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Yamaha A760 *CPR needed*

T

Thzar

New Member
Hello

I bought a Yamaha A760 with the information that it wont start.
And I had read gigapod restoration of a Yamaha A760 and thought maybe the one I "gonna" buy had the same fault. Just a simple fix with the RFI component. But no =/


I removed the RFI cap.
Checked the pots all showing some ohms.
Power it up, no bulb light. No sound.

I got the service manual and started measuring some control points.
Offset:
L: 2mv
R: 0.2mV

Main B+
Voltage: 49vdc

Photo coupler:
1.1 Vdc

Trafo Sec.
+-9V ac ?

Idlecurrent:
Left 2mv
Right 98 Vdc *wow*
 
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On that right channel, you have apparently taken a measurement from collector to collector. Or both the PNP and NPN transistors are shorted from collector to emitter, and at least one emitter resistor is opened (else it would be blowing mains fuses).
 
Shorted? You mean open don't you.Typicallly emitter resistors are less than 1 ohm. The A760 emitter resistors are .33 ohm. They burn open pretty much most of the time if there bad.
 
But I shouldnt get 98VDC at the NPN emittor if the emittor resistor was the only faulty one, right?

I guess the NPN and PNP is shorted.
 
I just checked the NPN in the right channel, and its dead -> shorted.
So its companion PNP is definitely shorted as well. But I will take a closer look.

Where do I find "new" 30 years old transistors?
•2SD845 NPN power transistor.
•2SB755 PNP power transistor
 
Oh..my.
I found a ugly spot.
Just on the right side of the relay.
The area is much darker than the rest of the board
Here have been some heat.
The area were covered by a rubber pad underneath.
Lots of corrosion.
Many of the parts had no connection.
Quite difficult to remove corrosion without damage the traces, you cant do it with a solder iron.
 
Pictures of the spot?
Thzar said:
Oh..my.
I found a ugly spot.
Just on the right side of the relay.
The area is much darker than the rest of the board
Here have been some heat.
The area were covered by a rubber pad underneath.
Lots of corrosion.
Many of the parts had no connection.
Quite difficult to remove corrosion without damage the traces, you cant do it with a solder iron.
Click to expand...
 
P1020297.jpg



P1020295.jpg

no joints at all for the resistors


P1020293.jpg

Not pretty
 
Last edited:
Right channel distorted A760

Hi,

I may have similar problem. The right channels for speakers A & B have a 'Hum' and the volume is low with a lot of bass.
I have electronics 101 and good with a soldering iron.
Where shall I look first?
Many thanks.
 
ikhan said:
Hi,

I may have similar problem. The right channels for speakers A & B have a 'Hum' and the volume is low with a lot of bass.
I have electronics 101 and good with a soldering iron.
Where shall I look first?
Many thanks.
Click to expand...

check the capacitors near the power supply for bulges on the top and/or goo on the bottom. thats a good place to start
 
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