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Yamaha A-760 II, DC-Offset rising up to 1200mV

darko

darko

... and never give up!
Hi AK'er,

This is a PITA Thing, I cant take longer alone.
Off Course, my Skills in Electronics Vintage Gear is not professional.

So, I need Your Help :-)

What is a common Failure, when DC-Offset goes up and up.
More warm, more DC.

When its cold, the DC is only 50-90mV (R,L).
After 1-2 Hour playing, the Sound interrupt.
The Relay is switching off. - good :-)

I search for Shematics - Modell A-760 II. No Way to find, ether on UPay.

First Thing is repalcing old Caps. I did it. The whole Set... 3 Days ...

Its a very nice little Amp, I would like to bring'em in good Shape again.
The Bulbs ( 14.5VAC-80mA ) are hard to find too. Iam in Switzerland :-|


In mostly Case, when Cap-Couppled Output is present,
-> WHAT CAN RISING UP SO CONSTANTLY the DC-OFFSET,
at the Speaker Terminals and Switch OFF the RELLAY ?

The Power Supply seems to be good. No Changes when warming up.


For ANY Help, I would appreciate very much :-)
Maybe also a Shematic for the MK-II, would be a very big Help.

Cheers,
Darko
 
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You might check if the pots used to set the DC offset are still good and replace if necessary (that is, if the amp has pots for that, but I would think so ?).
 
Hey Stoffie,

Nice to see You here :-)

Malhereusement pas...

Only B+/- and Bias L & Bias R

No DC-Offset Pots. Capacitor couppled.

The Big Ones are from "ELNA". 2x 15'000uF at 53V

I dont have this Range here. So I cant replace now.
Also the Scope is in the another House, where I stock Speakers too.



Greetings from the Zürich Area :-) Veeery rainy Days ... to cold ...


Cheers,
Darko
 
Thank You Stoffie, thats the right Type of this Modell.

( Links? Iam not sure about! Its the Icon "World, with Chane" I supose? )


I take a closer Look at this Shematic.
Will report here.

Cheers,
Darko
 
Ok, Manual ordered:

"Your Order Has Been Processed!

Your order has been successfully processed! Your products will arrive at their destination within 2-5 working days."

2-5 Days to waiting? Ok, no big Deal.

Will report here,

Cheers,
Darko
 
Yeah I've ordered a couple of times from them before and it can take a day or two, but usually only one day. :) Hope you can figure it out !
 
A-ha! :-)

I hope so. Iff not, I know a good Fellow.
He have fixed my Sansui 5000A ( this, with the first Driverboards, outch .. )

In every Case, the fine working TA-5650 are still here :-)))
( 1x4650,1x5650, 1x4810, 1xdef.5650, 1xdef.5550)


Cheers Stoffie,

Darko
 
Hi Guys,

Last Steps:

- Last Night replacing temporarly the 2 Big 15000uF Caps with others.(18000/63V)
:= Left Channel only 0.004VDC, rising slightly up and up slowly too, until the Right Channel is too high for Relay.
:= Right Channel ~0.070VDC, rising up fast, until Relay klicks out at 1.2VDC
( per Second: ~ 1-5mV higher and higher )

- Today after Noon, the Shematic is here :-)))))) ( take only 1Day to get'em )

- This Evening: ? Maybe a Try, to not smoking up the Driverboard. ):-)


Will rapport here.

Cheers,
Darko

ps: the left Channel sounds very great and clean -> :-))) ( old Dyna's 2-Way, Back close to Wall )
 
Ok, thats a improvement:

Have Tonight checked a few Voltages:

- Bias, Left CH. 6.6mV ->OK, DC: 0.004mVDC, stable
- Bias, RightCH. 6.7mV ->OK, DC: 0.953mVDC, stable at this Value.
- Relay is not klicking out
- B+/- Voltage: 47.5VDC -> Perfect :-)

Left Channel is still tooo high!

Now I have the Shematic AND - I can compare both Channel Values on the Driver Board and Control Board.

QUESTION, what is a Metal Plate Resistor??
 
Hey Stoffie,

Thank You, Iam too happy, when Things move in a good Direction ;-)

Now Iam reading about Resistors and try to configure out,
what the Reason can be, that a Voltage rising up with the Temperature.

I have some little Resistors here, they regulate the Bias -> Wow!
( Metal Plate Resistors, 0.220K/3Watt )

Ok, but the Bias Currents are fine at 6.6mV. Both.
It must be a Cap - but, I have changed already all on the Board.
More higher ( longer play ) Temperature, is more higher DC.

My Skills "should" improve rapidly, otherwise its only a Job ;-)


Cheers,
Darko
 
I'm no tech..... but could something be heating up and then opening up a loose solder joint? I'm thinking of the bias resitors or something similar. Perhaps re-flow some of the joints? A spray can of cold air might help diagnos this. Good luck with it!
 
Hi packrat,

Thank You for the Tips.

Yes, thats right. I used allready a half full Cold-Spray ( Tube?).
Have resoldered the Solder-Points very carefully.

The Thing is, I cant figure out with Temperature Up/Down.
Must be something other Issue.

I put here up later a Frame of the Shematic. ( Control Board ).
Maybe a "Pro" can give me some Advise.

CU later,

Cheers,
Darko
 
darko said:
Hi packrat,

Thank You for the Tips.

Yes, thats right. I used allready a half full Cold-Spray ( Tube?).
Have resoldered the Solder-Points very carefully.
.....
Click to expand...

I should know by now that anything I think up is pretty obvious to anyone else! Hope you can figure this one out. (My only other suggestion is maybe a diode or resistor in a regulator circuit has gone bad?)
 
Shematic Yamaha A-760 II - Control Board (Mainboard)

Here is a Capture of the whole Shematic:



The Zener-Diode D314 is a Orange-colored "HZ-6C1L"
Can a 6.1V do the Job too?

Cheers,
Darko
 
Its strange -

Now after Start, the right Channel is ~130mV, Left 4mV.
Than it starts sinking to ~70mV, Left Channel still 4mV.
Couple Minutes later climbing up to ~200mV. L:4mV.

The whole right Channel Part (its symetric architecture) is much warmer.

I cant measure a Difference in Supply Voltage for each Channel.

Its Time to get the Osziloscope back in Action :-)
Maybe some Oscilation is here in the Right Circuit ??

Fazit: its harder as it have looks at the Beginning ("aah, some dry Caps..?")


Cheers,
Darko

PS: its Friday :-)))
 
Have adjust the Optocoupler Trimmer from 0.908 to 1.200VDC

Readjust the Main B+/- from 49V to 47.5V ( after 2 Hour playin Diana Krall )

Readjust both Bias to 6.6mV

Result in: DC-Offset now ~30mVDC, Right Channel :-))) Left:3-4mVDC :-)))


It must be Diana ...


LOLLOL

ps: the new Bulbs are very nice to see in Function: 14V-80mA!
 
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