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Old 27th December 2012, 06:13 PM   #11
jaycee is offline jaycee  United Kingdom
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Having read the schematic now, it is a very simple circuit. Don't worry about the offset voltage unless it's higher than 50mV. You could probably use any PNP transistor such as a BC556B but the pinout would be different, so lets keep it simple and get an appropriate substitute part Stick with the 2SA970.
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Old 27th December 2012, 06:32 PM   #12
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Ebay has both of those transistors brand new.

2SA970-GR X 10 pieces, 6.40 inc delivery (from China)

or

2SA970 X 5 pieces, 3.20 inc delivery (UK stock) with best offer!

So either way its peanuts if it fixes the problem .

The latter would be here a lot quicker I reckon. If the GR are worth the wait Il hang on for them.
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Old 27th December 2012, 06:42 PM   #13
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Also check the transistors C05, 06, 07., 08 09 and C10. 7All of theese is capable of making noise in theese amplifiers.
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Old 27th December 2012, 07:01 PM   #14
jaycee is offline jaycee  United Kingdom
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Beware of buying such parts from eBay, especially if the seller is from China/Hong Kong. There is a high possibility of you getting fake parts.
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Old 27th December 2012, 07:07 PM   #15
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10x BC556B 1.38, delivered, London seller! Excellent feedback.

Maybe these?
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Old 27th December 2012, 07:15 PM   #16
jaycee is offline jaycee  United Kingdom
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They should work, but be aware the pinout is different. You'll have to be a bit creative on installing them. Compare the datasheets to see:

http://www.alldatasheet.com/view.jsp?Searchword=2SA841

http://www.onsemi.com/pub_link/Collateral/BC556B-D.PDF
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Old 27th December 2012, 07:27 PM   #17
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Creative? Is it just a case of figuring out which pins go where?
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Old 27th December 2012, 07:32 PM   #18
Mooly is offline Mooly  United Kingdom
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OK, so first things first and if your not sure on anything then ASK

All tests and measurements are done with NO speakers connected.

I'm assuming you have the amp with top and bottom covers removed for access. First step is a visual inspection for obvious problems such as dry joints. In particular look at the the leads on ALL the transistors. Spotting problems comes with experience but here's a couple of typical potential problems in these pictures. Notice how the solder is cracked in a ring around the component leads.

Click the image to open in full size.

Click the image to open in full size.

It would be useful to measure a couple of voltages at this point. Put your meter on DC volts and on a range high enough to read at least 50 volts. Connect the BLACK meter lead (make sure you have them the right way around in the meter too) to either the NEGATIVE speaker terminal or the metal chassis somewhere. Both should be at the same point electrically. Now with the amp on measure the voltage on Fuse 1 and Fuse 2. One is left output and the other right. Both voltages should be very very close to zero.

That's the first step. Report back with the voltage readings.

The second step is to see if the output stage is drawing current. Read the manual where it says "adjustment instruction". All you are doing here is measuring the voltage ACROSS each of the four 0.47 ohm resistors. For this you need to use BOTH meter probes of course so remove the black lead from the chassis and measure across each 0.47 ohm. Polarity of the meter here doesn't matter. All that happens is you will get either a minus or a plus result. The voltage should be around 2.4 MILLIVOLTS. Thats tiny so you will need the meter set to a range that can measure low voltages. The voltage may be somewhat wide of the mark and may increase as the amp is on. Thats normal. What we are looking for is a zero result or an excessively high result both of which indicate a problem.

Remember that an accidental short with the meter leads... I was going to say it happens to the best of us... (twitchy fingers )) will probably result in instant damage to transistors so take your time and be confident.

Again report back with the readings. We'll do it for BOTH channels so we know where we're up to. Those readings won't in themselves find the problem but they will give a good idea of the state of play and will get you used to handling the meter
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Old 27th December 2012, 07:36 PM   #19
jaycee is offline jaycee  United Kingdom
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Quote:
Originally Posted by peacocksuit View Post
Creative? Is it just a case of figuring out which pins go where?
Yeah. You'll have to bend the leads into the right places. Might need some insulation on the leads so they dont short.
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Old 27th December 2012, 07:42 PM   #20
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Ok mate just waiting on the test meter to arrive. Looking forward to have a good poke about in there now . Il let you know how it goes.

Cheers again.

Quote:
Originally Posted by Mooly View Post
OK, so first things first and if your not sure on anything then ASK

All tests and measurements are done with NO speakers connected.

I'm assuming you have the amp with top and bottom covers removed for access. First step is a visual inspection for obvious problems such as dry joints. In particular look at the the leads on ALL the transistors. Spotting problems comes with experience but here's a couple of typical potential problems in these pictures. Notice how the solder is cracked in a ring around the component leads.

Click the image to open in full size.

Click the image to open in full size.

It would be useful to measure a couple of voltages at this point. Put your meter on DC volts and on a range high enough to read at least 50 volts. Connect the BLACK meter lead (make sure you have them the right way around in the meter too) to either the NEGATIVE speaker terminal or the metal chassis somewhere. Both should be at the same point electrically. Now with the amp on measure the voltage on Fuse 1 and Fuse 2. One is left output and the other right. Both voltages should be very very close to zero.

That's the first step. Report back with the voltage readings.

The second step is to see if the output stage is drawing current. Read the manual where it says "adjustment instruction". All you are doing here is measuring the voltage ACROSS each of the four 0.47 ohm resistors. For this you need to use BOTH meter probes of course so remove the black lead from the chassis and measure across each 0.47 ohm. Polarity of the meter here doesn't matter. All that happens is you will get either a minus or a plus result. The voltage should be around 2.4 MILLIVOLTS. Thats tiny so you will need the meter set to a range that can measure low voltages. The voltage may be somewhat wide of the mark and may increase as the amp is on. Thats normal. What we are looking for is a zero result or an excessively high result both of which indicate a problem.

Remember that an accidental short with the meter leads... I was going to say it happens to the best of us... (twitchy fingers )) will probably result in instant damage to transistors so take your time and be confident.

Again report back with the readings. We'll do it for BOTH channels so we know where we're up to. Those readings won't in themselves find the problem but they will give a good idea of the state of play and will get you used to handling the meter
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