Hum in mix bus - PSU recap time or something else?

[kilmister]

Thanks for a picture Sintech, it explains a lot. I took me a while to understand how Mix Bus 0V earths are working, but after a long study (read: 24h at once) I finally managed to drop noise level at -80dBu. Looking from spectra analyzer hum is in pretty same level than hiss is.

I couldn't find any explanation for RV2 at Quad Bus Mix Amp card (82026) but after little trial and error I adjusted all four busses to have exact same resistance, 186R in my case. Does anyone know how they should be adjusted properly?
Now, I need a pint 

Cheers,
Paavo

[sintech]

Hey Paavo, it would be great if you could explain how the mix bus 0V are working, there's no real theory of operation anyplace.

And was the fix just adjusting RV2 on the 26 cards?

Just checking the drawings, E series version doesn't have the RV2 trimmer. I'm guessing this would be to do with the Quasi Balanced mix bus on the G.

[kilmister]

If I understund a thing right it goes something like this.
After signal exits from the channel motherboard via edge connector it gets own dedicated Mix Bus 0V next to it. These 0V lines are floated from Mix Bus GND (there's own GND for power) via parallel resistor arrays. One bay has 3K3 resistor on both sides of the edge connector board and one 2K2 in middle for the Quad Bus. Mix busses 1-24 and so on has little different values 5K something.

Next what I did was measuring noise with level meter and spectra analyzer while adjusting RV2 at QB amp card to archive lowest possible noise level. I also measured Quad Bus 0V resistances and matched them. Today I was too tired to testi anything anymore, but I'll double check things tomorrow. I think it's possible to adjust RV2's just by measuring noise level. It would be sweet to drop couple more dB's away 

-Paavo

[kilmister]

Ok,
Checked RV2 thing today again and also got response from Peter Higgs who confirmed it should be adjusted to lowest noise and that's it.


Btw,
Anyone able to scan the page Sintec posted?
I'm working on with pdf version of the Service Manual(s) but I don't have this page.

Cheers,
Paavo

[sintech]

Ok,
Checked RV2 thing today again and also got response from Peter Higgs who confirmed it should be adjusted to lowest noise and that's it.


Btw,
Anyone able to scan the page Sintec posted?
I'm working on with pdf version of the Service Manual(s) but I don't have this page.

Cheers,
Paavo

Hi Paavo, never seen this in any service manual before, but we are lucky to have Jim.

Took that picture with my iPhone from a pile of SSl binders in front of me, which are in some cases from a manufacturing point of view. Understandably so, as Jim was one of the "Original 13" manufacturing consoles as a cottage industry.