blowing fuse

[DrewV]

I'm looking at a concert pa control amp circa 1970's. The 3 amp fuse on the back panel blows immediately upon turning the amp on. The filter caps look and test good and there are no signs of burnt components. I tested the four 2N3055 transistors on the back panel one at a time, removing them from the circut with a transistor checker and all look good. Looking at the amp from the back, with the transistors in the circut, the one on the far left shows the collector going to ground. The transistor 3rd from the left shows the base and emitter going to ground. Does this situation indicate a blown OT? Any info would be appreciated.
thanks,
DrewV

[monolith]

Have you checked the rectifier? I am not entirely familiar with this model but it could be something to look into there.

[loudthud]

The very first version of the Concert PA from 1969 was rated at 90W. Later versions were rated at 120W I think. It's hard to sort out the progression from just looking at schematics. There were changes to the driver stage. I think early versions used a TIP29 but by the time of the Concert Lead and Concert Bass, a 2N3055 was used, the 5th power transistor you see on the back.

Not sure about which transistor is where but one will have a grounded collector and one will have emitter at ground thru .5 ohm resistor and base will measure 24 ohms to ground. The power supply for the output stage floats. It's not grounded. There is no output transformer, just a driver transformer with 10 wires (I think).

The four output transistors need to be matched for beta pretty tightly. Mismatched transistors probably just mean some DC on the output. If it blows a fuse, disconnect all the wires from the power transformer except the primary. If it still blows a fuse you have a bad transformer.