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Old 16th September 2005, 04:43 AM   #1
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Default help me narrow down the distortion in my marantz 2238B

hi folks,
this marantz exhibits a curious distortion where the left channel waveform jumps up and down, causing scratchiness - an intermittent DC bias in the signal. my research before opening it up indicated that the problem was unrelated to any controls and was present in the preamp output. so i began to look on the inside.

i traced the signal in and onto the tone control board, where i discovered a point exhibiting the problem and one capacitor which shows it only on one side. it was suggested to me that the problem was coming from farther down the audio path from there, but since it's also evident downstream where the signal arrives at the power amp, i haven't learned any more about its cause.

most of this research has consisted of sending in a sine wave and comparing the waveforms of a known-good or -problematic signal; two similar curves appear, one moving around. i use a very low volume so as to magnify the effect of the bias relative to the distortion. if i recall, the signal jumps around by as much as 10mV.

i'd love to hear what you all think i should do differently or further.
thanks,
aaron.


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Old 16th September 2005, 01:34 PM   #2
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Hi Aaron,
The circuits you are troubleshooting use feedback. Therefore the faulty signal will show up throughout the entire signal path in that circuit. There is are two buffer circuits to look at. Since it is working, the troubleshooting is much more difficult. I would pull CE09 or CE10 and check the signal at those points first. Once you figure out whether the problem is in the input buffer or the output buffer, replace both transistors at once in the defective buffer. It isn't worth tracing it any further than that. I'd be looking at those electrolytic caps also.

Note that all these electrolytic caps are very old. Why not just change them all while you are there? The cost will be low. No audiophile super caps, just new good caps.

-Chris
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Old 18th September 2005, 01:45 AM   #3
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hey chris,
thanks for your suggestions, right on about the feedback - that hadn't occurred to me. perhaps it would be a good option to disconnect the feedback and thus narrow in more easily on where the problem first appears. this might require the schematic, though (available just under $20 on ebay); i don't know how to locate the buffers you spoke of; perhaps you could comment on what CE09 and CE10 seem to mean - i already tested the potential across them, which was nil.

i am not at this point replacing all the caps because i still have serious doubts about my ability to get it working. i am just an earnest amateur who has not studied fine points of amplifier design - a lot of the time i don't know if i will need to extract parts for testing or can do them in circuit. it's a long learning process, i suppose... thanks again for your help.

aaron.
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Old 18th September 2005, 02:28 AM   #4
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Hi Aaron,
CE09 and CE10 are the capacitors that separate the two "halves" of this circuit. They should be marked on the board. Removing one lead and checking the signal will cut your work in half.

The buffers are composted of pairs like QE01 and QE03 on the diagram. It is possible that a coupling capacitor is causing your problem as well. Use logic and the forum here to home in on the problem.

-Chris
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Old 18th September 2005, 08:09 AM   #5
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Hi Aaron
You don't need to buy the schematic check here.
http://dummyload.com/marantz/schmain.html
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Old 29th September 2005, 04:11 AM   #6
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Default little progress

OK, made the donation and read over a couple of horrible books on fixing electronics. now back to the problem at hand.

chris, i determined that CE10 is on the relevant (problematic) left side. with the cap removed i only measured constant potentials at its sides - ie, no trace of the intermittent bias, so i figured the problem was problably in the second "half" of the circuit, the output buffer, if i am correct. however, i measured the base and emitter voltages of all four transistors and in no case did i see the interference.

i also tried shorting those two pins as i read in one of those books - that if such a move eliminated the problem, that transistor would be suspect. in all cases this moved the waveform, mostly making it very weak, but the jumping problem was apparent all over.

it is not clear to me if the feedback is contained within the tone circuit or spans multiple amp stages. should i start disconnecting the sections to try each separately? also wondering if i should now verify the condition of the output transistors - i haven't got one of those curve tracers, so through scoping their collectors with a given input signal?

thanks, all, for your help.
aaron.
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Old 29th September 2005, 02:00 PM   #7
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Hi Aaron,
Take a deep breath. You will measure the transistors to be okay. The reason is that they are mostly working, one may be noisy. Same goes for the caps. If the channel was dead the hints you were given would be helpful. BTW, never short transistor leads. In a different circuit it may have caused mass destruction.

Try to jumper from the positive of CE10 to the negative of CE20(I think, it's 1uF 50V) with another capacitor. Positive to the CE10 side. This bypasses the tone circuit. See if the cracking is gone.

If the crackling disappears, replace CE10 and 20, and all the other caps between them, except maybe the small eq caps. If the unit still crackles, replace QE06 and QE08. If the crackling persists, replace the small caps with the same values.

If the crackling is still there, the fault may lie before the tone control in one of the switches. Otherwise, replace CE10 to CE02. If there is still crackling change QE02 and QE04. The fault may be weirder, but I've suggested replacement in order of the parts most likely to cause trouble.

Don't feel bad, many techs hate this fault and have failed to repair this circuit properly, leaving it to a more experienced tech. But you have help on this board.

-Chris
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Old 13th October 2005, 06:20 AM   #8
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chris,

despite my repeated disappearance, i'm real psyched about the detailed set of suggestions you gave. i have not had the experience of systematically replacing components as you mention; i guess my tendency is to stick with the diagnostics until i can identify the fault for sure, but perhaps your method is more effective for tough problems.

i think what you read as CE20 is CE24. i tried the jumpering procedure and the crackling persisted. on a hunch since CE24 was still in the circuit, i jumped to its other terminal with interesting results: no AC signal at all, just a jumpy flatline on the scope. by this i concluded that the problem would seem to be outside this circuit.

however, i then jumpered the wirewrap terminals where the signal seems to come into and leave the board - JE02 and JE24. this eliminated the problem. so it seems the issue is on that circuitboard indeed. i guess my next step will be replacing all the caps in the left channel; and given that, i might as well replace the ones in the right channel too - but this is a slippery slope. perhaps i should, instead, remove the caps one at a time and check them individually somehow so that i know when i've found the problem? i mean, i think i can replace all those parts, but i'm not sure it's the most effective way to proceed.

thanks so much man! chris, you've been beyond helpful.
aaron.
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Old 13th October 2005, 02:17 PM   #9
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Hi Aaron,
Part of my rational is that time is money. Since the capacitors are both old and inexpensive, I'd just replace the lot. A more experienced tech may chase it down to the part (I am like that), but that approach becomes pointless and frustrating to a new tech. As you get more used to reading the clues in a circuit, you become more able to troubleshoot to a part. In some cases it's just well advised to change a group of parts. This is one of those cases.

When I tackle an old tube radio, for fun I'll chase down the actual defective parts. I will still change most of them. This is especially true in an amplifier or tuner since sound quality is generally improved at the same time.

So change the caps first Aaron, there is nothing wrong with that method.

-Chris
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Old 14th October 2005, 03:03 AM   #10
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great, chris. radio shack in my neighborhood didn't have the right ones (only nonpolarized for several of them) so i ordered them all online at much better prices in much greater quantity (10s). in a couple of days the tone control board will have all new electrolytics, and i will let you know how it goes. thanks again chris!!

aaron.
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