JD-800 ground loop hum

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Tekhed66
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JD-800 ground loop hum

Post by Tekhed66 » Sun Apr 24, 2022 4:00 pm

Hi all,

My trusty old JD-800 has developed an annoying ground loop hum... it never used to be a problem so I'm not sure what caused it.

I route the synth through a patchbay and into a mixer/outboard gear and then into my D/A interface... I swapped patch cables, audio cables, mixer inputs, power cables etc which didn't get rid of the hum .... I then finally bypassed everything and plugged the synth directly into my D/A and the hum persists.

I then inserted a DI box between the synth audio out and the mixer and also tried going directly from the synth audio out, into the DI and then into the D/A - in both cases when I lift the ground, the hum completely disappeared.

The synth doesn't get moved around in my studio (nothing does for that matter) so I'm at a loss to explain what might have happened. The synth and mixer hang off the same power board.

I haven't opened it up yet so before I do - is there something that I should obviously check first?... the power cable has an earth pin so I thought maybe the power socket on the synth might be old/damaged... is the case grounded internally?

Any help/suggestions - as always - gratefully received.

cheers and thanks.

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Re: JD-800 ground loop hum

Post by madtheory » Mon Apr 25, 2022 9:10 am

It does seem strange but it will be difficult to diagnose remotely. There is almost certainly some key info missing. I'll take a shot in the dark. It could be that the "D/A" has developed a fault. What's the make and model?

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Re: JD-800 ground loop hum

Post by Tekhed66 » Mon Apr 25, 2022 12:36 pm

madtheory wrote:
Mon Apr 25, 2022 9:10 am
It does seem strange but it will be difficult to diagnose remotely. There is almost certainly some key info missing. I'll take a shot in the dark. It could be that the "D/A" has developed a fault. What's the make and model?
Hi Madtheory ... I knew you'd come to the rescue (!!)

I don't think it's the D/A as all of the other synths are fine... no hum or other nasties... I haven't routed the other synths directly into the D/A but there's no noise or hum when they go through the mixer and then into the D/A.

So - to answer your question - the D/A is a 1st gen Scarlett 2i4 and it's been very reliable for all these past years.

I'll have a bit of a poke and see what happens (as you do)... the '800 was problem-free for ages so this noise is a real mystery.

If I crack it, I'll post to the forum and let everyone know what I found.

ps - late edit ... just realised I haven't tried running the synth directly into a separate amp and bypassing the D/A and mixer altogether ... I'll plug it into my guitar amp and check for hum... there's no noise or hum from the headphone out (not sure if there would be hum audible here anyway...)

to be continued...

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Re: JD-800 ground loop hum

Post by madtheory » Tue Apr 26, 2022 9:45 am

Don't think I'll be of any help here :lol: Laptop or desktop computer?

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Re: JD-800 ground loop hum

Post by Tekhed66 » Wed Apr 27, 2022 4:03 am

madtheory wrote:
Tue Apr 26, 2022 9:45 am
Don't think I'll be of any help here :lol: Laptop or desktop computer?
Ahh c'mon madtheory... you seem to know your stuff (!!!)

OK - the synth definitely has some weird grounding problem all by itself.... if I plug it directly into an amp (my trusty Fender amp in this case), there is a ground hum which I can eliminate if I plug the synth output into a DI box first and then lift the ground.... amp is silent.

I haven't opened it up yet as I'm fixing my M1 (again!!!!!!!!!!!!!!) (grrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr) so one thing at a time.... might have a chance to have a look this weekend.

Madtheory - computer is a desktop.

WHY did I become a synth player?????? ... why, why, why???????????????? :( :( :( :(

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Re: JD-800 ground loop hum

Post by madtheory » Wed Apr 27, 2022 10:34 am

I feel your pain. One of the great advantages of soft synths is they cannot have ground loops :D At least the issue is narrowed down to a fault in the synth.

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Re: JD-800 ground loop hum

Post by Yekuku » Wed Apr 27, 2022 4:09 pm

Hi TekHed,
is there any hum when using the headphones?
Do both Main L&R outputs produce hum ?
What about Direct outputs?
A broken ground connection on the output jack can produce hum.
It is all about sharing...

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Re: JD-800 ground loop hum

Post by Tekhed66 » Thu Apr 28, 2022 11:49 pm

Yekuku wrote:
Wed Apr 27, 2022 4:09 pm
Hi TekHed,
is there any hum when using the headphones?
Do both Main L&R outputs produce hum ?
What about Direct outputs?
A broken ground connection on the output jack can produce hum.
Hi Yekuku

The headphone jack output is quiet (hum-free) but there is a hum from both the L and R outputs (straight from the synth into an amp... hum is removed by a DI ground lift).

I plugged an audio cable into the L and R output and checked the grounding between them ... there is continuity between the L and R shield as well as the 'phones output shield and audio output shield.

I also checked continuity between the earth pin on the power cable and the shield of the audio out (both L and R) cable and this is also OK so it looks as if it's being grounded.

