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Marantz 4100 Right Rear channel noise

mgrisoli

mgrisoli

Active Member
Hey folks,

One of the channels for a Maratz 4100 quad that I'm recapping is making some noise.
I'm trying to troubleshoot and I isolated the problem to a section of the circuit...

This is a snippet of the P600 (right channel) schematics
upload_2021-9-7_14-0-0.png

The problem can be confirmed by measuring some transistor voltages:

H007 B: -31v, C: -11v (-0.01v), E: -31v
H610 B: -14v (-0.1) C: -14v (-0.5) E: 0.06v

R645 and R635 are not burnt but are showing overheating discoloration.

All other transistors of that section seem to be measuring nominal.
I have tested H621 and H625 off board and they seem fine as well.
I didn't find bad resistors.

If someone could help me on troubleshooting this I would greatly appreciate.
 
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merlynski said:
Describe the noise, please?
Click to expand...
It is not like the typical transistor crackle when the amp is idle and you hear those pops and hisses randomly
It is also not a constant hiss like a ripple current would cause.
It is a distortion that varies with the intensity of the sound, a little more pronounced when lower frequencies come in like drums and much more present when you increase the volume.
As if it is clipping or saturating way too soon.
 
Last edited:
petehall347 said:
0.5 ohms emitter resistors ok ?
Click to expand...
Yup, that IS the next question.
The driver transistors will output low level audio because the feedback and gain stage will push them hard enough to get some signal through their emitter resistors (R653, R651), but as the signal level rises they cannot drive hard enough without the current gain of the output transistors. Then you get a distortion glitch where one output transistor is conducting through its emitter resistor on one half cycle, and the driver transistor tries to keep up on the other half cycle . . . if you have an open emitter resistor, bad output transistor or an open connection to an output transistor, Those are the things to look for at this time.
 
Yup, you both are exactly right. R657 is as burnt as it gets.
I have replaced with a temporary res and it seems the problem is gone.
I need to place an order for the real thing now.
Satisfying (well, almost) is that I was capable of thinking about that possibility all by myself but I did a poor job documenting what resistors I tested.
This is always a humbling experience.
Thanks for the help so far, I let you guys know if I it all worked well.
 
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