New Project Marantz 2250b

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Well your readings seem consistent in that the combined voltage of the two would be around 90mv. That is far to high though. (on the diagram it is R732 and R733)

Also, you measured the DC offset at 0 volts for this channel (which is good on the face of it) but you also said you had no sound.

Different transistor characteristics of your replacements may account for this bias issue but we need to be more sure.

1/ What is the DC voltage between the base of H714 and the base of H715. Put you red meter lead to H714 and the black to H715.

2/ If you reduce the value of R718 (tag another 1k across it) does it help with the bias and allow it to be set correctly ?
 
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Yes, that's it. A quick check of the voltage.

The zener shouldn't be affecting things as long as its OK. Just compare the voltage across it with the on the other channel if you are not sure, or if you know the value then just confirm the voltage agrees within 10% of the marked value.
 
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Those sound realistic values and so that leaves the possibility that it is your replacement semiconductors having slightly different characteristics that is causing the high bias in this channel. That is something we see quite a lot when parts are swapped.

I would try reducing the value of R718 that is across the thermistor and see if that allows the bias current to be brought down.

Try something like 680 ohms to begin with. You could also just tag another 1k across the thermistor as a test which might be easier than swapping.

The bias problem would not give no sound however but that is something to look at later. Lets get the DC conditions correct first.
 
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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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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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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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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