# Need help ID-ing pots on Mordaunt sub



## Forddenial (8 mo ago)

I have a Mordaunt-Short MS907 W and there are a few issues that i have mostly traced to the potentiometers for the dials.

Symptoms
1. Hums like a bad ground
2. Crackle when dials are adjusted (any dial)
3. Sometimes the hum diminishes after dial adjustment (any dial)

Symptoms occur regardless of input style (rca or line)
I was going to start the repair by swapping pots but there is minimal ID markings.

Thanks for any help


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## mrexcitement (Jan 23, 2019)

I have used electronic contact cleaner lubricant , available from most electronics supply stores , this is one example https://www.3m.com/3M/en_US/p/d/b00034204/ , with very good success on many pots over the years but if the internal wiper or resistance strip is worn then only a replacement will fix it , once you remove them if you measure the 2 outside pins it should give you the total resistance of the pot unless it's really worn and then you would have to guess on the required value or find a circuit diagram that shows the value.


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## daloudin (Nov 2, 2020)

mrexcitement said:


> I have used electronic contact cleaner lubricant , available from most electronics supply stores , this is one example https://www.3m.com/3M/en_US/p/d/b00034204/ , with very good success on many pots over the years but if the internal wiper or resistance strip is worn then only a replacement will fix it , once you remove them if you measure the 2 outside pins it should give you the total resistance of the pot unless it's really worn and then you would have to guess on the required value or find a circuit diagram that shows the value.


THIS ☝ Before you go de-soldering those pots - hose them down in De-Oxit (dead and grounded preferably) and let them air dry after thoroughly and rigorously sweeping them full range while wet. Once air dry do the same with Fader Lube and then power it back up and see what happens.


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## Forddenial (8 mo ago)

Thanks for the tips! Ya'll are awesome


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## Forddenial (8 mo ago)

🤦‍♂️ Forgot: The pots move freely and do have what seems to be correct response in their respective applications. 
I'm still gonna grab the cleaner and lube though. I really don't want to have to replace the pots because the board is mostly glued down and there is a metric ton of it on the heatsink for the mosfets


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## mrexcitement (Jan 23, 2019)

Forddenial said:


> 🤦‍♂️ Forgot: The pots move freely and do have what seems to be correct response in their respective applications.
> I'm still gonna grab the cleaner and lube though. I really don't want to have to replace the pots because the board is mostly glued down and there is a metric ton of it on the heatsink for the mosfets


Well there is a very good chance the cleaner will make the pots perform like new again , what you have described is the perfect application for the cleaner.


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## Forddenial (8 mo ago)

I'm glad to hear that. Listening past the problem this thing still has lots of life to it and it really wants to play....if that makes sense 😅


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## Forddenial (8 mo ago)

Shot some cleaner in the pots last night and spun them a few times to ensure coverage, flushed with more cleaner.

Plugged it up this morning and....no joy. Still have the hum


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## daloudin (Nov 2, 2020)

Sorry, the cleaner was for the static crackling noise when you twist the dials, not the hum.

Most likely - You have a ground loop hum and the easiest way to identify that is to "lift" the ground. Use a 2 to 3 prong adapter plug for the power cable and see if the hum goes away.









Sent from my SM-G965U using Tapatalk


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## Forddenial (8 mo ago)

I was actually doing research before i even read your post and was about to go hunting for one of the 30 adapters i have floating around.
I appreciate the help and ideas, hopefully this fixes it


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## clange2485 (Dec 10, 2020)

Forddenial said:


> I was actually doing research before i even read your post and was about to go hunting for one of the 30 adapters i have floating around.
> I appreciate the help and ideas, hopefully this fixes it


If you can’t find one, you can take the ground pin off the power cord with a pair of pliers. I’ve got plenty of other iec cords i can give you.


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## Forddenial (8 mo ago)

I have a ton somewhere. They were gonna throw away a box of them at the store 😅

Just have to get around to cleaning up the piles in the garage




Edit: just for reference


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## Forddenial (8 mo ago)

No joy.
It doesn't matter if i power on in "auto (detect)" or just regular "on" it'll have that hum.
I'll upload vids.


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## daloudin (Nov 2, 2020)

Forddenial said:


> No joy.
> It doesn't matter if i power on in "auto (detect)" or just regular "on" it'll have that hum.
> I'll upload vids.


The 2 prong adapter didn't help? 

Sent from my SM-G965U using Tapatalk


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## Forddenial (8 mo ago)

Nope, still the same issues. I'll link vids shortly









test 1







youtube.com













test 2







youtube.com


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## daloudin (Nov 2, 2020)

Forddenial said:


> Nope, still the same issues. I'll link vids shortly
> 
> 
> 
> ...


So if you're not getting a ground loop from the wall then there's a ground loop on the circuit board somewhere. Usually this is from some component that's no longer grounded appropriately but without an oscilloscope to trace the signal path you will be blindly replacing components till "the blind squirrel gets a nut." Without a circuit diagram your even more in the dark. 

Those pots are as good a place as any to start - especially the sensitivity knob as it appears to have the most deleterious effect. 

Sent from my SM-G965U using Tapatalk


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## Forddenial (8 mo ago)

I actually DO have an o-scope. How would i go about testing? I was too busy flirting with that chick next to me to pay attention during this section way back when i took basic electronics 😅


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## Forddenial (8 mo ago)

So. Flipped it on "auto" and phased it 180⁰ and.........silence. No hum

Switched to "on" it hums all through the phase from 0⁰ to 180⁰


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## Forddenial (8 mo ago)

.......and now it works perfect on "auto", like new......but hums when switched to "on" 

😵‍💫🤯


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## daloudin (Nov 2, 2020)

You follow the signal path backwards from the FET outputs till the hum goes away. One component will have the hum on the output but not on the input and that component is injecting the noise. 

Now that you've found a component (the Auto/On switch) that can make the hum go away - you work with that... use the contact cleaner (DeOxit) on the switch. Spray it thoroughly and then exercise the switch then rinse and lubricate. 

Sent from my SM-G965U using Tapatalk


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## Forddenial (8 mo ago)

One of these days i'll have to have a discussion with myself about why i like older gear......


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## clange2485 (Dec 10, 2020)

Forddenial said:


> One of these days i'll have to have a discussion with myself about why i like older gear......


Don’t forget the price was right. You'll be working on that marantz next! 😂

I hope that isn’t the case but it hasn’t been used it years.


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## Forddenial (8 mo ago)

@clange2485 oh i'm not complaining about the price at all! I just know that it would be soooo much easier to get a Bash 300 or SA-250 plate and call it a day......yet here i am still searching the bowels of the interwebz for a damn schematic and splashing contact cleaner all over my room.

And don't jinx the Marantz, dammit. That receiver is a beast and plays its heart out 😆


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## Forddenial (8 mo ago)

So this seems to be the only real info i found








The board itself looks in good condition, so hopefully just replacing the caps fixes the hum. I'm guessing the filter caps are the large ones based on a quick eyeball of the tracings from the transformer plug on the board


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## Forddenial (8 mo ago)

-----ignore this, didn't realize the speaker unplugged from the board and i thought i fixed it--------


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