SOLVED: Outboard PSU for Alesis 3630 backfeeding -15VDC to AC line

[Blue Jinn]

I did the gate disconnect and socketed VCA mod to an Alesis 3630. I decided to just power it from an external PSU and removed all the power supply components and wired +/- 15VDC to a 3 conductor plug to the +15v, -15v, 0v where the regulators would have been. For some reason I thought this would be easier than locating a beefier 9v wall wart and swapping out diodes. The external PSU is just a prebuilt eBay bipolar supply installed in a repurposed PC PSU box. (It does look like the reverse diode over the regulators has been cut out of the circuit though.)

I had some problem and noticed a spark between the chassis and the rack. I measured -15VDC between the chassis of the 3630 and the rack, as well as a random AC line measured from a different power strip. Putting the PSU and the 3630 on the bench, the same, I measured -15vdc at the chassis of the power supply and at the hot and neutral of the IEC plug. On the 3630 I measure about 20k resistance between the -15 and chassis. I've never seen this before, and would like some insight as to what's causing this and the best remedy. It seemed to go away if the PSU board was left not grounded to the chassis box, but any info appreciated.

 

[pucho812]

Take some photos.
Sounds like a wiring issue.

show us the psu you purchased.
Add eBay link.

 

[ccaudle]

I measured -15vdc at the chassis of the power supply and at the hot and neutral of the IEC plug.

Referenced to what? I.e. one test lead was on the hot pin of the IEC plug, and the other test lead was on...?

 

[Evertide]

What part did you forget to remove ?

 

[Blue Jinn]

Referenced to what? I.e. one test lead was on the hot pin of the IEC plug, and the other test lead was on...?

To the 3630 chassis, (from one of the top screws)

 

[Blue Jinn]

What part did you forget to remove ?

Removed every single psu component on the 3630. I'll post photograpsh when I get home this PM and take it apart.

 

[ccaudle]

Were you measuring from a hot mains cable to the chassis? I don't think I would trust most voltmeters to reliably measure low voltage DC using a 120V AC line as the reference.

I think Pucho is right, we need to see the details on the power supply you bought. The fact that the problem goes away when you disconnect the power supply from the power supply enclosure would seem to indicate that the power supply is not wired like you think it is.

 

[Evertide]

Were you measuring from a hot mains cable to the chassis? I don't think I would trust most voltmeters to reliably measure low voltage DC using a 120V AC line as the reference

The unit had 9 Volt AC , there should not be a connection to the new input... ( if all that was power supply was removed... )

 

[Blue Jinn]

The unit had 9 Volt AC , there should not be a connection to the new input... ( if all that was power supply was removed... )

I removed all the power supply components inside and wired +15 0 -15 to the equivalent points on the PCB where the regulators were.

 

[Blue Jinn]

Here's a photo of the PSU and the bottom side of the 3630 board showing where I removed parts. Let me know if more pictures needed.

 

[Evertide]

Are there connections under the switch ?

 

[ccaudle]

This looks strange. What is going on with this connection?

 

[Evertide]

The isolation should be close to the board , bud it looks like 0 Volt ( green ) and +15 Volt ( red ) are connected , so why - 15 Volt ( black ) on the chassis ?

 

[Evertide]

Alesis 3630 Adapter Ac 9V 830Ma on Amazon...​

 

[verbos]

Looking at the supply picture, the red is +15, black is -15 and green is ground. Looks to me like on the 3630 PCB you have the black and green as the +/- and red as ground. Right?

 

[verbos]

Maybe not, I can't tell in the picture.

 

[ccaudle]

Maybe not, I can't tell in the picture.

The messy wiring makes it difficult to see what is connected to each location. Proper wire dress would help to evaluate what might be wrong.

 

[Blue Jinn]

The messy wiring makes it difficult to see what is connected to each location. Proper wire dress would help to evaluate what might be wrong.

Some of that is the photo. But I noticed I could dress it better. The connections are correct and not shorting.

 

[Blue Jinn]

The messy wiring makes it difficult to see what is connected to each location. Proper wire dress would help to evaluate what might be wrong.

Looking at the supply picture, the red is +15, black is -15 and green is ground. Looks to me like on the 3630 PCB you have the black and green as the +/- and red as ground. Right?

Yes, I have red +15 going to where pin 3 would have been on the 7815, green 0 v to where pin 2 would have been on the 7815 and black -15 to where pin 3 would have been on the 7915. The photo is bad, and I could have soldered better, but there's no shorting.

 

[Blue Jinn]

The isolation should be close to the board , bud it looks like 0 Volt ( green ) and +15 Volt ( red ) are connected , so why - 15 Volt ( black ) on the chassis ?

-15 black is connected to where pin 3 of the 7915 regulator was. That's exactly my question why there is -15 on the chassis.