DIY Marantz 7 Motorboating

[Ed_Chambley]

Schematic attached.  120Hz motorboating only on phono stage AND only when turntable is powered off, but preamp and amp still on.  2 chassis build with power supply seperate.  I have not installed a volume pot or step attenuator.  A similar build can be found here http://www.preservationsound.com/?p=3187


Of course, I have DC filiments, which is the only change I have made to the entire build.  I am using an Actual Marantz 7C for my layout guide, but I do have a seperate chassis housing the Power Supply.  Voltages all check to schematic within reason.


The most confusing to me is when I provide a signal with the same voltage maximum I receive from 3 different turntables  (moving magnet, no moving coil), scope reads better than I expected.  It is only motorboating when anything is connected to the inputs of the phono stage and powered off.  If I turn off my signal generator, but leave it connected.  Motorboat yes.  Unplug and stabilizes.


Also, I still need to order a stereo volume control for the build.  What is the best to use in this instance, a stepped attenuator such as an ALPS?  I guess it should be a 500KA stereo, but I have never used a 500Ka for Hifi, only 100K.  If someone knows, and I know some of you do, please let me know.  I have not ordered this simply because I want to use one with a remote receiver.


My main question I guess is could using 2 resistors, 470k temporarely in place of the 500Ka pot cause this?  Suggestions welcome.  Thanks!

[shooter]

isolated ground for your "input" device, I know that should be 60hz
the Kit pcb M7 I have used 12.6 for fil, it hummed, way less than guitar, but way more than audio.  I converted to DC, took like 10,000uF 4 stage RC just to get it where I was happy-ish.

you need something pretty sensitive, or good intuitive - which you have, try measuring VDC ground in to ground signal WITHOUT the signal inputted, so your meter is measuring between the 2 disconnected devices

[Ed_Chambley]

isolated ground for your "input" device, I know that should be 60hz
the Kit pcb M7 I have used 12.6 for fil, it hummed, way less than guitar, but way more than audio.  I converted to DC, took like 10,000uF 4 stage RC just to get it where I was happy-ish.

you need something pretty sensitive, or good intuitive - which you have, try measuring VDC ground in to ground signal WITHOUT the signal inputted, so your meter is measuring between the 2 disconnected devices

I understand where you are going.  Will do.  I used all MF resistors with all being double the schematic wattage, but stayed sane with caps.  I used Solen which I find to be quite nice for the cost.  Requlated 6.3 VDC filiments and isolated the phono section using a copper shield. With everything running it sounds great and really no floor noise to speak of.


Usually, when I get something going on like this I get tunnel vision simply because I wouldn't have intentionally wired anything wrong, but I can assure you I did something.  Maybe I connected the hip bone to neck bone.    I had read where this preamp did not get very good reviews.  Then I listened to one in very nice shape that had been rebuilt completely and to me it sounds like music should.  He loaned me a parts preamp, which is still really complete.  It needs servicing, but a great build guide.


The one I built has less floor noise than his original, but he has carbon comp mojo and AC filiments.  I can assure you carbon comp resistors lose all their mojo as soon as I solder one in place.    Ed the Mojo Killer.  Quit, you are harshing my buzz.

[shooter]

You'll find it, hum sucks, my tube transistor hybrid looked soooo pretty but hummed almost enough not to pass guitar standards, I didn't even bother, I just set the tube part aside for an ALL tube audio.

even on the SS I get a low hum (not bothersome) with NOTHING on the input, goes away with signal, I'm ASSUMING it's because the input is "open".

The hum is still there with a "floating" input, battery powered music, OR turntable straight from needle, plug in a charger, or set turntable to line out and humbegone

[PRR]

> 120Hz motorboating

Motoboating is usually few-Hz.

120Hz is simple power supply buzz.

Turntable power status should hardly matter (slight 60Hz hum when motor is powered). I suspect a wiring flaw.