Fender deluxe 112 repair

[sambo42]

Hi, my first post here, though been watching the blog for a while.
I'm trying to repair a fender 112 deluxe. The power transistors Q1,2,10,11 keep blowing as well as the fuse. The amp will power up for approximately a minute before this happens.
I've now taken the powers transistors out of the board and it lasts over 10mins without the fuse blowing.
Link to schematic: http://www.electronicstudio.net/au-schem-fender.html]
Thanks in advance for any ideas of what to have a look at next.
Sam

[SeanB]

Check the 4 power rails are there, and are about correct ( the 40V rails can be between 30 and 45V but the 16V rails should be within 1V of 16V.

Check U1 pin 6 is at about 0V, the power rails to the opamp are correct at about 25V, and check all the diodes CR5 to CR14 that they are not shorted or open circuit. Check R74,75 are the correct value, and C39,40 are still within value.

Very likely that the ouput stage bias is wrong, and these are the commonest components to fail and cause the bias current to bee too high, which will blow transistors.

[sambo42]

Hi, thanks for the advice. at work at moment. Will check in the morning. Realised I missed of the 'plus' of 112 deluxe. Its a fender deluxe 112 plus

[sambo42]

CHecked the power rails. Getting 45V and 16V. So i reckon they are ok. U1 pin 6 is 0V and the power rails are 30V, which as the 40V is 5V higher i think should be ok(not checked the opamp datasheet yet to check its MAX supply V) Diodes 5-8 read 0.5V correctly biased tho when tested the other way round show 2V. I was reluctant to take these out of cirtui as they are pressed into the heatsink. The other diodes all test ok. R74,75 show correct values. i dont have a capacitance meter  so could only check C39,40 for short circuit, they were fine and have also been recently replaced so their values should be ok.
Any other ideas?
Ive seen on a nother site the idea of placed an incandescent bulb in line with the mains power supply to act as a current limiter to allow powering up while not blowing anything, any thoughts on that?

[SeanB]

Dim bulb tester will work well to test. I would still check the 4 emitter resistors by replacing them, as they are setting the amplifier bias current, and there is no adjustment, and as they are very low in value they are hard to measure accurately. With the transistors removed check the voltage at the output TP19, and the voltages at the bases of each transistor. The mid rail should be 0V, and the bases should be 1V2 from the 0V rail. Check Q8,9 are not shorted.

[sambo42]

Hi, apologies for leaving this for a long time. Busy with work etc.
Not been able to get replacement emitter resistors yet.
However I've checked all the biasing diodes- all fine. Q8-9 checked.
R97-98 are getting extremely hot.
Checking the base V and TP19 now

[sambo42]

Base voltages 1.2V.
TP19 - 0V DC need make up a way to test with AC signal

[Andy Watson]

Have you checked the resistance from TP19 to ground?  I'm thinking there might be a short-circuit somewhere in the headphone switching/socket. The over-current protection is only applied to Q10 and Q11; Q1 and Q2 could be getting into the region of thermal-runaway.