LC II grounds and no or very faint sound

[Kubik]

Folks, I wonder if someone could shed some light on this.

(TLDR - LCII sound only works with board in case, not with board sitting on the desk)

I have an LCII that basically works but squeals a little. Originally, the sound was pretty bad, but I cleaned the cap fluid stuff and the sound became nice and strong. However, it was still squealing a little, so I decided it's time to recap. I replaced the capacitors around the 78L08 - and now I have no sound whatsoever. Actually, I can hear something really faint, but I can barely recognize it.

Anyway - I was checking the schematics, trying to understand how the audio part works, and I run into something I don't get. The 78L08 gets the input voltage from the 12V line through R114, which is 47Ohm resistor.
However, the audio part uses different ground that's not connected to power supply ground at all !!!
There's no connection between the two ground planes. Measuring the resistance between the two ground planes shows open circuit. Input voltage of the 78L08 is somewhere around 4V, not 12V.

This is obviously a nonsense, so I was wondering WTF. Based on the schematics, the audio ground is also used with filtering caps on the parallel port, so that got me thinking - is this the frame ground, so it's supposed to be connected to the frame of the power supply?

I should mention I was testing just the board alone on my desk, not the whole computer in case. I felt pretty stupid, but I moved the power supply closer to the board, making sure the metal of the power supply touches the frame of the video connector.
Surprise!
Nice and clean sound!
The only problem was testing the board outside of the LCII case!

Is this a known fact? Am I just being stupid?
Could it be that some folks complaining about no sound after recap are actually testing just the board on open bench?

 

[cheesestraws]

However, the audio part uses different ground that's not connected to power supply ground at all !!!

Yes, according to the schematic the audio is referenced to chassis ground, not to the normal digital ground. I agree that given the schematics I cannot see where the two connect, and it may rely on the case to do this (but I'm fairly sure I have LC IIs where the metal bit of the case is missing and sound still works?).

 

[Kubik]

That's what confuses me, it's not supposed to work unless someone modded the board by connecting both grounds on the board itself.

Anyway, I put the no-longer-squealing board into the case, turned it on... and it started to squeal again. I took the power supply out and bent the fingers of the metal sheet so that they make better contact with the power supply, and the noises are gone for now.

I think I am done with those LCII bastards. I like the small case and design, but they're too weird inside

 

[cheesestraws]

Hang on, no, there's something weird going on here, because sound definitely works on my rackmac - which is an LC II logic board in a rackmount case - but that has the logic board suspended on nylon standoffs within the case and unless I accidentally did it, doesn't have a chassis ground connection at all. I know that chimes because I tested that it did - I want it to chime so I know it's coming up properly.

I'll try to remember to get an LC II board out tonight to poke at a bit...

 

[Kubik]

You can check the ground planes on the burn-in connector (the pads near the SCSI connector). On my board, there's open circuit between pads 10 and 11 (digital / analog ground).

 

[Zarwox]

Normally I think there is a star ground point somewhere. At least for the LC475 it appears to be around the video connector. I have an LC board running outside the case right now and with nice audio and continuity between the grounds. Question is where...

This is what the LC475 schematic looks like.