Hi all,
I'm trying to repair a left channel Quad 405, board number 12368 iss 9.
which blows fuses. It had a number of burnt components including 4 of the transistors and 2 of the diodes D5 and D6 which have been replaced with 1N4148, and used TIP42C for TR7 and 8. It ran once at low volume but blew fuses as soon as the volume was increased. It blows fuses now all the time. All transistors test OK. and diodes too, and all resistor values are within tolerance ratings. Caps aren't shorted.
This amp uses current dumping, which I never came across before, and am still trying to grasp.
The negative rail on the defective board is 1.75K to ground with the TR9 and 10 collectors out of the circuil, by removing the transistor mounting screw. The negative rail on the good amp is in excess of 500K to ground. Maybe that is a fault indicator.
Is it possible to test it and bring it together in stages in order to find the fault? Quad mentions something about a 6 volt supply being used to power it in their service manual. I tried using 300 ohm 5 w resistors in place of the 4 amp board fuses but they had 16 volts across them when connected to the 50 volt rails.
thanks
Denis
I'm trying to repair a left channel Quad 405, board number 12368 iss 9.
which blows fuses. It had a number of burnt components including 4 of the transistors and 2 of the diodes D5 and D6 which have been replaced with 1N4148, and used TIP42C for TR7 and 8. It ran once at low volume but blew fuses as soon as the volume was increased. It blows fuses now all the time. All transistors test OK. and diodes too, and all resistor values are within tolerance ratings. Caps aren't shorted.
This amp uses current dumping, which I never came across before, and am still trying to grasp.
The negative rail on the defective board is 1.75K to ground with the TR9 and 10 collectors out of the circuil, by removing the transistor mounting screw. The negative rail on the good amp is in excess of 500K to ground. Maybe that is a fault indicator.
Is it possible to test it and bring it together in stages in order to find the fault? Quad mentions something about a 6 volt supply being used to power it in their service manual. I tried using 300 ohm 5 w resistors in place of the 4 amp board fuses but they had 16 volts across them when connected to the 50 volt rails.
thanks
Denis
Hi.
So I have some experience. You must miss something. There is some shorted or leaky device. There is only a few components to check so my advice. Don't guess what could be possibly wrong and start measuring. If you miss something it is possible that it is again burned something what you already replace....
And 405 won't work at 6V. You will need some +-20V at least.
Regards
So I have some experience. You must miss something. There is some shorted or leaky device. There is only a few components to check so my advice. Don't guess what could be possibly wrong and start measuring. If you miss something it is possible that it is again burned something what you already replace....
And 405 won't work at 6V. You will need some +-20V at least.
Regards
- Status
- Not open for further replies.