500 series rack PSU grounding question.

[Potato Cakes]

Hello, friends!

So I noticed that the on the edge connectors of my 500 series racks (Lindell audio, Fredenstein), the chassis, audio, and PSU ground are all shorted together. I haven't really noticed a problem with my preamps, but I'm finishing up an old LAZ 525 compressor, and one side of the output pot winds up shorting with the chassis ground when connected to either of my racks causing it to act like an attenuator instead of a make up gain. When I insert a 10 ohm resistor in line with the audio and PSU ground it seems to work better, but is still -10dB down from unity with the output (make up gain)  all the way down. I haven't built any of Jeff's 51X racks to be able see to do a proper grounding scheme for this type of system, and I wanted to check here first before I start ripping stuff apart like a mad man. Or a made man.

Thanks!

Paul

 

[john12ax7]

It sounds like something is wired wrong with your unit. Chassis, psu, and analog ground all will eventually tie together at one point. Different manufacturers will handle it differently, and it's a bit unusual to tie them together at the connector instead of further downstream, but eventually they will all connect.

So I would say double check the pot wiring.

 

[Potato Cakes]

That is what I was thinking as well. The pot seems to be wired correctly, I even took it out of circuit and wired it with test leads to see if experimenting with how it was connected would change anything. I guess I'll just have to keep staring at it till I find it.

Thanks!

Paul

 

[john12ax7]

Perhaps post a schematic. In an 1176, for example, the output pot is wired as an attenuator followed by a gain stage. Is the 525 the same way?

 

[Potato Cakes]

Here is the schematic. Another member here has built a couple of them and has confirmed it is supposed to act like make up gain and not attenuation.

I think its going to come down to pulling it all apart and redoing all of the leads between the different PCBs. I was hoping to avoid doing this by finding an misplaced connection between the boards, but it looks like that I won't be that lucky this time.


Thanks!

Paul

 

[john12ax7]

It looks like you have the pot followed by a fixed gain stage.  Is it possible to measure the signal at various points? It could be something as the wrong value resistor installed.

 

[Potato Cakes]

I'm going to undo all the leads so I can look at the boards a little easier. I hope it's something simple like you suggested, and it has always been that way when I mess up, but I just wish the obvious answer wasn't so well hidden.

Thanks!

Paul

 

[Potato Cakes]

So it's me being a stupid face. I actually had a working unit to begin with. For some reason, I thought that with the input and outputs all the way down, I should get unity gain, similar to an SSL (without the input control). Unfortunately, I didn't really realize this until I pulled it apart, moved some things around, burned out all the LEDs, broke a bunch of leads from excessive bending back and forth, and then it finally hit me that it was working properly all this time. I am an idiot. I cobbled it back together as I had it and it was compressing and quiet.

Misplaced components and crossed wires have always been the culprits of my problems in the past, but this is the first time that I built a unit correctly to begin with and didn't realize it.

All that aside, it is a pretty sweet compressor. After I finish some more projects, I may see if there is any interest for a group buy for more as I believe Peter will only do another batch for 25 sets or more.


Thanks!

Paul