• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

BA312 Distortion Fix

C

cjenrick

Active Member
pain in the behind, these discrete opamps that nobody sells except as a 99 dollar upgrade,

forced to buy a new tone board for 39.95 plus ship?

stalled out project due to waiting for the board to arrive with hopefully a good set of old parts?

Not anymore.

we did a recap on a 2250 B tone board PE01 and noticed one channel distorting.

sometimes worse, then sometimes moving around and getting better,,

checked all the caps, replaced all the outboard resistors,

same thing>1.jpg
 
Register to hide this ad
thanks to AK we found a pinout for the opamp,

then we scribbled in the outboard parts (black ink) , as a hunch, we wondered if the transistors drifted and could be brought back to spec,

we noticed that the bias on the first transistor is taken off the emitter of the output transistor via a 680K resistor>4.jpg
 
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 3.jpg hope this helps others with similar problems.
 
Last edited:
What about replacing those transistors in the PE-01 board with KSC1845/KSA992 pair?
 
yes i replaced those first, as sometimes a bad E-B junction can drag down and distort the signal preceding it.after replacing those transistors, the problem was still there.

so at that point, the only thing left was the opamp,

we then marked the opamp from the distorted channel with a small piece of blue tape, you can see it in the pic above,
this was just in case we were interupted after pulling both chips.

now after both chips are pulled and marked, we done swapped them L to R to see if the problem follows the chip.

sure enuff, the problem followed the chip. now we had the culprit, it wasn't the recap, or anything else we did to restore the board.

at that point, we knew that breaking open the chip and soldering it back together with new components was not an option.

so the only way out of this mess was to pray that we could fix it by way of the external components, how happy can one man be when our prayers were answered! because if that resistor did not work, then we were stuck going down a road that nobody wants to travel.

the scary thing is that the 2250 B sounded fine for a little while, then gradually got distorted. so a good burn in time should be done after all repairs to see if any demons aregoing to crop up.

there could be a lot of receivers out there that are borderline as far as opamp bias, maybe a distortion meter would be handy to check those opamps and perhaps tweak that 680 K. easy to tack on some leads and put different resistors in parallel to look and see if performance is improved.
 
Last edited:
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...
Click to expand...

Excellent troubleshooting; thanks for sharing.
Interestingly, the implementation of the BA312 in the Sansui 9090 receiver is biased higher with a 150kohm resistor.

Sansui 9090 BA312.jpg
 
Just wanted you to know I did what you suggested and changed the resistors from 680K to 470K and after I did, it fixed the distortion! Thanks again for posting this fix. :banana:
 
your welcome!

just fixed another 2275 with the exact same problem, unit worked fine on the bench, boss took it home to burn in/test for a week, distortion magically appeared,

used 220K resistors for R11 and R12, (blue arrow) had 680K in there, sounds fantastic!

wondering if, during recap, CE9 and CE10, 100uf/10V, (red arrow) might drag a little more leakage current as they burn in, thus upsetting bias point,

2275 ba312.png
 
Last edited:
we just changed those resistors on another 2275 just to be safe, this one had 1 M resistors for R11 and R12, we changed them to 270 K and took some bias current and voltage measurements. Marantz was obviously trying to keep noise down by running very low current on that first amp stage, 0.2 micros is pretty low, figure hfe = maybe 500 on first stage and collector current is about 0.1 ma.

side note: this 2275 has been sitting around for a few weeks after the rebuild, there was still some DeOxit liquid on the 7 pin connector when we pulled it off, we will be washing those dexoxit sprayed parts with some CRC QD electronic cleaner from now on, as we do not want any conduction between channels. or voltage nodes, etc

4.jpg
 
Last edited:
Just fixed a 2275 with one bad channel. Went from 680K to 330K on both channels and that did it again! Thanks for posting the fix. :bigok:
 
I know this is an old thread, but I would like to chime in with another success story changing the 680K's to a 220K. I had a left channel distortion that appeared out of nowhere after performing a switch cleaning and a general overall going over (new LED lamps, new velum paper, and cosmetics). I'm glad I ran into this thread. Thanks!
 
