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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Jan 2019, 11:42 pm   #1
Michael Maurice
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Default Pioneer SA9800 amplifier

I had a look at this amplifier yesterday. There is no output and the protection replay isn't coming on.


Checks showed that the output from the RH channel was at -53V. The -ve supply is -60V.


A check on the output and driver transistors showed no short circuits.


Now these amplifiers are complex and not easy to work on. In fact you cannot work on it when connected to the output transistors as there isn't enough room.


Note that the output is not at either supply rail, nearly by not.


Personally I think it could be serviced by connecting the board, without the power transistors to a split supply and looking to see what happens at the output. When I'm satisfied that the two outputs are +0.7V and -0.7V respectively, then refit it to the amplifier and see what happens.


However, operating the amplifier without the driver board (and making sure nothing was touching or shorting) the relay still didn't come on.


To make things more difficult, the owner has had it repaired three times!


At the moment I've declined to repair it, so what should I do try it or leave it. My money is on Q4 and/or Q6



These amplifiers in good condition sell for around £1200 on ebay.


I've included diagrams for you
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Old 23rd Jan 2019, 12:15 am   #2
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Default Re: Pioneer SA9800 amplifier

I can open the right-hand diagram but not the left-hand one (just get an eternal whirling arrow on a black background as it tries to download.

THere are lots of things in an amplifier like this which can ram the output against one end stop, starting with Q2 open base and gone leaky.
Q6 if not mirroring would let the lower current source Q5 pull everything afterwards downwards. Or an open circuit long tail resistor under Q4 would kill all Q4 current, turn off the mirror and let things sink.

Running the board off reduced voltage from a dual bench supply is the way I'd go.

Running without output transistors is feasible if you arrange resistors to set things up with scaled-back quiescent to suit the drivers AND if you reinstate the global feedback, but from the driver output midpoint.

It doesn't look too terrible

What would worry me is what were the previous failures and were correct components fitted.

A case for going slowly and inspecting very carefully, but do-able and not much worse than any other big solid state amp.

It's a bit oddball in places, but nothing to justify those prices!

David
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Old 23rd Jan 2019, 9:30 am   #3
phut bang
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Default Re: Pioneer SA9800 amplifier

i seem to remember repairing one of these a few years ago with same fault it was not sound output stage causing problem but an electrlytic on protection relay voltage line a few other caps were relaced at same time the faultry electrolytic had slight bulge on top othere were beggining to show signs of leaking around leads at bottom of caps with the age of this amp now i would be looking at replacing the electrolytics in power supply
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Old 23rd Jan 2019, 1:26 pm   #4
Michael Maurice
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Default Re: Pioneer SA9800 amplifier

Maybe the protection is faulty too, but it would definitely not be causing the RH O/P stage to dive -50V DC .
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Old 23rd Jan 2019, 2:13 pm   #5
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Default Re: Pioneer SA9800 amplifier

With the output so far off, the protection circuit should be keeping the speaker relay open, so the protection department may just be doing its job. It's a case of fixing the amp then seeing what the protection circuit does.

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Old 23rd Jan 2019, 3:05 pm   #6
Michael Maurice
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Default Re: Pioneer SA9800 amplifier

Quote:
Originally Posted by Michael Maurice View Post

However, operating the amplifier without the driver board (and making sure nothing was touching or shorting) the relay still didn't come on.

By that i mean also completely disconnected from the protection circuit.
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