Discussion in 'Marantz Audio' started by hirscwi, May 9, 2017.
Yes, the unit in question is champaign faceplate. s/n 2801
Thank you and good luck getting your questions answered. Dennis
This chart helped me solve an issue with a 2245 amplifier, which is similar to the 2270. It had widely drifting bias, which started after replacing the original H760 bias transistor with a MJE243G. And bias could not be set correctly at R763. After viewing Tom's list, I decided to try the KSC3503. It's now possible to adjust the bias at R763, and it is reasonably stable now. With the MJE243G it was drifting from 2.0mV to 8mV.
This 2245 is an early champagne face with the older style 2245/2270 amp board. Tomorrow I'll try this on the other amp and see if this fixes the same issue with it.
Thank you Tom!
It's becoming puzzling for me.
Meanwhile I bought Hitachi D669 that I am pretty sure are fake. You can't see it in the pictures but they don't look or feel like NOS at all. Like knocked off yesterday rather. I have never seen such shiny and thin (flat) legs on transistors either.
Could be fake or could be parts which failed at final test.
You are wanting to match Vbe? You can buy a Atlas Peak DCA55 which will measure Vbe. You'll have to buy several transistors to test, or find a modern equivalent, they measure closer than the older transistors because the manufacturing tools have changed in many ways and the process has been well developed. A couple of replacement transistors have been suggested in earlier postings, as well as advising not to buy transistors off the internet because of counterfeit or known bad or fake parts.
Alternately, you can check with a local community college and see if they have a curve tracer, learn how to use it and measure Vbe, find a friend who can loan you a curve tracer or DCA55 or meter that can measure Vbe.
Lastly, you can make up a circuit to measure Vbe or pay Little Diode and get some genuine good parts from Jolly Old England across the pond.
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?
The 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.
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.
Overly high bias will generate excess heat due to increased current draw. I'm nmo amp guru but I don't think distortion rises but perhaps the current will rise far enough to just run away and cook the amp, taking out the outputs most likely.
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 5/10mV
^^^ That's quite normal.
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.
My other 2270 with the round tube like bias/temp. compensating transistors 2SC984 goes over the target with trimming resistors turned to the limit.
But max of 13mV when should be 10mV is no tragedy, is it? Marantz never gets hot really, if you had other brands you know what I mean.
From my experience with other amps setting correctly the primary voltage (35V here) is most important for heat.
I don't think so, as long as it's stable.
Yeah, I think I will stay with the original cylindrical transistors for now.
BTW, this mounting outside the original bracket seems so lame, but hard to figure out anything else or maybe there are better ideas around
Randy is correct (as is usually is), the bias (idle) current is dependent upon serial numbers.
o s/n<3901 – J753 and J754 – set R763 for 5mV
o s/n >3901 - J754 and J760 – set R763 for 10mV
I 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.
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.
Yes, the transistor goes between the bracket and the heat sink (radiator). I cut the bracket off because it is not used with the new transistor. I bend the 'tab' so that it presses the diode against the heat sink as the bracket is now the thickness of the new transistor body away from the heat sink.
The white paint on the trimmers is to secure them from moving after they were adjusted.
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