Soundcraft 8000 maintenance

[diko2]

Hi,

I'm working on a Soundcraft 8000 for some weeks now.
So happy doing this, i'm learning a lot.

Checking input and output frequency response and THD with REW.

I was a little perplex when i found that the main mix out got the worst high frequency response of all the bus.

I've made a lot of measurement and i found that the path:
MIX insert return to MIX OUT
was worst than
AUX MIX insert return to AUX MIX OUT

Strange because on the schematic those 2 paths are the same (same IC, same values).

So i check the board and found some differences between schematic and real board.
First: some IC differences.
TL072 instead of NE5532
and NE5532 instead of TL072

After a quick swap test: no changes.

Then i saw that feedback capacitors  are not the same.
On the worst path the feedback capacitor are (film?) 220p

instead of ceramic 22p.

When i say worst,  it's only -0.5dB @20kHz (starting at 4kHz).

And when i measure from a channel input to Mix output it's only -1dB@20kHz
Sure i could live with that but i want to understand.

Is there a good reason for those IC and capacitor value changes from schematic and production?
Can i change those 220p to 22p?
What kind of capacitor should i use in there?

Thanks

Morgan

 

[moamps]

Can i change those 220p to 22p?
What kind of capacitor should i use in there?

Yes, change those to 22pF.  C0G (NPO) ceramic type will be fine.