Studer 169 channel gain loss; seems related to mute/solo circuit

Hi all,
One of my Studer 169 console's channels has about 10dB less gain and considerably less headroom than all the others.
The gain loss seems to occur after the mute/solo circuitry. Both the mute and the solo were working OK in this channel though.
When I bypass the mute/solo (lift pp1-12 and connect pp1-13 to pp1-11 on the schematic) the gain and headroom is restored.
Just wondering if anyone has any clues as to what could be going wrong? The circuit seems to be doing what the manual describes. 
Thanks in advance for any clues!

Here is the mute/solo section description attached:

"mute/solo section description". Fig 7.4.15. Measure voltages both ways around those three transistors. Likely one is shorted.

Thanks PRR!! You were right. 
It was Q9, an e112 JFET
I'd checked the flow the -15vdc and it seemed to be exactly as the manual suggested.
But the gate to source resistance measurement was very different than the working channel I compared it to. I initially missed this because it was the same digits but KOhm instead of MOhm  :-[

Hello

Yes, what I will suggest as you still have signal, it happen those jfet become leaky with the time.
In case you don't have spare right now and need to use the desk, it should work fine if you remove it.
Maybe just more "pop" went switching, and attenuation not >90dB as stated.

Best
Zam

Thanks Zam.
It's very helpful to know that they tend to go leaky with age.
It's too bad they're so expensive...
I was able to salvage one from my only spare channel (which has problems of its own, namely that it's acting like an oscillator and a radio receiver--I'll have to deal with that another day).

ethervalve said:

It's too bad they're so expensive...

Click to expand...

J112 ??
what is your source ?
Best
Zam

Oh the J112 is equivalent to the E112? That's a relief!

I wasn't able to find the E112 datasheet. The NOS E112s seem to sell for a lot: http://www.ebay.ie/itm/MOOG-E112-ORIGINAL-VINTAGE-FIELD-EFFECT-TRANSISTOR-P-N-62-534-/231844780163

You wouldn't happen to know the pin and  crimp header types that Studer uses it its modules?  I've been using  generic DuPont style female headers  and it isn't the most secure fit. 

ethervalve said:

Oh the J112 is equivalent to the E112? That's a relief!

Click to expand...

Yes it is. At schemo fet is J112.
Just say Moog....17$ for a fet... 
I should have some in this package (e112) I may put them on ebay :-X
J112 is TO92 package
Best
Zam

I also have a channel on my 169  that behave like a radio receiver / oscillator. Any clues where to start looking?

I would say the best way would be to trace the signal with an oscilloscope...