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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Sat 04, 2017 7:31 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
So I'm convinced the problem must be somewhere on P400.

All caps on the board being new (apart from the two .47uF films which cannot go wrong apparently), what's left as possible faulty are the transistors and resistors: I can envisage a carpet-replacement operation here.
Most of you would probably go about it in a different manner, but with the very little knowledge I possess I'm resigned that I have to compensate with work and spending on parts. Fair enough, only hope it'll get cheaper as I learn.

Anyway, transistors do measure ok-ish but I went on replacing H401, 402, 405 and 406 with KSC1845s anyhow, because I happened to have some available. Not obvious improvement though.
Most of the resistors measured rock-solidly within specs, but R403, 404, 407, 408, 422 and 423 were fluctuating a bit weirdly. I've never measured resistors before so I really don't know whether that's the sign of them being bad. Again, I've replaced R403-404 because I had 150 KOhms available.
Glad to announce there's noticeable improvement, very encouraging. I've placed a lengthy order at Digikey with most resistors known to mankind ("having bits around house just in case" seem to be a good policy for the time being) and looking forward to replace R407, 408, 422 and 423.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Sun 05, 2017 4:12 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 483
Location: Fayette County, Pa
Since you are doing a complete swap out of all the parts I will bring up another consideration. What type of PC Board does your amp have? If it is glass -epoxy you should be fine. But some of these amps use the phenolic boards and I have seen these become contaminated over the years causing leakage between traces. This will skew the specs, and can be next to impossible to completely clean.

You have layouts and component specs and since you are replacing most of the parts you might be ahead to simply fabricate a new board using the layout of the old one. That would allow for testing and verifying everything before simply installing it as a functional block into the amp. Use a glass-epoxy board and you will be better than new. There is no easy way to test isolation between traces without removing components, but if the board shows any discoloration or signs of residue you might want to consider this option.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Mon 06, 2017 7:33 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Thanks for suggesting that CaveRat, only that I don't think I'll need replacing more than R407, 408, 422 and 423. Regarding the material of the board, not sure what it is made of but looks solid enough. The traces are holding well so far, never had a problem yet.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Tue 14, 2017 6:54 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
The smooth sound is restored after swapping a few pairs of resistors. Very pleased with the result, couldn't wish for better.
At some point I also got to replace a set of four transistors, only to find the resulting sound was very unnatural (higher gain than it should) so decided to revert to the originals.

Btw, is there a reason why some of the resistors can be easily measured on the board (and show proper value), while others show wildly wrong values, fluctuating ones or entirely O.L.?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Thu 16, 2017 4:57 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 483
Location: Fayette County, Pa
It depends on where they are in the circuit and what other components are connected to them. A simple example, suppose a cap and resistor were in series. One could take a resistance reading on the resistor and get the correct value. (Assuming the cap is not leaky or shorted) because the meter uses DC to read the resistance, and as far as DC goes that cap is an open circuit just like having one lead clipped. However if other resistors or components are attached, your DC meter voltage would also go through them altering the true value. Example, a resistor and coil in parallel would read through the coil winding as well as the resistor.

If you have a semiconductor attached to the resistor any reading would also go back through the semi. The readings could change depending on whether the semi was forward biased or reverse biased. That would depend on the polarity of your test leads.

Another factor would be residual charges on caps. Even though the circuit may be shut off, a residual charge could be sitting on a cap somewhere. This would skew the reading unless the resistor was out of the circuit As the charge dissipated the reading would change in value.

Three reasons why you never check resistor values in circuit. Always remove at last one side and read only through the resistor in question.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Sat 18, 2017 4:35 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Thanks Caverat, now it makes sense why sometimes they read right but sometimes not.

Now that you mentioned that "as far as DC goes that cap is an open circuit ", you reminded me of another matter: I contemplate replacing a non-polarised cap from a speaker's crossover with a pair of polarised caps of the same value, mounted anti-parallel.
Would you advise for or against that?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Sun 19, 2017 3:56 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 483
Location: Fayette County, Pa
That will work, but remember the values of caps in series will be half that stated. i.e. two 10 mf caps in series equals 5 mf. Adjust the values accordingly.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Sun 19, 2017 6:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4067
Location: Boston, MA USA
Many recommend not using an electrolytic capacitor at all in this application, now that film capacitors of the appropriate rating are much smaller and less expensive than in the past. Film capacitors are inherently non-polarised.

-David


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Fri 24, 2017 6:20 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
I have quite a few electrolytics available so trying to make use of them as a first option.
I'm aware of capacitance getting halved in series and doubled in straight parralel, but the position I was referring to is the one below. I called it anti-parallel, but it may be known as something else.
Attachment:
002991.jpg
002991.jpg [ 2.09 KiB | Viewed 1208 times ]

At second thought, I realise that a good result can be achieved by using two caps (of twice the needed value) in series like this:
Attachment:
00298.jpg
00298.jpg [ 1.53 KiB | Viewed 1208 times ]


Or perhaps, two pairs of series of the right value as an overkill. Could there be any benefit in doing that, say having one series connected + to + and the other - to -?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Mar Sat 25, 2017 3:00 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 483
Location: Fayette County, Pa
I would agree regarding film caps, I prefer them as well. But if you are going with electrolytics put them in series. If you actually measured what happens with the AC signal you would find that for half of the signal one cap is actually wrong polarity with respect to the voltage applied. what saves it is the other cap which is correct. The one that is correct protects the other one from current flowing backwards. In a way you might think of the series arrangement like diodes connected back to back. No current flow because each blocks the other. But since this is AC there will be the capacitive effect rather than complete blocking. Thus the theory behind why this works. But a film cap is much simpler.


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