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G6700 FET polarization

faccan

faccan

New Member
Hi Everyone,
I'am repairing a G6700 and I have some problems as the circuit in manual is not like the one implemented.
Power Rail of the FET circuit is +31 V and -32V (not + 18.2 and -18.2V as is in the schematics)
At the Source Pin of each FET ( 1 to 4) the is 6 V (not 11.3V as is in schematics) (I think this is no right it should be 11.3v)
R15 / 16 is 3.9 K ( and not 3.3k as in schematics)
I guess that + - 32 V are ok as this is the newer version of the board without the zeners, but I think there is a problem at this stage of the circuit as VS of the FET should be 11.3V
Could someone please help me in confirm my guess.
If I'm right, do you know where could be the problem.
Thank you very much in advance.
Regards.
 
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Don't believe the schematics, Sansui changed things all the time during production of these units, there are many many different versions of the F2980 board, and only one bulletin we know of.
If it is original and has not been butchered, then throw the schematic away, its next to useless.

What is the fault? is it working and passing audio? Is it meeting power output and THD specs?
 
Thanks for the feedback. When I recieved it all O/P TR were blowed out. Almost all fuse resistor were open, the main fuse had been replaced by a solded wire, etc etc etc.
I replaced evething burnt and I replaced 4 variable 100 ohm resistor (Vr1 to 4) for 25 turn trimpot. I meassured almost every component in the driver section.
I tested the amp with a 70 Watt Bulb in serie with the main power (220 Vac here) but it showed a kind of problem ( as it remains bright)
I disconnect both main rail supply 63V from both channels and I removed 22 Ohms resistors ( R43 R44 R49 and R50 ) that feed the first stage of the driver amp.
Then I tried with DBT again, and now the beast started to wake up! Of course almost all the stage of the driver circuit were without supply except for the FET's and thats where I measeure the diferences of voltage.
I tried adding R43 and R49 to feed TR01/TR03 TR05 and TR07 and TR09 but I meassure more than 4V over those resistors so I turn it off because think that more than 0.5 amp for this stage is too much. (4V / 22 omh)
I will take out and recheck TR07 and TR09 just to be sure the problem is not there. Any help or advice will be welcome!
 
They did change the power supply in the front end of the driver stage at some stage, I would have to cast my mind back, I believe they increased the voltage and changed the constant current circuit for the FET.
I wouldn't go ahead and think the FET is at fault they very rarely fail or give trouble....

I will digest your post above and see if I can work out what is going on when I get some free time...
 
HI!!!! Some update here!
In a previous repair attempt, someone swiched the position of the main diode bridge.
He/She also change the position of the cables that go to the main capacitors (avoiding the boooom) but He/she didn't change the cables that feed R44 / R433 and R49 / R50, therefore this stage of the circuit received the inverted voltage.

Keep working.

I will update soon
 
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