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BA312 Distortion Fix

cjenrick said:
we changed the 680 K to 470 K and Viola! distortion gone.

that transistor needed a few more micro amps of base drive, thats all.

poor guy was probably designed right on the edge of starvation.

now we have a way to fix distortion problems without the hassle.

and maybe we can tailor gain both of the channels for equal level. that is the next project, compare output voltage as related to the bias resistor.

we View attachment 1942165 hope this helps others with similar problems.
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I just stumbled on this thread, and I will definitely have to swap out my resistors also! I've always had mysterious distortion on my 2275 in one channel. What value have you settled on at this time? Same 470K? I'm seeing some posts with other values that also worked. Thanks!!
 
absolve2525 said:
I just stumbled on this thread, and I will definitely have to swap out my resistors also! I've always had mysterious distortion on my 2275 in one channel. What value have you settled on at this time? Same 470K? I'm seeing some posts with other values that also worked. Thanks!!
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I used 430k because it was what I had on hand and was reasonably close to the original 470k that was mentioned by @cjenrick.

It does seem that others have used lower values, such as 220k, and the value of 150k was used by Marantz themselves in the 2325, and 150k was also used by Sansui in the 9090. So it appears we have a range from 470k down to as low as 150k. Like I said, 430k is working so far for my 2275. Do let us know what you use and how it works out for you.
 
I am resurrecting this thread again just to thank the OP. Troubleshooting a customer's 2250B, I tracked right channel distortion to that channel's BA312. In process of looking for a BA312, I stumbled upon this thread. I changed the 680k resistors to 470k for both channels and the right channel distortion is gone. I did both channels 'just because'. Anyway,... many thanks to the OP!
 
harlest said:
Would a 470k 1/4 watt work? I happen to have 1/4 watt on hand but could order whatever is needed. I checked the service manual and 1/4 watt is fine.
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That value should work for you, and the 1/4W rating is totally fine since these resistors see negligible dissipation in this circuit. This resistor is biasing the base of the input transistor in the BA312 - in other words only a tiny amount of current is passed.

The resistance value is not hypercritical, the principle is just that it should be lower than the stock value (and yes this is a great PM item when working on a PE-01 board). I have used 430k myself in 2275's and 2250b's very successfully, and from my research there have been other AK'ers who have used even lower values, such as 220k. And most telling of all, the value of 150k was used by Marantz in the 2325, and 150k was also used by Sansui in the 9090. So the evidence suggests we are fine within a range from 470k down to as low as 150k.
 
GlasgowGrip said:
That value should work for you, and the 1/4W rating is totally fine since these resistors see negligible dissipation in this circuit. This resistor is biasing the base of the input transistor in the BA312 - in other words only a tiny amount of current is passed.

The resistance value is not hypercritical, the principle is just that it should be lower than the stock value (and yes this is a great PM item when working on a PE-01 board). I have used 430k myself in 2275's and 2250b's very successfully, and from my research there have been other AK'ers who have used even lower values, such as 220k. And most telling of all, the value of 150k was used by Marantz in the 2325, and 150k was also used by Sansui in the 9090. So the evidence suggests we are fine within a range from 470k down to as low as 150k.
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Thanks for the reply and explaining that resistor value. My 2250B sees everyday use so I have to keep her going.
 
 
Has anyone measured the removed 680K resistors to see if they had drifted high? Just curious.
What a great fix. So far, I've never had a BA312 give me trouble, but I'll be sure to remember this, and probably just change those resistors as a preventative measure. I love little things that advance the restoration of these units to a higher level.
 
@tarior: These are just bias resistors, the two pairs I have removed all measured fine (well within spec). This seems to be a case of some sort of "drift" in the transistors embedded in the BA-312 packages, where changing the bias point brings them back into a normal operating state.
 
GlasgowGrip said:
@tarior: These are just bias resistors, the two pairs I have removed all measured fine (well within spec). This seems to be a case of some sort of "drift" in the transistors embedded in the BA-312 packages, where changing the bias point brings them back into a normal operating state.
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Wondering if changing resistors to fix the problem is the right thing to do.
Since the BA-312 transistors seem to be drifting, wouldn't replacing them be the right thing to do?
 
kwayde said:
Wondering if changing resistors to fix the problem is the right thing to do.
Since the BA-312 transistors seem to be drifting, wouldn't replacing them be the right thing to do?
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I'd say Dealer's choice, depending on your specific case. Just my opinion.

If changing out a resistor with a 20-cent metal film doesn't solve an existing sound quality problem, then your only choice is to replace the BA-312. Otherwise, it seems we are maximizing originality of the unit by extending the life of the BA312 with a cheap resistor swap. I will say that when I first ran into one of these cases in a 2275 and the resistor swap fixed it, I was ecstatic!

And of course if you do happen to find an outright failure of a BA312 then you are replacing it. The BA312 is long obsolete, so you probably need really good luck to find any new old stock. Hence there are 3rd-party replacement boards that "should" equal or exceed the original performance.

FWIW, I designed my own BA312 replacement PCB using SMT components, but haven't actually needed to build one yet. If I ever get around to building one, I'll post some measurements.
 
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