There are two different methods depending on your amp board and I believe it's to do with units of serial number being below 3901 or above it but I'm not quite sure of the number. Take a look at the service manual error sticky thread, I believe the info is in there.So it is the matter of setting the idle current, right? I can set with no problem the 35V primary voltage, then DC Offset fairly close to zero.
Now the Idle Current, however I set it it goes up and up seems to never stop. Same in my 2270 with the round compensating/bias transistor 2SC984 and my other 2270 with the flat one 2SC496. Should it be 5mV for 2270 as stated in manual? I think I saw somewhere it should be 10mV rather.
And what if it's much too high? Will it overheat the amp? Cause more distortion?
Yeah, good, J760 is easily accessible. I may have some success. But when I turn on from cold I have like 1mV and then it goes in 20 minutes up to the desired 5/10mVThe setting depends on where you measure. If you measure between J754 and J760 as recommended by many, the setting is 10mV. Are you giving it lots of time to stabilize? I find that it takes 20-30 minutes to stabilize. I keep an eye on it during that time and constantly adjust it to something under 10 mV. When it rises above about 15 mV, I turn it back down to 9. Eventually, it should stabilize. If not, there is trouble somewhere. One suspect would be the bias transistor H760. For better insight into this, read this thread where expert member Ecluser has an in depth analysis. He believes the design is bad and recommends moving H760 from the main heat sink to the smaller driver heat sink, and has good theory behind his recommendation. Good reading for anyone having this problem.
http://www.audiokarma.org/forums/index.php?threads/marantz-2270-bias-wont-go-below-15mv.322206/
I wonder if KSC2690 would work theoretically, Vbe aside. I don't remember why I bought these some time ago, I like it as it is plastic encased all around without collector at the back like most.If you can't find D669, the following work but don't have as low Vbe, I don't think. KSC3503; JE243G; 2SC495; 2N4923
My other 2270 with the round tube like bias/temp. compensating transistors 2SC984 goes over the target with trimming resistors turned to the limit.^^^ That's quite normal.
Yeah. This is good. Thx. You mean the new transistor goes between the bracket and the radiator with the screw through its hole? You don't really need to cut the bracket, do you? I reckon you cut off the part holding the cylindrical transistor, I don't really get what and why you did to the part holding the diode though that it looks so ... differentI add longer leads to the replacement bias transistor, cut the old transistor bracket and reattach it to the heat sink with the screw going through the hole of the new bias transistor. Don't forget the mica insulator and thermal compound.
Sorry for the poor pictures.
Tom
View attachment 933940 View attachment 933941 View attachment 933942
Yeah. This is good. Thx. You mean the new transistor goes between the bracket and the radiator with the screw through its hole? You don't really need to cut the bracket, do you? I reckon you cut off the part holding the cylindrical transistor, I don't really get what and why you did to the part holding the diode though that it looks so ... different
BTW. I see these white paint marks on the trimmers for marking their position. I have them too in one 2270. I thought it had been made by a user but it seems it was in production. And in mine the marks are where the trimmers are turned to the limit for smallest voltage possible so they had to realize there was a problem with the bias being too high and never too low.
I made Vbe measurements a while ago using an Atlas Peak DCA55:
2SD669=0.68V
MJE243G=0.70V
2SD612=0.70V
2SC495=0.72V
KSC3503=0.76V
YMMV,
Tom