Hello and thankyou for looking i am in need of some help with this amplifier....
I can email the service manual which is a meg size approx in pdf
Started to fix left channel as it was dead right channel was good....
Took output fets out they were shorted(irfp240)
Got new ones replaced pre drivers with a close equivalent ,checked resistors
signal diodes the norm...Put new fets in ...
Turned on no blown fuses this time but sound is very quiet and
very distorted......
While i was checking over on the left channel nothing leaped out
getting 3.9 mv bias....
Then the right channel blew AAAAAAAAAAAAAAHHHHHHHHHHHHH
was not even testing it took fets out of right channel and what a surprise both shorted fuse replacement time also.....
Today i checked right channels predivers and removed the fets from other channel put them in and now doing same as other channel..
You can connect speakers up to the outputs and hear same
distorted sound whether they are being driven by an output stage or not..............
Ihave jumped the relays so i have ruled them out ...
I coonected up pre out to another amp both channnels fine
Arcams volume,tone control all work normally so must be in the power amp stage............
Any ideas would be great.......
Thankyou......
I can email the service manual which is a meg size approx in pdf
Started to fix left channel as it was dead right channel was good....
Took output fets out they were shorted(irfp240)
Got new ones replaced pre drivers with a close equivalent ,checked resistors
signal diodes the norm...Put new fets in ...
Turned on no blown fuses this time but sound is very quiet and
very distorted......
While i was checking over on the left channel nothing leaped out
getting 3.9 mv bias....
Then the right channel blew AAAAAAAAAAAAAAHHHHHHHHHHHHH
was not even testing it took fets out of right channel and what a surprise both shorted fuse replacement time also.....
Today i checked right channels predivers and removed the fets from other channel put them in and now doing same as other channel..
You can connect speakers up to the outputs and hear same
distorted sound whether they are being driven by an output stage or not..............
Ihave jumped the relays so i have ruled them out ...
I coonected up pre out to another amp both channnels fine
Arcams volume,tone control all work normally so must be in the power amp stage............
Any ideas would be great.......
Thankyou......
Hi,
Checking FET's isn't easy. Problem is the gate can charge up from static etc and give confusing readings, sometimes different every time you test them.
The IRF types should read ( use the diode range on DVM ) as open circuit drain to source one way round and then reversing the meter leads it should read a bit like a diode the other way. They also should not read to the gate. You will have to read them out of circuit.
Using FET's means the driver stages are pretty high impedance which can limit any further damage.
Checking FET's isn't easy. Problem is the gate can charge up from static etc and give confusing readings, sometimes different every time you test them.
The IRF types should read ( use the diode range on DVM ) as open circuit drain to source one way round and then reversing the meter leads it should read a bit like a diode the other way. They also should not read to the gate. You will have to read them out of circuit.
Using FET's means the driver stages are pretty high impedance which can limit any further damage.
Steve,
Depending how the FET's failed you need to do a basic check of all the low value resistors in the output stage. Anything under 1k including those 0.22 ohms.
I know you spoke of replacing all the transistors, but first lets try and get some idea of what happened. Check also those zeners.
Do any of the small transistors read faulty at all.
Suitable replacements would include the 2N5551 and 2N5401 which were in stock last time I ordered at www.cpc.co.uk
I can send you the data sheets for these as the pin outs are different.
We have to start somewhere
Get measuring !
Depending how the FET's failed you need to do a basic check of all the low value resistors in the output stage. Anything under 1k including those 0.22 ohms.
I know you spoke of replacing all the transistors, but first lets try and get some idea of what happened. Check also those zeners.
Do any of the small transistors read faulty at all.
Suitable replacements would include the 2N5551 and 2N5401 which were in stock last time I ordered at www.cpc.co.uk
I can send you the data sheets for these as the pin outs are different.
We have to start somewhere
Get measuring !
arcam
Hi Mooly thankyou for your help so far i possess a towers international ,brilliant little book had it for about 15 years for tranny cross referencing,yeh i have got to twist 2 legs on those american 2n,s to make bce as they are cbe ....what is not shown in diagram is a
2sc2240 which sits in air above the the two ..0.22 ohm resistors..the base is connected to one side of those ..0.22 through a 10 ..ohm resistor...collector is connected to a diode not sure yet whether it is a signal or a zener and emitter connects to other side of those 0.22 ,,,looking at schematic it points to q12 (2sc2547) but q12 is elseware in board further near front end....Regards
Hi Mooly thankyou for your help so far i possess a towers international ,brilliant little book had it for about 15 years for tranny cross referencing,yeh i have got to twist 2 legs on those american 2n,s to make bce as they are cbe ....what is not shown in diagram is a
2sc2240 which sits in air above the the two ..0.22 ohm resistors..the base is connected to one side of those ..0.22 through a 10 ..ohm resistor...collector is connected to a diode not sure yet whether it is a signal or a zener and emitter connects to other side of those 0.22 ,,,looking at schematic it points to q12 (2sc2547) but q12 is elseware in board further near front end....Regards
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