Hi. Where did you get colored schematic?
cjenrick said:
your welcome!

just fixed another 2275 with the exact same problem, unit worked fine on the bench, boss took it home to burn in/test for a week, distortion magically appeared,

used 220K resistors for R11 and R12, (blue arrow) had 680K in there, sounds fantastic!

wondering if, during recap, CE9 and CE10, 100uf/10V, (red arrow) might drag a little more leakage current as they burn in, thus upsetting bias point,

View attachment 2181170
Click to expand...
 
I also would like to report another success story for this method with a Marantz 2275:

Right channel seemed fine, left channel was distorted, quickly isolated the PE01 board as the culprit. Analyzer showed typical values of 0.02% THD on Right channel but >10.0% THD on Left. Probing the PE01 board showed that the sine wave on the Left channel was rounded off on the negative half of the waves from the output pin of the BA312 (HE01 pin 6) while Right channel sine looked normal.

Recapping the board made no difference, and changing out the final gain stage transistors (HE03-HE06) on the board with KSA922/KSC1845 pairs made no difference. At that point I had nothing to lose so changed out the Left channel bias resistor as described by @cjenrick... Changed RE11 from 680k to 430k and the distortion vanished! In fact, the Left channel was now much quieter than Right on the analyzer, so I then changed the Right channel (RE12) as well and now they are both looking great on the analyzer! I am going to let this soak for a while to see if there are any temperature effects or other changes and will report back. Hopefully it is stable for a long time...
 
ocdecio said:
FWIW the 2325 uses 150k:
Click to expand...
That's very interesting, @ocdecio, thanks for pointing that out! That also matches the 150k bias resistor used by Sansui in the 9090, as pointed out by @Leestereo above in post #8. So all of the component values on your 2325 schematic are identical to the 2275, except they decreased RE11 to 150k and slightly decreased RE15 to 910 ohms. They also changed the voltages shown at pins 1, 3, 5 and 6.

So the premise behind this entire thread is that the BA312 can be "revived" by tweaking the DC bias resistor, therefore increasing the bias voltage and current available to the input transistor through pin 2 of the BA312. Pin 2 is shown at 0.57V for both the 2325 and the 2275, and I can report that with the new resistor values in place I am measuring 0.589v/0.588v on the L and R pin 2's, and the performance is really fine. It now measures <0.003% THD in both channels with a 1kHz signal into AUX and the PreOut's driven to 1Vrms.

Sadly I did not record the BA321 pin 2 voltages before I changed out the resistors - and I have no intention of putting the old ones back in now. I will say that if I have another Marantz PE01 board cross my path in the future (e.g. 2275, 2250B, 2325, etc.) I will be sure to thoroughly record measurements of the BA312 voltages before and after I touch anything.
 
GlasgowGrip said:
So the premise behind this entire thread is that the BA312 can be "revived" by tweaking the DC bias resistor, therefore increasing the bias voltage and current available to the input transistor through pin 2 of the BA312. Pin 2 is shown at 0.57V for both the 2325 and the 2275, and I can report that with the new resistor values in place I am measuring 0.589v/0.588v on the L and R pin 2's, and the performance is really fine. It now measures <0.003% THD in both channels with a 1kHz signal into AUX and the PreOut's driven to 1Vrms.
Click to expand...
I wonder how many BA312 could have been "fixed" instead of resorting to discrete versions.
 
ocdecio said:
I wonder how many BA312 could have been "fixed" instead of resorting to discrete versions.
Click to expand...
That's a GREAT question! I do wonder how permanent this "fix" might be. If this fix works because the input transistor has "drifted", does that mean it will continue to drift and eventually the distortion will return? Maybe eventually the BA312 drifts so far that increasing the bias current will no longer help or will introduce other problems? I will keep my eye on this one, and in the meantime will likely prepare to go the discrete component module route (3 NPN transistors/4 resistors per channel).
 
You must log in or register to reply here.
Back
Top Bottom