Marantz SA8001 Modification to Eliminate Distortion - Page 2 - diyAudio
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Old 27th July 2010, 02:59 AM   #11
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I have found it ill advised to use any transistor with a KTC prefix, honestly. I avoid them. But, no, have never even seen a 2875.
As to the grounded collector configuration, I do very strongly consider that 'upside down' for an NPN transistor with a positive 'mute on' voltage, as these are generally done, and can say it is an absolute FACT that 2SC2240's only hold up well as muting devices when they are grounded emitter configuration.
And it is also quite common for JFET's to be used in the same capacity, and I have NEVER seen any muting jfet of any type go bad or cause any noise at all. I consider them ideal for the purpose.
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Old 2nd August 2010, 03:07 AM   #12
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Originally Posted by stephensank View Post
Looking at the OS schematic, there is NO WAY that circuit should be able to clip at 2.8V RMS. It should be able to swing damn near 7VRMS or so without clipping. I
So, I quite firmly believe that the problem is actually the muting transistors. I know for a fact that 2SC2240, an otherwise nice transistor, does not like being connected as an 'upside down' muting transistor, with grounded collector like this. When thus configured, they will ALWAYS eventually starting leaking current to ground.
I very strongly suggest that you either disconnect them, or replace them with 2SK117 or 2SK170 jfet, provided the mute line goes negative when unmuted, which it darn well should. I do this on all Nakamichi Dragons I rebuild & they work brilliantly; totally silent operation with no current leakage at all, and no discernible effect on sound. This is to replace the 2SC2878's, which do hold up reasonably well in that funky grounded collector muting config, where I used to install 2SC2240's in grounded emitter config, but the jfet's work better & never go bad.
Other than the stupidly chosen muting xstor config, looks like a nice output stage to me, if rather conventional.
Sorry folks been away building speakers and just got back to this. Your potential diagnosis is right along with what I thought the errant behavior of the unit would point to. The headphone tap is before this muting circuit, it contained "less" distortion but still distorted noticeably on hot CDs. This left the problem upstream and a very easy fix was replacement of the coupling resistors from the DAC. I have 4 other Marantz players, and now this one is the only one I can count on to totally not distort with new hot CDs. To my ears at least, there has been no sacrifice to quality. The lesser output also allows me to use it with vintage pre/amps without distortion peaks.

Not sure how it got this way, but I think your suspicion makes sense. It still pops when you stop or eject the unit. Not sure about that, but not worth messing.

