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Old 10th Nov 2020, 7:55 pm   #1
bigfathairyvika
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Join Date: Nov 2009
Location: Cambridge, Cambridgeshire, UK.
Posts: 489
Default 2nd Pioneer A80 Amplifier Repaired

For this repair the amplifier had already been worked on and two output transistors were definitely popped.

TIP: Before removing anything other than the top and bottom covers, set the speaker control to "A" and remove the knobs and front panel. Then undo the nuts for the speaker selection control and the phono impedance selector. Remove the two screws on the front left and right of the control panel and loosen the side screws. The front control panel can be carefully tilted forward. Carefully unclip the other ends of the two selectors and again very carefully remove the two cable assemblies taking care not to bend the cable at either end. The two assemblies can be put somewhere safe as they can very easily be damaged left in the chassis whilst working on it.

As usual, the two channel driver boards are removed completely.
Both channels had R39 and R40 open circuit.
C20 ( NP, feedback ) was replaced as I had plenty of spares and saves time later.
No transistors were faulty but there was a lack of negative rail.
R42 and R43 10R in the supply rail were replaced even though only R43 had gone open.

The four resistors mentioned were all very close to capacitors glued down and were also partially covered with glue.

R20 which supplies most of the preamp/drivers also had gone high to about 18k from 100R, when the outputs short the drivers shift loads of current which causes the resistors to heat up. The driver transistors are plenty capable of taking the extra current and survive ok.

Although this fixes the driver boards, there are two smaller boards holding the dynamic power supply drivers and one on each had gone open circuit.
Q101 2SC2275 in both cases, replaced with 2SC2073.
Also on this one the .22R double emitter resistor pack was open circuit and replaced with two .22R 5W ceramic resistors.

The amp uses wire wrapping throughout and I just cut the wires to make easy removal of the boards. Soldering the wires back.

Note that when soldering the wires to the headers on the driver boards either make a good mechanical connection first or you have to hold the wires still for a while as they hold the heat so the solder takes time to solidify.

Finally after getting both channels working, there is the balance control on this one that decided to go open circuit at multiple points on the track.
Initially I thought volume control but the alps control is surprisingly reliable on these.
I did a quick swap of the balance and volume boards with a 3rd A80 of which I've been using as a part donor for now until I either repair the pot or get another. ( These are dual gang with one track reversed ).

For initial testing with the driver boards out, they can rest over the back of the amp on a piece of card board for insulation.

When setting the bias current, do this with the bias diode attached to the heatsink and give the amp 10 minutes to warm up.

Surprisingly the recommended setting for the voltage across the resistor is 22mv, + or - 10mv, but it is easy to get it within 2mv. Also ignore the manual when it says to set VR1 and VR2. The adjustment trimmer is VR1 on both channels.

TIP: When removing the driver boards be very careful of the bias diode clipped onto the heat sink. The wires can break off right at the case.
And whilst working on the board desolder this device and remove to a safe place as it is easy to bend the wires whilst moving the board about.

TIP: Before replacing the two selector assemblies, wrap insulation tape tightly around the cable/switch joint to protect the joint from flexing.

Final TIP: The Transformer is ****** heavy and in one corner so only stand on bench with transformer at bottom. eg: on the left side.

The 3rd A80 has yet to be fixed, it appears to have quite extensive component damage from one or two resistors catching fire!


Ta ta for now.

Mark
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Old 11th Nov 2020, 1:05 pm   #2
Mooly
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Join Date: May 2011
Location: Lancashire, UK.
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Default Re: 2nd Pioneer A80 Amplifier Repaired

Well done, these are quite a complex and somewhat diy unfriendly item. Back in the day I bought an A80 based as much as anything on its 'paper' specifications.

I was always a little amused by the dynamic power supply and the fact that the normal 'low voltage' rail was something like -/+60 volts DC.

Given that we generally listen at powers below around 3 to 5 watts as an average... it just seemed nutz

They were certainly a solid lump and that transformer was awesome. The only failure I had on mine over many years and countless hours use was the filament bulbs for the red/green power switch. Seem to remember that failure of one bulb causes the other not to work. It was a very long time ago now.
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