Troubleshooting a console mute issue

[ethervalve]

Hi All,

My linked Studer 169 Consoles are finally up and running quite acceptably. Just one really annoying bug to iron out:
Channel 4's mute behaves more like a 10dB pad. So even though its own Solo works correctly (i.e. all other channels get muted), when I solo any other channel, #4 will only be attenuated by about 10dB rather than the expected heavy duty muting (>90dB).
I've made some extender cards and I was able to determine that -15v is correctly switching at the gate of Q8.

(here's a hi-res schematic of a channel card if anyone cares to look at it: http://www.mediafire.com/i/?0cg0t7a7523p25g )

Any pointers on how I should proceed? Should I just replace Q8,Q9 and Q10? Any idea what would cause this failure in the first place (it was like this when I got it)?
Thanks in advance for any tips.

 

[JohnRoberts]

Looks like a series -shunt mute... So when muted both Q8 gets turned off and Q9 gets turned on.

Look at gate of Q9, it should go to ground or near 0V (from -15) at the same time as gate of Q8 goes to -15V

Incomplete mute could be;
-Q8 shorted or note turning off
-Q9 open or not turning on
-bad solder connection or impedance between Q9 source and ground.

I am not a huge fan of having a JFET in series with the audio path when not muted...  but that's life.

JR

 

[ethervalve]

Thanks for the great info JR!
I'll check for all those possibilities when I get home.
Just out of curiosity, what's the downside of "having a JFET in series with the audio path when not muted..."? Noise issues? Reliability?

 

[JohnRoberts]

Thanks for the great info JR!
I'll check for all those possibilities when I get home.
Just out of curiosity, what's the downside of "having a JFET in series with the audio path when not muted..."? Noise issues? Reliability?

Off the top of my head I would be concerned about linearity (distortion). But this is just speculation, I should also ASSume the original design was competent and did not load that node such that JFET Rds on matters.

I have used JFET shunts for mutes in consoles and other products before but without the series JFET, so I didn't get as much attenuation, but but better linearity.

This is just my opinion in passing without inspecting the full circuit or bench measurements. If it sounds good to you it is good...

JR

 

[ethervalve]

Oh that's interesting. Sometime I'll try doing a RightMark test before and after the FETs and see if the distortion is measurably different.

I should say I'm upgrading from a Tascam M520 console which (despite its many charms and clever routing features) had the most useless, noisy mutes known to man (for mixing, I had to flick faders up and down with my fingernail!)

 

[JohnRoberts]

Oh that's interesting. Sometime I'll try doing a RightMark test before and after the FETs and see if the distortion is measurably different.

I should say I'm upgrading from a Tascam M520 console which (despite its many charms and clever routing features) had the most useless, noisy mutes known to man (for mixing, I had to flick faders up and down with my fingernail!)

It is worth noting that a perfect hard mute would click as the waveform gets interrupted mid zag.... There are ways to mitigate that and they may do a slow mute to improve that...

Consoles are all about the details and what you choose to optimize over what else...

JR