Neve 1290 gnd wiring, Whatever i do i'm getting hum!!!!

[hitchhiker]

Are your 10468 input transformers cases grounded correctly?  I remember once having a hum when the screw got loose where the wire was attached to the transformer case.

 

[Spencerleehorton]

i feel like there needs to be a much better way of explaining exactly how this should be all done, its very confusing and there isnt a lot of information out there which is clear?

People say follow the EZ1290 manual and it'll work, well in the manual it says to have 3 different chassis GND this only works if your using the exact PSU which the manual uses i suppose? ive adapted an SSL 9k psu pcb but the principle should still be the same.

other people are saying you have to star ground your output transformers?

Could we put together a definitive way of getting this right please?

 

[MagnetoSound]

Sorry if my earlier post wasn't clear. I just read it again and the wording is indeed clumsy.

100/120 Hz hum is post-rectification ripple. It is coming from the PS.

Most likely cause is bad regulation on the supply rail. It seems like you have a good ground path back to the PS, so I suspect ripple coming down the supply rail itself.

Can you substitute another power supply?


Ian, your attachment is not visible.

 

[Pusch3l]

Hi Spence,
I think you use an adjustable PSU with LM317 for the +24v rail?
Got into the same situation twice and both times because the regulator was working over its regulation limits. Try to lower the Voltage from the Rail.  +23.0v should be fine because you start with 22.66v for Biasing the Pre's according to the manual.
That made my 4 Channels quiet

Greets
Sebastian

 

[Spencerleehorton]

Ah, now this sounds more like it but I’ve only powered one channel now and it’s just the same?

A Psu redesign I think is needed!!!

Could someone suggest a decent upgrade please?

I’ve also noticed there is no heat sinks on the LM317 so will change them .

Also could someone explain the 48v filter cap please, I hadn’t used one? I’ve put in a 68uf 63v at the moment?

 

[Spencerleehorton]

How would I go about putting in a adjustable pot within the ssl 9k Psu?

This is what I’ve done Psu wise

https://groupdiy.com/index.php?topic=35965.0

 

[ruffrecords]

read what sorry ian?

Oops! Used the image tag rather than the URL one. I meant this:

https://drive.google.com/open?id=0B_n67A1hN3qtSzNkQUhhdHR1TkE

Cheers

Ian

 

[Spencerleehorton]

Thank you Ian, some good reading there.

I’ve found this:

https://groupdiy.com/index.php?topic=28409.0

Also is there a way to test for rail ripple?

 

[Spencerleehorton]

I’ve changed the regulator it was a TL783 for a LM317.
And left hand channel sounds much better but right hand channel still much noiser!

So hopefully proves it’s a pay issue.

I think I need to take the Psu out of the box now and see if I can get both channels being quieter?

 

[MagnetoSound]

Oops! Used the image tag rather than the URL one. I meant this:

https://drive.google.com/open?id=0B_n67A1hN3qtSzNkQUhhdHR1TkE

Cheers

Ian

It is an excellent piece and very thorough. If it could be put somewhere in the forum docs for easy reference it would be fantastic.

 

[Spencerleehorton]

Right progress, I swapped the input txf and now have completely clean channels!

Need to work out why I was getting that noise from them?

 

[Spencerleehorton]

Another question I’ve been wanting to ask, on the 2503 output txf there is copper tape wrapped around the bobbin on the outside? What it’s use?

 

[Spencerleehorton]

I’m going to try copper tape around them and also potting them in a steel or mumetal box will help!!

 

[ruffrecords]

It is an excellent piece and very thorough. If it could be put somewhere in the forum docs for easy reference it would be fantastic.

I have started a Grounding tab under the Tech Docs section and added it there.

Cheers

Ian

 

[PRR]

> on the 2503 output txf there is copper tape

Shield on an OUTput transformer is so it does not spew signal back into sensitive low-level stages.

This is not your hum.

 

[Spencerleehorton]

No it needs to be within a case, I’m going to look into metal box or mumetal case for them.

What thickness mild steel would work? 1mm, 2mm?

 

[ruffrecords]

I’m going to try copper tape around them and also potting them in a steel or mumetal box will help!!

I am not quite sure what you think that will help with. I thought you had fixed the hum problem. Output transformers were never potted or screened at Neve,

Cheers

Ian

 

[Spencerleehorton]

This is input txf on the Neve, sorry to be unclear, I’m getting a few mumetal sheets and will build some cans for them, then potting with some semi hard resin, should sort the job.

And yes the copper tap didn’t do anything really!

 

[Ilya]

I think you will need some special treatment after bending mu-metal sheets. They loose their properties after stress, AFAIK.

 

[Pusch3l]

Spence,
I thought about getting a mu-metal case for the input transformer you send me. But i will look out for a ready-made box  and pot them into bees wax. I think theyre not that expensive because they dont need to be that big.
So even if that fails i could get the transformer out of the box again

In the meantime you should look into 1 or 2 threads from DaveP. He tested shielding-stuff and transformer positions to reduce or cancel hum-problems. Maybe that will help you solving all issues

Greets