I haven't opened it up yet so it might be that the output jack could be damaged (dirty?) ...

The synth is never moved and it has been quiet as a mouse for years... for this to suddenly happen is just weird... mind you, it's one of those weeks... my computer died unexpectedly yesterday and refused to boot... I took it to the local computer shop to get checked, they plugged it in and it booted OK (now that it's working I'm backing up like crazy but that's another story).

I'll try and open up the 800 on the weekend and see if there is anything obviously wrong visually.

cheers.

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Re: JD-800 ground loop hum

Post by madtheory » Fri Apr 29, 2022 9:57 am

Tekhed66 wrote:
Thu Apr 28, 2022 11:49 pm
I also checked continuity between the earth pin on the power cable and the shield of the audio out (both L and R) cable and this is also OK so it looks as if it's being grounded.
That could be the problem. Signal ground and chassis ground are best kept separate. Inspect inside the synth for loose wires, loose screws and corrosion. You will probably have to check under PCBs, and definitely around the PSU area.

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Re: JD-800 ground loop hum

Post by Tekhed66 » Sat Apr 30, 2022 9:06 am

madtheory wrote:
Fri Apr 29, 2022 9:57 am
Tekhed66 wrote:
Thu Apr 28, 2022 11:49 pm
I also checked continuity between the earth pin on the power cable and the shield of the audio out (both L and R) cable and this is also OK so it looks as if it's being grounded.
That could be the problem. Signal ground and chassis ground are best kept separate. Inspect inside the synth for loose wires, loose screws and corrosion. You will probably have to check under PCBs, and definitely around the PSU area.
This I did not know.... y'see madtheory, you CAN pull that rabbit out of the hat :D

I still haven't opened up the case.. I'm up to my eyeballs in disassembled Korg M1 at the moment (I had 7 keys that were extremely quiet even when played hard... I think I accidentally bent a couple of tabs when replacing the keyboard felt not long ago... cleaned, scrubbed, lubed, straightened and re-assembled the keyboard... switched on... still 1 dead key.... here we go again...)

I'll get stuck into this JD800 very soon.

Thanks again.

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Re: JD-800 ground loop hum

Post by madtheory » Sat Apr 30, 2022 6:12 pm

It's just a guess. The schematic will show what the grounding scheme is. Good luck and looking forward to seeing what you discover!

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Re: JD-800 ground loop hum

Post by Tekhed66 » Tue May 10, 2022 11:18 pm

madtheory wrote:
Sat Apr 30, 2022 6:12 pm
It's just a guess. The schematic will show what the grounding scheme is. Good luck and looking forward to seeing what you discover!
A super-quick and brief update...

I opened up the case and had a squizz... I was hoping to find something glaringly obvious but it all appeared OK.

The chassis is grounded to the earth pin ... no obvious wear and tear on the soldering joint or the lug so there goes that one... in fact, the most serious thing I found was that one of the two restraining bolts of the power supply transformer had come out and it was being held down by a single bolt... it had also twisted slightly so after straightening it and one M4 bolt and nut later, it's all good to go.

I switched on the synth and tested each output individually by plugging directly into my guitar amp... I tested the L (mono) , R and direct L, R and they all hummed... while it was humming away, I gently prodded each of the plastic jack cases with my trusty wooden spoon hoping to find a loose connection but for the most part, it remained unchanged though once when I prodded the direct R output, the hum seemed to reduce and once it briefly disappeared completely but I don't know if this means it's a broken connection at the jack or it's the culprit.

I'm going to take the jack board out and have a look later in the week... maybe repeat the above with the board completely isolated from the chassis.

I'll post an update if I don't electrocute myself ;)

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Re: JD-800 ground loop hum

Post by madtheory » Thu May 12, 2022 1:18 pm

Clean the jack sockets with a burnisher and reflow the board. Plug in an audio cable so you can measure throughput with a meter. Measure the electrical resistance while wiggling the plug. It should be zero at all times. If it isn't, replace the socket.

Admittedly, the problem is common to all the sockets. So the above may not work. Or it could be the same issue in all the sockets! Not impossible.

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Re: JD-800 ground loop hum

Post by Tekhed66 » Fri May 13, 2022 2:40 am

madtheory wrote:
Thu May 12, 2022 1:18 pm
Clean the jack sockets with a burnisher and reflow the board. Plug in an audio cable so you can measure throughput with a meter. Measure the electrical resistance while wiggling the plug. It should be zero at all times. If it isn't, replace the socket.

Admittedly, the problem is common to all the sockets. So the above may not work. Or it could be the same issue in all the sockets! Not impossible.
Good advice... I'll give it a clean and see what happens... the jack sockets are the most wear and tear part of the whole synth so it makes sense that there's a mechanical fault somewhere... I can't find any obvious electronic problems eg. leaky capacitors or burnt resistors... the PCB looks very clean.

Will let you know what I find.

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