A760II schematic needed

[sesebe]

Hi guys,
Can somebody help to repair my amplifier?
It is an Yamaha A760II. The power transformator have shorted inside and I do not know what voltage I need for a new transformator.
The amplifier is rated at 100W/8ohm and the filtering capacitors are Elna for Audio 15000u/63V.
The power supply is stabilised on primary side with thirystor and optocupler (Yamaha Xpower I think).
A schematic will be verry good for servicing (I do not know the reasone of shorted tranformator).
I have the schematic for Yamaha A760 but it is very different.

Thank you in advance,
Bogdan

[thaumaturge]

Eservice has parts one and three of it. Don't know what's missing.
Yamaha A-760 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics (part one)

[sesebe]

I have that schematic and is not the right schematic.
The internal layout of the amplifier is diferent.

For example the input of the final amplifier at A760 is clasical - single ended and the input of final amplifier in A760II is diferential simetric,
A760 have 4 capacitors and A760II have 2 bigger capacitors.

You can see some diference between this two models.

Bogdan

[homemodder]

Are you sure the transformer is shorted and not the rectifiers or the output transistors ??, it would be better to remove the transformer to be sure.

[sesebe]

Hi,

With output of transformer disconected from amplifier I measure ~20VAC and ~43VAC on output winding, the input was conected to original triac circuit.

The output is duble so the voltage must to be the same.

[homemodder]

Well there you have the answer, the transformer secondary should have +- 43VAC although it seems a little on the high side in view of 63v caps. For 100 Watt stereo you should look at least at 500VA rated one but I know many commercial manufacturers use lesser to reduce costs and for protection.

Before installing new one make sure that other circuitry was not the cause for the failure of the transformer, there might be a shortcircuit somewhere.

[thaumaturge]

I'd check the transformer with direct mains drive, not through triac. 86V center tapped would give you about +/- 60VDC rails. 88 would be closer with diode drops. I'd suspect the triac circuit before the transformer.
Doc

[sesebe]

Thank you for replay's but,
In a good transformer, if I load one of the secondary to generate a drop in voltage from 45-60AC to aroud 20VAC, on the second secondary output I expect to have a big drop of voltage.
My ideea is that the voltage should be much high that I measured.
Another big problem is that the original power supply is made to work from 100VAC (Japonia). My adapter transformer is at 1/2 result that I power the input with ~115 -120VAC.
The first adapter (for 100VAC out) was broken in the same time with the transformer.

Bogdan

[sakis]

THE ...A760 schematic as seen in the picture with 4 capacitors is available .... email me to have it ....


( took a quick look at it but i didn't understand what exactly the control board does .... i presume that something has to do with regulating the input of the trafo or so )

[sesebe]

Thank you Sakis but I need schematics for version with 2 big capacitors.
For version with 4 capacitor I already have.

Bogdan