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Marantz 2245 p750 Board Help

P

PAZee

New Member
I could use some help with my amp rebuild and troubleshooting. I have a Marantz 2245 Serial Number 31730. recently I've noticed diminished audio quality on the right side. I've been able to determine it is the p750 board.

For background I've used the rebuild kits from peace.love.and.music of Ebay to replace components on power supply board, both p750 boards. I also replaced the output transistors with MJ21193G & MJ21194G from the same vendor. All components tested OK and well. Left side p750 board sounds great (although I can't still adjust idle current, another issue after I fix the right side, I suspect the original output resistors, yes using the corrected test location).

Right side board is what sounds terrible. I gave this a bit more work, checking for the usual lifted traces, etc.. I've also gone ahead and replaced the variable resistors (R 762 and R 763), I also pulled and replaced the ASO resistors (R794 and R795) (one measured below spec). The output resistors R788 and R789 were just above spec (~.3ohms so also replaced these guys). Board rail voltage measures right about 37volts, so I think I have clean power coming in.

I got my hands on a scope and with a load on speakers, I see a nice notch in the center of the 1kHz sine wave, which tells me something isn't working. My first thought might have been the ASO crossover, but I'm not sure that's the case or not.

I am looking for some advice on my next approach, as I've pulled a leg on each resistor to confirm function, and pulled and rechecked a bunch of stuff on the board with no smoking guns.

Some pictures attached below. I have done some searching and reading on the forum with marginal success, but I'm at wits end and hoping that one of you guys might point me to how to use the scope and/or voltage testing to properly adjust and get my sweet sound back. Pictures of the board are attached.
Scanned_20250910-1636-01.jpg

Scanned_20250910-1636-03.jpg
 
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I am not able to adjust idle current on this bad right channel, measuring .1mV. I was also not able to idle current on other channel (measures .1-.2 mV) but waveform on left channel is good and sound is OK.
 
Suggest solve your bias issue first, it will take care of the crossover distortion.

Carefully measure dc voltages at the following points on the P750 left then right, black meter probe to chassis/GND
Measure to 1 decimal point

J753
J754
J755
J756
J757
J758
J759
J760
J761
 
I appreciate the help, here's my numbers:

J753 =36.15 V
J754 =11.9 mV
J755 = 36.09 V
J756 = 12.0 mV
J757 = 12.3 mV
J758 = 0.1 mV
J759 = -36.19 V
J760 = 11.2 mV
J761 = -35.94 V
 
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J754 & J760 both look low, but to be honest I don't know much about quasi amps so will take a while to do some study. Did you perform the same measurements on the other/working channel? The differences would then be noticeable.
 
check both R788 and R789, schematic has 0.2ohms(???) check if resistor is open/out of spec.
 
So non-working (right side with crossover distortion)
J753 =36.15 V
J754 =11.9 mV
J755 = 36.09 V
J756 = 12.0 mV
J757 = 12.3 mV
J758 = 0.1 mV
J759 = -36.19 V
J760 = 11.2 mV
J761 = -35.94 V

Working side (left side) no distortion (but note that even though it sounds OK I can't properly adjust idle current on this side)
J753 =36.15 V
J754 =-1.6 mV
J755 = 36.10 V
J756 = -4.0 mV
J757 = -2.0 mV
J758 = 0 mV
J759 = -35.80 V
J760 = -4.4 mV
J761 = -35.84 V

I have pulled and replaced both R788 and R789 (with 0.2 ohm resistor ) I've double checked these after install as well and they seem OK.
 
Are you sure you are measuring the correct test points for setting idle current? There are errors in the service manual idle current procedure for some models, and I believe the 2245 is one of them. Search the Marantz service manual error thread and make sure you are testing the correct pins.
 
I appreciate the comment, yes I believe I'm testing the correct points, if tested per the incorrect service manual, I think you measure the rail voltage.

I want to set this to 10mV. I am using J754 and J760 and results in .1 or .2mV. I have also tried to attach one probe to the metal of top output transistor and one to the bottom, which tests the same thing.

Being that this version of the board has some jumpers and not resistors, I would expect that the bias voltage might be too high out of the gate, but this is not the case.

It would be super helpful if someone could help point me to other points on the board that I can test and verify voltages. I can't seem to find a good place to help troubleshoot and determine if I might have a lifted trace, etc..
 
1000014660.jpg
Posting another image of my sine wave with some higher output/volume. Definitely a crossover issue but still having trouble figuring out how to troubleshoot this.
 
I've got low confidence due to schematic errors, should leave to the Marantz guru but... some questions,

Output H001 Vbe should be 500-700mV, you measure 50-60mV.
Possibly H758 is off.
Plz measure H758 b/c/e voltages to GND. Repeat for H756

The scope trace in post #11, which pin was being measured? Where is the negative half of the sine wave?
 
H758
b=11.22 V
c=-35.89V
e=4.1mV

H756
b=-35V
c=11.25V
e=-35.7V

scope trace at post #11 is the positive and negative from the speaker output, using a 8ohm load and 1kHz sine wave as input.
 
Something about your voltages to H001/H002 don't look right.
Can you please tell me which terminal the ECB of H001 & H002 connect to on the P750 board for both sides?

EDIT: I think it is OK once the correct schematic/layout was determined.
 
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