Anybody here have repair experience with the Digi PRE

[skylab001]

I recently bought a Digidesign PRE (blue sliver model) locally for what I would consider the going rate of a good condition unit. Tested before I purchased because no returns on cash sale, of course guy said it was perfect 😒 but I missed something, got it home to find out that on one channel (in the lower region) between 9db-24db of gain the volume drops a few db then rises a little until at 24db it jumps back up to the level it should be equivalent to the other channels. It’s most noticeable on line level sources that live in that lower area rather than mic levels and that’s why I think I missed it.

I’ve opened it up and began looking at it. Contacted Avid to see if I could get a schematic(long shot I know) and I was refused. I don’t think I’d be asking here if they had given it to me. I’m not trained with electronics but have done plenty repairs over the last 20 years. I have repaired most of the items in my studio when needed, but most companies have provided schematics or I’ve found them on the web.

I was hoping to find someone with some knowledge that might elaborate on how the gain stage works, is it just remote controlled relays and resistors stepping the gain or is it more complex? It seems like it’s a multilayer board so it’s been hard trying to directly trace the circuit. Some of the smd components are nearly impossible to read so it’s tough to tell what they are, ie, diodes or transistors. If anybody could point me to the right section of the board I should be looking or at least tell me what kind of voltages I should expect in certain areas so I can get this problem sorted. Feel free to send me a PM.

Sending it in for repair is out of the question, it would probably cost more for a tech to open it up than I paid for it.

[JFreak]

Just forget that channel then. If you haven't done work on surface mounted electronics, this is wrong box to start trying.

[musicman691]

Originally Posted by JFreak

Just forget that channel then. If you haven't done work on surface mounted electronics, this is wrong box to start trying.

Agreed. I've done a lot of surface mount work and even with the right tools like a very specialized soldering station you can muck things up VERY fast. And no schematic to boot?


SMD's are fragile. If I had to guess this unit was dropped at some point and maybe a few components are cracked. I'd get a hold of the seller and let him/her have a good what for for selling bum merchandise. Make the seller famous on Twitter or Facebook.

[skylab001]

I’ve done plenty of rework on smd boards, that’s not the issue.

[skylab001]

Mostly hoping somebody can chime in that has worked on a PRE and knows the circuit.

[musicman691]

Originally Posted by skylab001

I’ve done plenty of rework on smd boards, that’s not the issue.

If that's true then you know that without a schematic you're royally screwed.


You still need the proper tools to do the job - you have them? Like a proper soldering station and a magnifier?

[Darryl Ramm]

I doubt folks will pop up here with schematics. And there has been past requests all over the Internet looking for Pre schematics. Including this great thread from a few years back https://www.gearslutz.com/board/geek...t-problem.html

But with these sorts of things you just often don't need a schematic in full detail. You know it's got digital controlled gain/attenuation that is not working. Mentally giving up on getting a schematic may help you. You should be able to crudely trace stuff from the digital side and from the analog side and it will meet somewhere. Even without detailed circuit schematics you should be able to follow stuff by the parts used. Somewhere there will be a stepped attenuator or something obvious. Logic analyzer (or simple logic probe if it's not serial data controlled?) on the digital side... is that signal getting there OK? If so then look at the analog side, it's either that part or stuff a little downstream.

On the analog side, follow the suspect signal path between components with a scope you will work out what you need as you go along.

You can also just send a common signal input into multiple channels including the bad one. And start poking around in that area of that convergence of digital and analog and comparing the suspect and good channels. Sometimes you can solve stuff without knowing nearly anything.

If you want to play with it go for it. The worse you can do is wreck the pre or electrocute yourself.

I wish Avid and other vendors would just publish circuit diagrams and repair manuals for their old gear, certainly EOL gear. The Pre almost qualifies as old gear in my book.

[albee1952]

Given the vintage of the unit, and Digidesign's common issues back when it was made, I would open it up and try to find every connection that uses those infernal white nylon connections, unplug and replug every one of them a few times each(it may not help, but its free)

[skylab001]

Originally Posted by musicman691

If that's true then you know that without a schematic you're royally screwed

Well that’s not really the case. I thought all engineers had rework/soldering/desoldering stations, scopes, frequency counters and function generators and at least basic repair skills. Maybe I expect too much. Isn’t that all part of the job?

Crisis averted, seller told me he had a second unit and was willing to swap them, but when I tested it I found other issues in that unit. In the end he just gave me my money back. 👍🏻 Turned out to be a real stand up guy and I’m thankful for that.

[musicman691]

Originally Posted by skylab001

Well that’s not really the case. I thought all engineers had rework/soldering/desoldering stations, scopes, frequency counters and function generators and at least basic repair skills. Maybe I expect too much. Isn’t that all part of the job?

Crisis averted, seller told me he had a second unit and was willing to swap them, but when I tested it I found other issues in that unit. In the end he just gave me my money back. 👍🏻 Turned out to be a real stand up guy and I’m thankful for that.

Actually most engineers don't have rework stations/soldering gear - that's the job of a technician or wireman. Or basic repair skills. I know because in my day job in another space/time continuum I worked in aerospace as an engineer; I also had all the reworkability/skills of a technician but I was a rare bird. Most engineers where I worked (RCA Aerospace in East Windsor NJ which no longer exists but the company has morphed into Lockheed Martin) had no idea of how to solder or basic repair skills. They told a technician what they needed done and the tech or wireman took care of it.
SMD's make things even worse because it's impossible to identify what a part is for the most part. Not having a schematic makes it even harder; doesn't matter how skilled one is or what skills one has.