Hi
Thanks for all the good advice so far. Purchased the parts and may have a go at doing the blown side this weekend if I have time.
Do I need to put any thermal grease on the insulator pads for the transistors?
The power transistor at the "front" of the board has limited rear access so I will bend the pins to get the correct height, etc. by fitting it to an accessible transistor point on the heatsink and then solder it without the screwing it to the heatsink.
The other transistors can be soldered after screwing them to the heatsink after I have refitted the circuit board the chassis.
Hope that makes sense.
Thanks for all the good advice so far. Purchased the parts and may have a go at doing the blown side this weekend if I have time.
Do I need to put any thermal grease on the insulator pads for the transistors?
The power transistor at the "front" of the board has limited rear access so I will bend the pins to get the correct height, etc. by fitting it to an accessible transistor point on the heatsink and then solder it without the screwing it to the heatsink.
The other transistors can be soldered after screwing them to the heatsink after I have refitted the circuit board the chassis.
Hope that makes sense.
Sure on both sides of the insulator pad thermal grease (non cunductive) must be applied. or you can put it on the backside of the transistor nad on the backside of the insulator pad (side to heatsink).
I would not use thermal grease. Never necessary - for a little Rotel too. And you will have got a cleaner "work"-)
Blowing fuse question
Hi all
Another question. When I was given this amp I put a low value 2a fuse (8a is the normal fuse)in the right hand side in a naive attempt to test it. This side had no visibly burned components unlike the left channel which had the burned resisitors but it blew straight away. Does this mean anything for the diagnosis?
Hi all
Another question. When I was given this amp I put a low value 2a fuse (8a is the normal fuse)in the right hand side in a naive attempt to test it. This side had no visibly burned components unlike the left channel which had the burned resisitors but it blew straight away. Does this mean anything for the diagnosis?
Of course, this means you also have to check the power transistors and the driver transistors for defekt pn-junctions, the same test as for the burned channel.
If you talk about F981 and F982 from the power supply, and if you did this check you described above, then it's normal that the fuses blew. These fuses are protecting both power supplies, left and right channel. After you repaired the burned channel, this would be a good test if both channel work now. For this test you should use a slow-blow fuse at nearly the requested rating of 8A, since you have to charge 4x 8200µF in the power supplies and simply the inrush current can make the fuse blow.
Since you have dismount the board from the chassis, why not check the power transistors and the driver transistor from the other channel inboard for working pn-junctions (0,4-0,7 v voltage drop).
Since you have dismount the board from the chassis, why not check the power transistors and the driver transistor from the other channel inboard for working pn-junctions (0,4-0,7 v voltage drop).
Thanks, just trying to be prepared and get as much information as possible before I actually do anything.
Hi
Fitted the new transistors on the left side only and replaced the resistors. Not soldered them in yet. Will do that tomorrow. Don't want to worry about it before I go to bed if it goes bang!
The blood on the white resistors is mine. I had cut my finger on a broken light bulb!
Fitted the new transistors on the left side only and replaced the resistors. Not soldered them in yet. Will do that tomorrow. Don't want to worry about it before I go to bed if it goes bang!
The blood on the white resistors is mine. I had cut my finger on a broken light bulb!
Attachments
Hi all powered on. Fuses (6a only) did not blow but Q617 (driver transistor) fried with lots of smoke. This is the same transistor that was fried prior to attempted repair. Right channel seems ok.
Any ideas?
Any ideas?
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Sorry to hear that.
Q617 is the bias transistor, connected to Q613 and Q615. You will have to check these transistors,from there til the power transistors again and usuallly all other parts in this area on the schematics. Check also for shortage of the solder joints of the transistors , you already changed
Q617 is the bias transistor, connected to Q613 and Q615. You will have to check these transistors,from there til the power transistors again and usuallly all other parts in this area on the schematics. Check also for shortage of the solder joints of the transistors , you already changed
The bias voltage transistor (Vbe multiplier) is a shunt regulator that sets the bias voltage to <<5V
It should never blow up.
Except if the transistors linking it to the supply rails have shorted.
You need to thoroughly check all the other transistors, BEFORE you next power ON via a Mains Bulb Tester.
It should never blow up.
Except if the transistors linking it to the supply rails have shorted.
You need to thoroughly check all the other transistors, BEFORE you next power ON via a Mains Bulb Tester.
Is the burned area not conductive now? i also had problems in the past with errors like this, when the pcb material was burned, i hardly can make it work again.
Hi. Made an MBT. Still powers on and not blowing fuses but same transistor burns. It is VR601 that is smoking afterwards. Tested transistors as suggested and seem OK. Progress in that it powers on. Transistor still burned despite MBT.
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