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Help diagnosing noise in a Marantz 7 circuit

Z

Zach Austin

New Member
Hi All,

First post here.

I have a Marantz 7 pre-amp that has persistent noise to it. The audio affliction is composed of both a high frequency whine and a lower frequency rhythm, with the whine similarly being of higher decibel and the rhythm being similar to the tapping pattern a jackhammer would make (albeit quieter). I am wondering if anyone can help me trouble shoot what's the matter? I can trace an analog circuit, I know my way around a multimeter and a soldering iron, and I'm not afraid to open the Marantz up. I do also have an ancient EICO tube tester (calibration status unknown and doubtful). Long story short, I'm happy to get my hands dirty finding and addressing the root of this problem, and if anyone has any recs on how to begin pinpointing the issue...well, that'd be a heck of a lot better than tracing and checking the entire circuit schematic or continuing to listen to this.

Anyone willing to test their know how to help, here is what I can tell you:

1. The noise is present irrespective of if there are any inputs plugged into the pre-amp
2. The noise is present with the volume turned all of the way down, but turning vol/gain up does notably increase the volume of the noise.
3. The noise is present in both channels.
4. Turning the treble up serves to increase the noise (irrespective of channel).
5. Turing the bass up on the right channel increase the noise to a point and then decreases it once you pass the 75% mark or so. Turning it up on the left channel increases it only, but less dramatically than the right.
6. If you touch the cover to tube "v6" (right most tube from the front/ left most from the back), the whine gets far worse. Even just having an object close to tube v6 makes it worse (hand, wall, etc.)
7. The noise is not originating in the amps.
8. Changing the tubes doesn't fix the problem.
9. Removing the tube cover to tube "v6" makes the noise worse and the sensitivity to the proximity of objects greater.
10. There is no combination of settings that I can find that make the noise go away.
11. The noise is not 100% uniform. The whine part fluctuates in and out and shifts in volume (pitch stays mostly static, to my ear). Occasionally this is just random distortion layered on top (like the sound produced when flipping between radio stations).

What the heck is going on? Bad ground somewhere in the circuit associated with tube "v6"? I'm happy to investigate further if specific additional insight would be helpful)

Thanks for any thoughts, brainstorming, etc.

-Zach
 
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petehall347 said:
welcome to AK
Click to expand...

Pete, thank you for the welcome.

The upload option is fighting me on allowing an audio file upload. I tried m4a (iphone default), and converting to mp3, was told by the posting page that both were improper file extensions. If you know a file type that will upload, I'm happy to convert.

Otherwise, you can access a folder with both the m4a and mp3 versions here: https://www.dropbox.com/sh/hrtdhntscleelt6/AABvpB4k7c5DUiwbhfF4CHsca?dl=0

Many thanks.
 
also curious what the power supply is or looks like. Some pics of the unit might lend a clue here.

It does have a very digital-like sound. Outside interference or if this uses a switch mode power supply that isn't well filtered would make sense here.
 
To me it sounds like oscillation with external interference mixed in..
I would check all grounding for the treble, bass and loudness compensator pots; if you have shielded wire to/from these pots, make sure they are properly grounded too; check the all the solder joints on tubes 3 (both channels per schematics); check solder joints on caps connected to those pots. It feels as if there is an open loop antenna around tone controls components. To confirm the outside interference, you need something like a faraday cage. Wrap you amp in plastic wrap first; then put a layer of aluminum foil, ground the foil. Power the amp and hear the output. If it was outside interference, with the foil shielding it should be no longer present. If you still hear the same sounds, it is likely your preamp is oscillating...
Good luck!!!
 
The noise sounds very computer related. Monitors, mice, keyboards, etc. How about a pic of your amp and everything around it?
 
prelius said:
To me it sounds like oscillation with external interference mixed in..
I would check all grounding for the treble, bass and loudness compensator pots; if you have shielded wire to/from these pots, make sure they are properly grounded too; check the all the solder joints on tubes 3 (both channels per schematics); check solder joints on caps connected to those pots. It feels as if there is an open loop antenna around tone controls components. To confirm the outside interference, you need something like a faraday cage. Wrap you amp in plastic wrap first; then put a layer of aluminum foil, ground the foil. Power the amp and hear the output. If it was outside interference, with the foil shielding it should be no longer present. If you still hear the same sounds, it is likely your preamp is oscillating...
Good luck!!!
Click to expand...

Regarding the antenna comment, funny enough, the system picked up some radio signal the other day. CB radio in a heavy truck or something of that like sounded like what was briefly broadcast. Thanks for the detailed diagnostic approach. I will follow it when I have time.
 
Hi all, thank you for the responses, and apologies for my slow reply. It's been a busy two weeks.

Here are some pics of the preamp, both where it rests normally and photos of the internals (the wires in the back are usually run more cleanly). First time I have seen the insides. Amazing attention to internal detail went into this.
IMG_9353.jpeg IMG_9355.jpeg IMG_9356.jpeg IMG_9357.jpeg
 
dinger said:
The noise sounds very computer related. Monitors, mice, keyboards, etc. How about a pic of your amp and everything around it?
Click to expand...

Photos provided separately.

Nope, there are no other electronics around it like computers, TVs, monitors, etc. The only other electronics that are in the vicinity are the two amps, the turn table, and the lights on the shelf.

Proximity of my phone, airplane mode, etc. don't make any difference.
 
Zach Austin said:
Photos provided separately.

Nope, there are no other electronics around it like computers, TVs, monitors, etc. The only other electronics that are in the vicinity are the two amps, the turn table, and the lights on the shelf.

Proximity of my phone, airplane mode, etc. don't make any difference.
Click to expand...

I assume you pulled all the tubes and clean sockets and pins? :idea:

:)
 
Another source of noise are aging light dimmers. I once found a bad one in my house (and my hi-fi is in the garage). It was mostly evident on AM stations but worth a mention.
 
All of my vintage gear will occasionally make this exact noise if my cell phone is nearby and exchanging data..

I'm no tube amp expert so take my advice with a grain or two of salt.

V6 in the 7 is a cathode follower and provides isolation for for both high and low frequency filters. This is mentioned in the manual.
Screenshot_20210201-120808_Drive.jpg

While its open is a good time to inspect the tube sub chassis rubber mounts and clean, clean, clean. The filter switches in the front are open to the elements and are notorious for needing much cleaning. Best to use qtips as deoxit isn't going to do much if blackened. Also clean the output level pots on the back of the unit.

Inspect all bumblebees for cracks. You may have to carfully rotate them to fully inspect Pay close attention to the .22ufs that come of the cathode of V6 for the high frequency rolloff switch. Clean all sockets and get those selector switch stacks nice and shiny.

Like I said, I'm no expert but a lot of issues can be cured with a cleaning. I spent 3 days just cleaning all the switches and pots on my 7. At the very least, it allows you to rule switches out of the troubleshooting process.
 
Some new appliances will inject noise also. All those new "smart" appliances can cause problems. I had to move my system to another wall due to my new fridge that was on the opposite side of the wall injecting noise. Do try turning off all cell phones, routers, and any other wireless gizmos around the house and see what happens. Then, if the noise stops, fire each one up one at a time until the noise appeares. This is how I found that my fridge of all things was causing the problem.
 
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