Fender stage 100 amp

[bluefox616161@hotmail.com]

this amp was being used at a outdoor event and being powered by a generator. the generator ran out of fuel and when it was restarted this and another amp no longer worked. would blow the fuse as soon as it was turned on. i checked and the voltage coming out of the transformer is correct going into the main board the amp has 3 tip147 transistors and 3 tip142 transistors at the top mounted to a heatsink. one of the  tip147 transistors (part q15) had a blow hole in it and the large 0.47 ohm resister below it was burned out (open circuit) part r95 so i replaced the resister and all the transistors and the it still blows fuses when i turn it on.  how can i test the transistors to be sure that they are working?  another question is they all have sill pads on the back of them so i guess it keeps them from shorting to the heatsink but they all show 0.3 or so ohms from the transister  back to the heatsink is this normal?   the heatsink is screwed onto the board and had a rather large trace going to one of the screws
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCQQFjAB&url=http%3A%2F%2Fsupport.fender.com%2Fschematics%2Fguitar_amplifiers%2FStage100_Stage100H_Stage160_schematic.pdf&ei=VQqKVbSTEcyqsAWHkYeIDA&usg=AFQjCNGQ6cCxuT1XLGULEnus3axV8HowUA&sig2=XVKEpeSnyNbAkbgMC1l2ig

[Shock]

You use either the ohmmeter or a diode test function found on most multimeters. These are darlington transistors refer to the datasheet which will show the diagram.

Then google or search on youtube on "how to test transistors". You need to test the transistors out of circuit if a fault is still present in the amp.

The thing to be aware of with darlingtons is they can have a protection diode between collector and emitter so if you do a diode test in both ways across those pins and it's conducting then it's also no good.

Testing the transistor out of circuit will also tell you if a pin fully conducts to the heat sink with both a diode and ohms test in both directions. If it's really ok for the transistor to be directly connected to the heatsink depends on the circuit so the schematic normally reveals all. If a silicon insulator and a screw insulator is present this is a strong indicator to not physically connect them (this means also using non conductive thermal grease).

Anyway it sounds like there is a short there somewhere so check all diodes/transistors and resistors as if they are failing something is causing too much current to flow. You want to rule out the speaker if it's a culprit as well.

[Shock]

If you want further assistance take some high resolution photos of the circuit board (both sides) and link to the schematic, with that information the chances someone helping you increases substantially.

[RobertHolcombe]

When you say you tested the voltage on the secondary side of the mains transformer I assume that was with it disconnected from the board, and the fuse didn't blow. If so, test for shorts between P1/P2/P3 and P4/P5/P6

[bluefox616161@hotmail.com]

yes i tested the transformer with the secondary side disconnected from the board. i will pull the transistors out when i get home from work today  and let you know what i find.

[jaysgr]

Hello guys, I have a problem with my Fender Stage 100 and I think it is the power transformer but I'm not sure. If I move slightly the secondary cables toward the transformer I get noise at the loudspeaker. I mean it dramatically interrupts with the magnetic field.

Also, I made measurements of the secondary cables of the transformer when disconnected from the board.
The results are:

Red 11.6 VAC
Red 31.9 VAC
Yellow 10.2 VAC

Blue 11.2 VAC
Blue 14.6 VAC
Brown 1.5 VAC

I guess my transformer has gone bad? The two reds shouldn't be the same and the yellow zero volts?
Please, I would appreciate any help.
Thanks!

[N2IXK]

Where are you measuring those voltages in relation to?

ASSuming the transformer has 2 centertapped windings (as the color code would suggest)  You should measure some voltage between the 2 like colored wires, and half as much voltage between either of the like colored wires and the odd colored wire of each secondary.

If you suspect a bad physical connection, use an ohmmeter in continuity mode across the winding while wiggling the leads.

[jaysgr]

I am measuring this voltages in relation to the chassis. Is this a wrong way to do it?

[Shock]

I am measuring this voltages in relation to the chassis. Is this a wrong way to do it?

Yes, and what you are doing, without a clue, is da-da-dangerous!

I suggest stop now, start a new thread. Describe the fault and include what you think is occurring, take some good close up photos and upload or link to the circuit diagram or schematic.

This will get you the best help and you have a far better chance of repairing it.

[jaysgr]

Ok Shock , thank you. I will start a new thread and maybe post a video of my problem.