Marantz PM-55 - humming on MC mode

Hello,

I've had problem with non-working phono amplifier. Capacitor C409 and C410 were swollen and there was cold joint near transistor Q401 - I replaced Q401 and Q402 with place.
Phono is working now, but there is problem with MC hum very similar to Brown noise on left channel (upper track on schema where Q401 is). On MM there is very little noise, but where volume knob is near 80-90%, so it's acceptable.

What I have done to this moment?
I replaced resistors R431 and R432
Solder again all joints for this channel from schematic
Replacing with the places all transistors Q401,Q402,Q403,Q404 left to right etc.
RCA terminal is ok.
Recap all others electrolytic capacitors.

Could it be problem with opamp NJM2068DD? Other resistors may generate noise? I have no idea and any others clue what it might be.
 

Attachments

  • Screenshot (3).png
    Screenshot (3).png
    1.1 MB · Views: 76
  • Screenshot (4) (1).png
    Screenshot (4) (1).png
    586.7 KB · Views: 74
I'm don't understand what "Brown" noise is. Random noise/hiss?

If the problem is hum, the opamp or resistors won't be the problem. Try exchanging left and right phono inputs to confirm hum problem is within the amp. Inspect wiring, soldering between board and RCA jacks.

NJM2068DD is the opamp in the phono preamp (Q405)?
 
Last edited:
That's not random noise/hiss. It's humming similar to this sound all the time on left channel. The sound is not similar for typical problem with lack of ground. Increasing volume, increase humming.

https://pl.m.wikipedia.org/wiki/Plik:Brownnoise.ogg

I checked RCA terminal. I had even desoldered it and the humming is still able to hear, so it's not problem with cables and turntable. All solder points were checked, resoldered.

Yes, njm2068dd is the phono preamp
 
And you've swapped phono input cables? Problem should remain in left channel if fault lies in amp.

Another diagnostic test is to fabricate two shorted RCA plugs and use them so that you can check for hum with turntable completely removed from suspicion.
 
Last edited:
Yes and that's exactly what happend. Problem is still and only in left channel. Other inputs like cd, aux etc are ok for both.

When I click mono button on amplifier, the sound goes to the right channel, so it's something wrong with this signal line. With and without plugged cables the noise is the same for me.
 
Last edited:
Ok, Here are the news.

I've got new (from my friend) opamp NJM2068DD. We resolder it to my board and there is no hum. One more check with my old opamp and humming started again in left channel.

In summary, I have to order a new opamp. Do you recommend another one or better to stay with NJM2068DD?
 
Excellent! I surmise original must have failed and must have passed power supply hum into signal path. I erred in claiming opamp would't be at fault. :confused: Interesting that other half still worked.

Data sheet says that original NJM2068DD is very fast and has very low distortion. Your post seems to suggest you were given same type from your friend--- am I missing something? I'd stick with original part, though the friend's part might be fine. Can you clarify?

Again, good work!