I just received this amp from a neighbor asking me to have a look at it, reportedly it is crackling on the left channel with no sound except that.
I rarely do repairs having had bad experiences but since I know him well I agreed.
I opened the amp up and immediately got a whiff of burned parts and sure enough two of the small resistors on the left channel pcb were fried beyond belief.
I do have the other channel that works and found the same two resistors, red brown brown code but cannot find this value on the schematic which I down loaded.
It is hand written copy but I do not see this value on the diagram. I do not have a lot of diagnostic tools but is there a way to check the ouputs in place. It must have been a heavy short of some kind to fry those parts like it did.
Anyone have a similar issue like this, it appears that the left board has been repaired before as one big cap has been changed and there are burn marks on the board in other spots.
The biggest issue that the resistors burned the board itself and took out eyelets for soldering making the repair that much worse.
I feel I have opened a Pandora"s box on this one, any help or advice would be appreciated, this why I do not do amp repairs, I only build a few as a hobby.
Cheers Wayne
PS I will try and take some pics and include them sometime today..
I rarely do repairs having had bad experiences but since I know him well I agreed.
I opened the amp up and immediately got a whiff of burned parts and sure enough two of the small resistors on the left channel pcb were fried beyond belief.
I do have the other channel that works and found the same two resistors, red brown brown code but cannot find this value on the schematic which I down loaded.
It is hand written copy but I do not see this value on the diagram. I do not have a lot of diagnostic tools but is there a way to check the ouputs in place. It must have been a heavy short of some kind to fry those parts like it did.
Anyone have a similar issue like this, it appears that the left board has been repaired before as one big cap has been changed and there are burn marks on the board in other spots.
The biggest issue that the resistors burned the board itself and took out eyelets for soldering making the repair that much worse.
I feel I have opened a Pandora"s box on this one, any help or advice would be appreciated, this why I do not do amp repairs, I only build a few as a hobby.
Cheers Wayne
PS I will try and take some pics and include them sometime today..
Assuming this is the schematic, the burnt resistors will be the base resistors 100 Ohms (brown, black, brown) and you will have to replace the drivers and the output transistors: BC639, BC640, 2SD396, 2SB776. The 0.2 Ohm emitter resistors are also likely open.
This amp has no output protection so it will blow up if the output is shorted. I would modify the circuit by adding current limits but it's a bit more complicated.
This amp has no output protection so it will blow up if the output is shorted. I would modify the circuit by adding current limits but it's a bit more complicated.
Attachments
https://www.sugdenaudio.com/service-repair-centre
If you are not experienced, you may want to cut your losses now and return the amp.
It's no shame that the damage is too extensive for you to repair, many would not succeed with this.
They probably shorted the speaker wires together and blew up the output stage.
If you are not experienced, you may want to cut your losses now and return the amp.
It's no shame that the damage is too extensive for you to repair, many would not succeed with this.
They probably shorted the speaker wires together and blew up the output stage.
Last edited:
I should add that you need to check the Vbe multiplier BC549, but unless the resistors 47R, 10R, 10R are damaged, it's probably ok. Bringing up a repaired amplifier requires careful steps to detect more damage. The problem is that most typical part failures will result in more damage if the amp is just turned on again, and you destroy new parts that have just been replaced. One step is to bring the amp up without the output transistors and without a load. The drivers alone should provide the expected voltages without drawing excessive current. Another technique is to place a ~40 Watt incandescent lamp in place of the fuse to limit fault current. If the amp is healthy, a small current will not light the lamp.
I should also comment that BC639 and BC640 drivers are too small for this application. A better choice would be BD139 and DB140, with small heat sinks attached. The outputs 2SD396 and 2SB776 are likely an availability and price problem, so I would use something like 2N3773 and 2N6609, or perhaps more modern parts if you can find something similar in the TO-3 package.
This amp was made when such amps blew up all the time and carries many of the design faults that caused those failures. An expert can correct those problems but assuming you get it working, you can expect to it to fail again before long.
I should also comment that BC639 and BC640 drivers are too small for this application. A better choice would be BD139 and DB140, with small heat sinks attached. The outputs 2SD396 and 2SB776 are likely an availability and price problem, so I would use something like 2N3773 and 2N6609, or perhaps more modern parts if you can find something similar in the TO-3 package.
This amp was made when such amps blew up all the time and carries many of the design faults that caused those failures. An expert can correct those problems but assuming you get it working, you can expect to it to fail again before long.
Since this is a high-bias, or virtual class A type amplifier, the OP might need some big incandescent lightbulbs to get any useful limiting, just for the idle current.
Thanks for the replies.. I took the advice given and returned the amp to my neighbor.. I suggested he take it to a local repair shop which has a good reputation for an estimate.
Thanks again for all the comments Cheers Wayne
Thanks again for all the comments Cheers Wayne