arcam delta 290 trouble in power amp section please help - diyAudio
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Old 23rd September 2008, 12:34 PM   #1
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Default arcam delta 290 trouble in power amp section please help

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......
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Old 23rd September 2008, 04:48 PM   #2
Mooly is offline Mooly  United Kingdom
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Is the same amp you keep starting new threads for You naughty boy !!

If you have the manual email me. No promises mind, the last Arcam I fixed here ended up a real marathon job.
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Old 23rd September 2008, 05:24 PM   #3
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Default arcam

Hi there thanks for reply Mooly could i please have an email address so i can attatch manual......Regards Steve
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Old 24th September 2008, 06:17 AM   #4
Mooly is offline Mooly  United Kingdom
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Check your email
You can also click on my name (Mooly) to contact me.
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Old 24th September 2008, 01:21 PM   #5
Mooly is offline Mooly  United Kingdom
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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.
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Old 24th September 2008, 05:54 PM   #6
jaycee is offline jaycee  United Kingdom
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To be honest the FETs in the right channel were probably stressed too, and thats why they failed. Replace all of them - its not like theyre expensive. You are probably best replacing the drivers too.

Use a light bulb in series with the live wire to limit the current.
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Old 24th September 2008, 06:09 PM   #7
Mooly is offline Mooly  United Kingdom
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Not a good image
Attached Images
File Type: jpg img263.jpg (80.1 KB, 121 views)
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Old 24th September 2008, 06:21 PM   #8
Mooly is offline Mooly  United Kingdom
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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 !
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Old 25th September 2008, 06:48 AM   #9
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Default 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
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Old 25th September 2008, 06:49 AM   #10
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Default ARCAM

Hi jaycee thanks very the very valuable tip
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