Marantz Model 1120 high filter noise issue

[rlisin]

I have a Marantz Model 1120 here that I got for maintenance (cleaning, adjustments etc.) and to investigate noise issues. I managed to do everything except this:

1. Less important: there is minor background noise (both HF noise and hum) in the preamp section (there is less noise when PRE OUT and MAIN IN jacks are disconnected). Nothing to worry about, it's inaudible 50 cm from speakers, but if there are known causes that could be eliminated, please let me know.

2. Important: When high filter is engaged, much louder background noise appears in both channels. Sounds more like white or pink noise than hum. It is clearly audible more than 2 meters from speakers. When low filter is engaged, some hum appears, but it's minor, less important. Loud background noise disappears again when high filter is turned off.

Noise issues exist regardless of the position of volume and balance pots and the muting switch, but that's normal because these controls are before the rest of the preamp, only the source selector and phono stage are before them.

My tech checked all components in the high filter circuit, they are OK. Well, one resistor wasn't, but he replaced it and that did not remedy the noise issue. There was not much to check, the circuit is only a few resistors and caps per channel. He also checked the PS, no apparent issues there (one transistor was suspicious, but he replaced it, again no change). He checked the high filter switch itself, it has good contact. We're out of ideas as to what could cause this noise. Any ideas? Is there anything we missed, or maybe something that might test well but still cause this?

This is the part of the schematic with filters highlighted:

And this is the amp, with my 1060 on top

 

[airtime]

the only suggestion I have is recheck that common ground coming in that connects to caps c111 and c112. I don't think it's the caps being it's both channels. but check where the rest of that common ground is coming and going to. And check the grounding as well.

 

[rlisin]

Thanks for replying. Tracks have been checked by my tech too, they were fine. He would have spotted a missing ground (I think).

I had a close listen and determined that while the filter-related noise occurs in both channels, it is a bit louder in one channel. Also, I noticed that there was slight additional buzz when touching the buttons that engage these filters. That could indicate a grounding problem, but I've seen it in other Marantz amps and could never isolate it. I have a Marantz 1070 that has this buzz when I touch the balance pot, no other issues whatsoever.

As it appears not to affect the function of the filters (they do cut their respective frequencies in both channels) or the sound (no distortion, nothing missing except the frequencies that should be cut, and the noise is negligible when music is playing), and the owner uses line sources only (no turntable) so he does not use these filters anyway, after consulting the owner we decided to leave it as is. I've sent the amp back to him today. If it gets worse we will have another look, but it probably won't.

 

[ubaldo]

HI, I've been a Marantz gears electronic engineer and restorer for over 25 years.
I have great respect for the engineering of Marantz for the quality of a lot of their projects, I refer in particular to the amplifiers built at Sun Valley in the 70s.
But about the model 1120, I can say that I had the opportunity to restore some tens of these devices, and ALL had problems with Hum and Pink noise using the filters Low and High.
The problem in my opinion lies in the type of realization, the wiring and the long interconnection board that collects internal noise like the flux dispersed by the power transformer, by the power supply and other sources.
There is no way to eliminate this defect, and I have managed to minimize this problem by constructing a small Faraday cage to be installed inside the M1120 which shields and greatly reduces this type of disturbance, but does not completely eliminate it.
If anyone is interested, I can without charge provide the design of the cage made by me ... or I can supply the aluminum cage at a price of 50 $.

 

[rlisin]

I suspected it was either the wiring or the circuit itself picking up noise from the transformer.

I can without charge provide the design of the cage made by me

If you can share it here or send it to me via PM, it would be very helpful. I will share it with the owner of the 1120.

 

[airtime]

Would replacing those wires with shielded wire help?

Or finding a wire shield? I've seen those is units other than Marantz.

https://www.amazon.com/Techflex-PTN...ocphy=9030084&hvtargid=pla-307290756181&psc=1

 

[ducati2]

HI, I've been a Marantz gears electronic engineer and restorer for over 25 years.
I have great respect for the engineering of Marantz for the quality of a lot of their projects, I refer in particular to the amplifiers built at Sun Valley in the 70s.
But about the model 1120, I can say that I had the opportunity to restore some tens of these devices, and ALL had problems with Hum and Pink noise using the filters Low and High.
The problem in my opinion lies in the type of realization, the wiring and the long interconnection board that collects internal noise like the flux dispersed by the power transformer, by the power supply and other sources.
There is no way to eliminate this defect, and I have managed to minimize this problem by constructing a small Faraday cage to be installed inside the M1120 which shields and greatly reduces this type of disturbance, but does not completely eliminate it.
If anyone is interested, I can without charge provide the design of the cage made by me ... or I can supply the aluminum cage at a price of 50 $.

I sure could use the plans for that faraday cage! Please send it on to me. Wondering where exactly it would be placed as well…