Problems with a A-760 Yamaha

[flambart]

Hi, I'm trying to restore an old A-760 Yamaha. I've had to replace all the final TR, the drivers and a number of other resistors, transistors and so on...a real mess.

Now, I think I've checked all the semiconductors, but I still have a 45 V voltage on the emitters and bases of every driver and final TR. The power supply is set to -47v/0/+47v.

Anyone has any idea ? I've found some schematics, not so clear but helping me anyhow

Thanks in advance for your helping hand, Paolo

[sakis]

a very detailed 3.1MB schematic is available ( just email me to have it ) but i ma sorry to say that this amp is quiet complicated to repair ...you need to work with original parts and extreme caution

kind regards sakis

[padamiecki]

Originally Posted by sakis

a very detailed 3.1MB schematic is available ( just email me to have it ) but i ma sorry to say that this amp is quiet complicated to repair ...you need to work with original parts and extreme caution

kind regards sakis

Hi Sakis!
Got spoiled A-760 II (with 2 caps in power supply section).
Please e-m me detailed sch because those from eservice are incomplete and lowres. Thank you in advance! padamiecki@poczta.onet.pl

[sakis]

sent ...watch it ...i don't think that is going to be easy
please post your findings here for feature reference and for other forum members to use

kind regards
sakis

[padamiecki]

Originally Posted by sakis

sent ...watch it ...i don't think that is going to be easy
please post your findings here for feature reference and for other forum members to use

kind regards
sakis

Thank you for detailed schematic, however is is slightly different to II version.
Now I can read it easier.

• The symptoms of failure: no sound
• checked: output trans, driver trans: OK!!!
• after powering up output relay switches on with characteristic click.
• weird: measured resistance BE output tran (unsoldered - measuring on pcb board) shows 25 Ohm

unfortunately no time to work on it harder...

[padamiecki]

looked once again

• and the rely do not click, I was wrong before.
• Rail voltages are exact +-48VDC
• TP1, TP2 are +36, +38VDC
• TP3,4 zeroVDC

looks like something is realy burned...
Looking at A-760 schematic and tried to find the parts inside mine A-760 mk II I saw the parts numbering is completely dfferent. The predriver from tripple darl output stage is hidden somewhere and not on main heatsink
will start digging deeper when have time...

[padamiecki]

Hi!
Came, saw, fixed...

The main reason of the failure was sold solder through the tran showed on the picture. His neighbor was slightly collapsed too.
I guess that during production process the soldering wave was too lean, it soldered the part with too low tin alloy. During normal operation those trans became very hot, and poor soldering was overheated which resulted with crunchy solder junction. During shakes, vibrations, etc. the junction started to lose the tin what resulted in breaking the connection.

The second reason of failure was bad connection in the driver connected to a socket with the wires. After I touched this, the rely clicked and the amp started working.

I was lucky to repair it, I was thinking to make it ~1 month…

I hope this will help future repairers

Now I will start to improve the amp, I saw corroded wire near the main cap; I think it might be dried…

My last mark is that is really hard to repair MKII version with A-760 schematic. The parts have different values and designations

Thank you Sakis once again!