PM4000 trouble shooting help

[Mike Caldwell]

I'm helping work on an old ( like there are any new ones) a PM4000 that has hum issues. All of the aux outs have a light to moderate amount of hum on them, turning the aux masters up and down does increase and decrease the hum. The main stereo and group outs have slight amount of hum. The matrix outs are clean with all the matrix main levels up and the matrix inputs all down, bringing up any of the matrix inputs will bring the hum into the matrix outs. All of the channel inputs are off the auxes are off. Changing power supplies did not help.

My theory is that possibly there is a channel module that is causing the the hum directly on the busses regardless of the mode the channel is in.

Short of pulling all of the channel modules does anyone have any ideas?
Can you pull and replace the channel modules with the board powered up?

Thanks
Mike C.

 

[Tim McCulloch]

Re: PM4000 trouble shooting help

One of our 4Ks developed a hum after it kept tipping... and landed lid-down on the floor. We tried a bunch of stuff and got rid of about 80% of the noise.

For us, it appeared that the chassis ground was no longer 100% intact. We took a ScotchBrite® pad and cleaned all of the metal-to-metal contact surfaces, sprayed DeOxIt and re-seated all the ribbon connectors. But the remaining low level hum was still there and worsened when there is anything remotely Pin 1 related. I recall finding a loose XLRM output connector that, upon regaining intimate contact with the sheet metal, quieted things down some as well. Or we may have just moved some wires around that needed moving....

That all said, I think you're on the right track. I recall being told that the 4K would tolerate hot-swapping of modules but it's been so long ago that I can't remember who told me that. It's probably worth a power cycle to not screw up things any worse. Try pulling all the input modules and see if you have the hum. If it's still there you can eliminate a lot of PC board inspection...

A final thought - both of the PSUs are likely about the same age. I suppose it is possible to have bad voltage regulators and/or filter caps by now.

Maybe JR can give us a designer's view of potential causes.

 

[Geri O'Neil]

Re: PM4000 trouble shooting help

I'm helping work on an old ( like there are any new ones) a PM4000 that has hum issues. All of the aux outs have a light to moderate amount of hum on them, turning the aux masters up and down does increase and decrease the hum. The main stereo and group outs have slight amount of hum. The matrix outs are clean with all the matrix main levels up and the matrix inputs all down, bringing up any of the matrix inputs will bring the hum into the matrix outs. All of the channel inputs are off the auxes are off. Changing power supplies did not help.

My theory is that possibly there is a channel module that is causing the the hum directly on the busses regardless of the mode the channel is in.

Short of pulling all of the channel modules does anyone have any ideas?
Can you pull and replace the channel modules with the board powered up?

Thanks
Mike C.

First, no, you cannot hot swap modules in the PM4000s (you could on the old PM3000s, can't believe I remember that). There is a pico fuse that will blow and need replacing. I did it only once. This info comes from one of the great analog console gurus, Rex Ray.

Aside from that, it's pretty easy to pull the modules and then replace them one by one in order from outputs to inputs to see if there is a particular module causing problems. I would do general housekeeping things first. You'll have to pull all of the modules to perform the following chores. Use star washers on the screws on the back of the module where it screws to the frame (hence, the 4K "Star-ground mod"..) While the modules are out, tighten everything in sight, especially ground wires that are attached to big copper blocks. Also, check inside the PSU and look for a copper block with ground wires screwed to it (I realize that swapping PSUs didn't help, but again part of the housekeeping). Again, make sure they are all tight. And I use Caig Deoxit to spray module contacts and their mates inside the frame.

From here, I'd start checking individual modules.

It's been a while since I refurbed the ones we had, I'll post back if I remember anything. I am in no way a guru on these things, I learned what I know from several phone calls with the ever-generous-with-his-time Rex Ray. I tried not to bug him too much, but when I did, I had a notebook and pen ready. Talk about a walking encyclopedia....

Geri O

 

[Mike Caldwell]

Guess what I did today

Thanks for your suggestions Tim & Geri.

After gutting, cleaning, deoxing, reseating and reassembling there was a noticeable improvement. No one module stood out as being outright bad but more of just a cumulative effect of the 52 modules with slight grounding issues. Didn't dive into power supplies today. I still want to try the star washers on the module input panels.

 

[Richard McMenamin]

Re: Guess what I did today

Used an old Soundcraft Series 800B with similar noise issues.

On days when it was particularly bad I would take the output of an unused aux, patch it back into an unused channel, polarity reverse it, then feed it back into the auxes until they went quiet.

Assumes you have a spare aux bus, a spare channel, and that the noise is the same on every output.

Thankfully I have not had the need to do this in many years.