Thanks.
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Old 2nd August 2010, 04:13 AM   #13
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The headphone thing actually confirms my diagnosis of bad muting transistors. The headphone line is broken out from the output of the final stage, isolated from the muting xstors by just a 75ohm resistor. So, while there would be less distortion to the phones, there definitely would be some. Also, the popping clearly is being caused by dc leakage from the bad transistors. This combo of symptoms is very precisely what happens when you use 2SC2240's as muting devices in grounded emitter config., in my experience.
If you don't pull those xstors out, they will progressively start distorting the signal at lower levels, and leak more dc voltage. If you do replace them with 2sk117 or 2sk170 jfets, you simply have to 're-pin' them. These jfets, looking at the labeled side, are DGS(drain, gate, source, equivalent to collector, base, emitter), whereas the 2SC2240 is ECB, so arrange the leads accordingly.
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Old 2nd August 2010, 01:32 PM   #14
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Quote:
Originally Posted by stephensank View Post
The headphone thing actually confirms my diagnosis of bad muting transistors. The headphone line is broken out from the output of the final stage, isolated from the muting xstors by just a 75ohm resistor. So, while there would be less distortion to the phones, there definitely would be some. Also, the popping clearly is being caused by dc leakage from the bad transistors. This combo of symptoms is very precisely what happens when you use 2SC2240's as muting devices in grounded emitter config., in my experience.
If you don't pull those xstors out, they will progressively start distorting the signal at lower levels, and leak more dc voltage. If you do replace them with 2sk117 or 2sk170 jfets, you simply have to 're-pin' them. These jfets, looking at the labeled side, are DGS(drain, gate, source, equivalent to collector, base, emitter), whereas the 2SC2240 is ECB, so arrange the leads accordingly.
Thanks Stephen, this is a good strategy since the popping is certainly not welcomed by my speakers. I will put them on my next Mouser order along with new 1.2K resistors. I still think this unit could use a little trim to the outputs to use with my older gear, but I will do that at the RCA's.
Thanks again.
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Old 31st August 2010, 03:04 PM   #15
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Quote:
Originally Posted by stephensank View Post
The headphone thing actually confirms my diagnosis of bad muting transistors. The headphone line is broken out from the output of the final stage, isolated from the muting xstors by just a 75ohm resistor. So, while there would be less distortion to the phones, there definitely would be some. Also, the popping clearly is being caused by dc leakage from the bad transistors. This combo of symptoms is very precisely what happens when you use 2SC2240's as muting devices in grounded emitter config., in my experience.
If you don't pull those xstors out, they will progressively start distorting the signal at lower levels, and leak more dc voltage. If you do replace them with 2sk117 or 2sk170 jfets, you simply have to 're-pin' them. These jfets, looking at the labeled side, are DGS(drain, gate, source, equivalent to collector, base, emitter), whereas the 2SC2240 is ECB, so arrange the leads accordingly.
Thanks for this helpful instruction, the parts came from Mouser Electronics - Electronic Component Distributor and now the 8001 is working wonderfully again. The JFETs had to be turned around and leads 1-2 swapped. I decided to go with 2.2K ohm resistors instead of the original 1.2K ohm DAC couplers. This is so I can use the unit with some older amps without it distorting. So far, the sounds is as good as I remember it and without any distortion whatsoever. Thanks again for guiding a good fix, now documented for others. The light blue resistors are the 2.2K ohm metal film DAC couplers.
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Old 1st September 2010, 02:49 AM   #16
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Very happy to help. Now all you need to do is get rid of all those horrid Elna caps before they start leaking & corroding the hell out of your board. You probably still have a few years before the leakage starts, but you never know. If you do replace them, use Nichicons if you want the unit to last, and sound good.
Oh, and if any of those Elnas are Duorex model series, I would replace them immediately!
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Old 8th October 2010, 02:55 AM   #17
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I forgot to turn the CD player off while I swapped in the tube amp today (the amps were off). The right channel is now distorted and softer than the left channel. I guess it did not like the swap. Changed cables and turned it off a few times. I think the JFETs on the right got damaged. This has to be one of the most fickle CD players I have ever owned.

Back to the bench
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Old 8th October 2010, 04:13 AM   #18
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I can't think of any likely scenario where doing what you did would cause any damage to the player circuitry. I certainly can't imagine it damaging any of those K170 fets. Although it looks like a pretty sound design, if the final output stage is borderline-stable, I could see one of the output transistors failing, or some other nearby component. I would do a dc voltage check at the junction of the final 2SC2240/2SA970 output transistor pair. If you find significant dc voltage, then the offset would be causing assymetrical clipping, and the indication would be a damaged component or two in the stage. I would advise, in any case, replacing these output transistors with 2SC2235/2SA965, for the sake of reliability and current capability.
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Old 8th October 2010, 07:52 AM   #19
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Tube amp ? Could there be something causing residual high voltage on the tube amp inputs that zapped the CD output stage/muting etc.

Something like a 0.22uf input cap etc somehow charged up to whatever
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Old 8th October 2010, 05:55 PM   #20
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Thanks for the guidance. Yes, the tube amp am sure had somethig to do with it. Often, I can feel some stray electicity when I touch its chassis, so I am sure something discharged through the RCA cable. At least I can just change the JFETs for starters. In the future, I will be more careful. I also will check for DC and consider changing those trannies.

Even after the fix, it poped at the end of CD or upon eject. Whereas before it popped all the time, like between tracks.
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