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Old 27th Sep 2020, 8:17 pm   #1
bigfathairyvika
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Location: Cambridge, Cambridgeshire, UK.
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Default Pioneer A80 Dynamic Power Supply Audio Amplifier

Well, after many hours fault finding and removing and testing components I managed to fix a Pioneer A80.

These amplifiers really are a pain in the rear end, especially when your not the first person to have had the cover off.

I have three of these to repair , one down two to go.

The now working one started as "no sound".
Removing the cover it was obvious that the power transistors had been desoldered to check for shorts.
They were indeed shorted C to E.
A spare set supplied by the owner were fitted and the amp run up with a variac.
The amp went immediately into protection mode.
So, I down loaded and read the Pioneer amplifier repair document ( produced by pioneer ) and spent a few days reading. ( its a long document )

The amp was going into protection due to dc on the output. So as per the document checking of transistors and resistors in the driver circuit was required.
After checking numerous components I found several resistors (150R) reading high, one fault transistor and a diode. All parts now obsolete ( R's excepted) so equivalents fitted.
Still into protection, so checking the dynamic power drivers , one was shorted C,E AND B. Again, unobtainium, equivalent fitted, luckily so far the transistors involved are just for DC control. Also on this board two resistors open circuit. Blimey these things are almost as bad as smps...

Amp now out of protection but moving the volume control off zero results in protection due to an oscillation.
So, going for it this time , all the capacitors are stuck down with a rock hard glue, took a while to remove and clean up the board and replace them.

Woo hoo.! Working, set bias, check dc offset and on to testing.

On reassembling the amp now decides it liked protection mode so uses it again.

Boards out, wires checked, several joints resoldered, no luck. I then recut the ribbon cables which use clamping connections to the boards, this solved it.
Boy I hate those connectors.

So, after a few simple repairs of the power light and a dicky selector switch it all works.

The only thing not repaired as the customer is happy with, is a very very slight bit of dc scratchy from the volume control on the left channel. This only lasts for about 30 seconds, so something it warming up as explained to the owner it could take ages and ages to fault find this , so wait till it does it all the time then fix. ( it isn't the volume pot needing a clean ).

Wish me luck I've another two to repair.....


Mark
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Old 29th Sep 2020, 9:57 pm   #2
Julesomega
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Default Re: Pioneer A80 Dynamic Power Supply Audio Amplifier

Well done! Pioneer seemed blissfully optimistic about how much heatsink is needed for a regulator dissipating several watts
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Old 30th Sep 2020, 8:59 am   #3
vinrads
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Default Re: Pioneer A80 Dynamic Power Supply Audio Amplifier

Well done Mark nice write up there, good luck with the other two ,at least you have had some experience with the first one . Mick.
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