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New Project Marantz 2250b
New Project Marantz 2250b
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Old 12th February 2018, 09:49 PM   #51
pjf610 is offline pjf610
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Hi Mooly, since i have to pull out the board what would be a good replacement resistor on this there is no easy way around this so since the board is out might as well try to get it right the first time. This is what is in there. Also these are the transistors i am using in place of the blown original MJ21193/MJ21194
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Old 13th February 2018, 07:42 AM   #52
Mooly is offline Mooly  United Kingdom
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New Project Marantz 2250b
Well the problem with this is that because the semiconductors are slightly different we just don't know for sure, and so we can not easily derive or calculate a suitable value. It has to be done by trial and error.

These older circuits rely heavily on the characteristics of the original parts and there just isn't enough leeway in the adjustments to accommodate alternatives.

Its an interesting project but it needs the time and effort putting into it.

Can you not get at the part from the top of the board ? Tag some wires to the resistor already fitted (leaving it in place) and bring them outside the amp. Then you can try different values and see what seems suitable.
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Old 13th February 2018, 10:34 AM   #53
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OK will try this, back too you soon
Thanks
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Old 13th February 2018, 01:02 PM   #54
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Hi mooly well I added the 1K resistor and was able to get it down to 5.1mv
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Old 13th February 2018, 01:05 PM   #55
pjf610 is offline pjf610
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The R719 is extremely sensitive I mean extremely sensitive
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Old 13th February 2018, 01:08 PM   #56
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The closest I can get to is 7.9/8mv
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Old 13th February 2018, 01:37 PM   #57
Mooly is offline Mooly  United Kingdom
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New Project Marantz 2250b
8 mv isn't to far off the mark and equates to 40 milliamps bias current through 0.2 ohms. That is good enough to allow you to carry on troubleshooting for now. When it is all fixed you may need to look at reducing the value of resistor a little further.

The next step is to confirm the DC offset as measured at the junction of the two 0.2 ohms is OK. It should be less than 100mv.

If that is OK then the amp should be basically functional and you should have audio.
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Old 13th February 2018, 02:45 PM   #58
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Is that the R732 and R733 that I am measuring the DC offset from??
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Old 13th February 2018, 04:39 PM   #59
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If so I get 3.5 and 3.6 receptively
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Old 13th February 2018, 05:31 PM   #60
Mooly is offline Mooly  United Kingdom
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You are measuring the DC voltage from ground to the junction of those two resistors. You should not see more than a 100mv.

You should also check the voltage across the zener H701 and compare ithe reading to the good channel. They should be similar to within a volt or so.
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