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Old 27th March 2008, 08:53 AM   #1
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Question Duson A10 - help needed

Greetings,

My friend has a Duson A10 power amp that is in need of repair. Does anyone have a schematic or any other information about this amp? From the only tidbit of information I was able to find on the Internet, it appears that the amp was made in the Netherlands in the early 90's and that it is heavily biased into class A.

What I do know is that the output devices in one channel are blown - the amp uses two pairs of Motorola MJ15015/16 per channel (well, three pairs actually - one pair appears to be used as a pre-driver as it is sitting on a separate heatsink from the other two and runs much cooler). I have equivalent ECG parts on hand (ECG 388 / ECG 68) - are these good enough to use, or should I try to get originals from OnSemi? Do they need to be matched? Any ideas as to what is the correct bias for these? I am measuring about 0.15V across the emitter resistors on the good channel, but I have some doubts that this is what it should be.

And, I have to say that I was rather surprised that the entire circut uses cheap 5% carbon compound resistors everywhere. There's not a single metal film resistor (let alone boutique parts) inside, and furthermore, the choice of output transistors seems to suggest that it was not built with quality in mind, yet it is supposed to be an expensive class A amp. Can anyone shed some light?

Cheers,
Doug
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Old 27th March 2008, 04:18 PM   #2
jaycee is offline jaycee  United Kingdom
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ECG parts aint good for squat. Use the proper OnSemi transistors. If you order them in a batch from OnSemi then they should be pretty closely matched.

The fact that it has adjustable bias suggests it's not really Class A, but rather Class AB with a high bias current. What are the values of the emitter resistors?
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Old 27th March 2008, 08:30 PM   #3
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Quote:
Originally posted by jaycee
ECG parts aint good for squat. Use the proper OnSemi transistors. If you order them in a batch from OnSemi then they should be pretty closely matched.

The fact that it has adjustable bias suggests it's not really Class A, but rather Class AB with a high bias current. What are the values of the emitter resistors?
Thanks Jaycee - that's what I thought. The emitter resistors look like they're 0.47 ohms (the markings have faded because of the heat, but I can discern an 'R47' on one of them).

Doug
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Old 28th March 2008, 01:53 AM   #4
jaycee is offline jaycee  United Kingdom
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That makes for about 320mA of bias... possibly half that, if you measured over both resistors instead of over one.
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Old 28th March 2008, 08:40 AM   #5
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Quote:
Originally posted by jaycee
That makes for about 320mA of bias... possibly half that, if you measured over both resistors instead of over one.
No, that's across each one - so am I in the right ballpark then (i.e. leave the bias as it is)?

Also, now that I have dissasembled the whole unit I am thinking about replacing the PS electrolytics as well. The amp is dual mono and each supply uses five 4700u electolytics in parallel per rail (so that's 10 per channel). I can get 8200u caps of the same voltage & temp rating that are identical in size as the current caps. Is this something that I should do at this time - how long do electrolytics normally last? There's no signs of can expansion or leakage around any of them, but they have been constantly cooking for 15 years... or am I really wasting my time (and my friend's money) here considering the parts quality in the amp?
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