually a shorted screen causes a blown screen resistor.  
Check tubes. One is probably dead and causing an imbalance of primary DC current which allows you to hear the ripple on the power supply.  
6/17/2003 6:06 PM
The ripple (more like a pure ac sinewave) is present without the power tubes installed.  
thanks for suggestions, it'll have to wait til i get home to look at it again.  
the pisser is there's no ac there without tranny connected, as soon as it is the ripple comes back.  
somethings definitely breaking down, cause as i was bringing ac up w/variac it was holding until ac in got to be about 30volts or so.
6/18/2003 6:58 AM
Re: Help with bizarre problem
Check your tranny for shorts between the primary and secondary, or between primary and the frame.
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