Locating the hiss in a Peavey PLM 8128

[miRthkon]

I recently acquired a used Peavey PLM 8128 8-channel MIDI programmable line mixer to be able to incorporate some sophisticated re-routing into my live rig via foot control.

Feature-wise, the unit is perfect but unfortunately every single output (including the phones) produces a significant amount of high-frequency hiss. These units are from the early 90s and are no longer serviceable by Peavey. Before I go take this to a repair shop, I was wondering if anyone could offer some advice as to how I'd go about finding the source of this noise. I'm reasonably savvy on a technical level, so I thought I'd attempt a DIY repair before forking over more cash (likely as much as I paid for it in the first place).

Any help would be greatly appreciated!

[JohnRoberts]

It was not one of my products (I worked in a different engineering group), but IIRC it was not very quiet when working properly.. I suspect the only way to make it quiet is to turn it off.

Sorry... I don't even recall what they used, perhaps some early digital pots.

It might be useable in a mid/lo-fi path...


JR

[miRthkon]

Thanks John, I suspected as much considering the time period. Early 90s digital gear definitely had the tendency to be noisy. That said, while doing research on MIDI-controllable line mixers, I read several posts of others claiming to have successfully integrated this unit into their rigs (within the last decade and from the looks of the rest of their gear, I found it hard to believe they would have tolerated the kind of hiss I'm getting, hence my attempt to resolve what I hope is just a repairable issue.

I'll keep at it for now.

thanks again,
Wally

[JohnRoberts]

I don't know details but I expect it is digitally controlled analog, so not limited by early A/D conversions but more likely limited by a relatively low operating voltage of circa '90s digital pot or multiplying DACs. So perhaps a +5V supply or at most +/-5V for audio path.

New lower noise ICs could help... but that sounds like a major project. Improved DPOTs or whatever are not likely to drop in and use same control protocols.

Good luck..

JR