If you don't want to go to the trouble of removing the two transistors, remove and replace the four diodes (1N4001-1N4007) and two capacitors (1000μF/50V). Make sure you get the orientation of the diodes and capacitors correct.

Of course, you cannot test the amplifier with no power and no fuse.

View attachment 304753

thanks. but I dont have replacement caps and diiodes yet so i will remover the transistors first

 

Yes unsolder the two transistors on heatsinks, and with the fuses and power connected, measure the voltages across the capacitors.
You can test the transistors on diode test, across the collector and emitter terminals, it should read open circuit, .

View attachment 304754

will do dave . but the fuses will blow again ?

 

They shouldn't blow without the transistors connected, .

 

disconnected the transistors . 1 - 0.25 swapped polarity 0.25
2 - 7.18 " no reading .

 

If you're reading across the C/E terminals then one of them is blown if it's reading 0.25V on diode test. They should read open circuit both ways.

 

disconnected the transistors . 1 - 0.25 swapped polarity 0.25
2 - 7.18 " no reading .

so I powered up with the transistors disconnected and re-connected the diodes which I had clipped to test . it blew a resistor . see centre of pic .

 

If you're reading across the C/E terminals then one of them is blown if it's reading 0.25V on diode test. They should read open circuit both ways.

ok will order one of those . thank you

 

so I powered up with the transistors disconnected and re-connected the diodes which I had clipped to test . it blew a resistor . see centre of pic .

There might be other things wrong with this, check the other transistors, don't power it up until you have checked the other transistors and small diodes. The resistor looks like 100 ohms?

 

Like I said, replace the four diodes and the two capacitors.
Also replace the two transistors, 2N3055. But do not install the transistors until we give you the go ahead.

 

There might be other things wrong with this, check the other transistors, don't power it up until you have checked the other transistors and small diodes. The resistor looks like 100 ohms?

ok . will order those parts and get back. thanks for all the help today

 

so I powered up with the transistors disconnected and re-connected the diodes which I had clipped to test . it blew a resistor . see centre of pic .

Did you have the speaker connected? If so that wasn't a good idea as it would try to power the speaker through the resistors that used to be across base and emitter of the power transistors.

 

If the hum sounds to be 50 / 60 Hz I would be looking at the filter caps and yes a schematic would be nice.

Ron

If the speaker is pulling in all the way I'd expect to find the transistor (amp) shorted. The hum may be coming through due to heavy loading on the amp circuit. The filter cap is worth replacing possibly due to simple aging. But until we solve the shorting issue the hum should be a secondary consideration (my opinion).

ok . dave . so I will try and replace those first off . thank you

Word of caution - don't replace anything until you fully diagnose the problem. You could end up blowing out new components if whatever caused the initial failure has not been rectified.

If these are the diodes you are testing, then it sounds like they are faulty, i would remove one end of each diode and check to confirm it.

Yes - definitely pull one lead from each diode before testing. Testing "In Circuit" often yields false information.

They shouldn't blow without the transistors connected, .

I suspect this to be true, but I've blown fuses because of a shorted bridge rectifier (the cube type). Replaced that after being certain there was no other issues with MY amp.

ok will order one of those . thank you

Again, fully diagnose the problem before you start throwing parts at it.

As MrChips said:

•••replace the four diodes and the two capacitors.
Also replace the two transistors, 2N3055. •••

Replacing just one blown transistor will likely lead to the second one blowing out a week after you've fixed everything and you think things are good from there out. Like tires on a car, you don't replace just one unless you don't care about reliability. Caps get old. Diodes can get hurt. Transistors can fail with age. Starting fresh with those items is highly recommended. Both by MrChips and by myself.

 

Did you have the speaker connected? If so that wasn't a good idea as it would try to power the speaker through the resistors that used to be across base and emitter of the power transistors.

no speaker was connected when it blew the resistor .

 

so I snipped the diodes . got readings one way of . 0.63 0.67 0.67 0.684.

Those readings would be forward drop in a diode check with a DMM. That tells me that the diodes were probably OK, which is what I suspected. I also suggest not immediately putting in tyhe fuses and switching the power on, because it will pop the replacement fuses just as fast. Jumper the fuse connections with a couple of 1K resistors, after removing the output transistors. But there will need to be diodes, maybe the snipped ones can be re-connected. Then the supply voltages can be checked.

 

will do dave . but the fuses will blow again ?

I think that they probably will fail quickly once again.

 

no speaker was connected when it blew the resistor .

That's another clue. Check that the bias is working. At a guess, the two blue diodes in series set the bias. Check that they are OK.

 

If the speaker is pulling in all the way I'd expect to find the transistor (amp) shorted. The hum may be coming through due to heavy loading on the amp circuit. The filter cap is worth replacing possibly due to simple aging. But until we solve the shorting issue the hum should be a secondary consideration (my opinion).
Word of caution - don't replace anything until you fully diagnose the problem. You could end up blowing out new components if whatever caused the initial failure has not been rectified.
Yes - definitely pull one lead from each diode before testing. Testing "In Circuit" often yields false information.
I suspect this to be true, but I've blown fuses because of a shorted bridge rectifier (the cube type). Replaced that after being certain there was no other issues with MY amp.
Again, fully diagnose the problem before you start throwing parts at it.

As MrChips said:

Replacing just one blown transistor will likely lead to the second one blowing out a week after you've fixed everything and you think things are good from there out. Like tires on a car, you don't replace just one unless you don't care about reliability. Caps get old. Diodes can get hurt. Transistors can fail with age. Starting fresh with those items is highly recommended. Both by MrChips and by myself.

ok. thanks for that . would those diodes be the same spec . just buy four of the same ? where's a good place to buy components these days ..just on ebay ?

 

The small blue diodes look like 1N4148 type,

Digikey, Farnells, Mouser, RS, , all do these transistors and diodes.

 

Those readings would be forward drop in a diode check with a DMM. That tells me that the diodes were probably OK, which is what I suspected. I also suggest not immediately putting in tyhe fuses and switching the power on, because it will pop the replacement fuses just as fast. Jumper the fuse connections with a couple of 1K resistors, after removing the output transistors. But there will need to be diodes, maybe the snipped ones can be re-connected. Then the supply voltages can be checked.

ok. thanks . have resolded the diodes back in .

 

The small blue diodes look like 1N4148 type,

Digikey, Farnells, Mouser, RS, , all do these transistors and diodes.

ta dave