Line level pot affects Mic level in REC Mode

[tlmaen]

Hi, doing some maintenance and checked the whole console for faults.
I notice when I am checking the Channels in REC mode, the Line Level Pot affects the Mic level by a few db (maybe 3d, not +/-20db).
Not on all Channels, but about 80% of the Channels.
When I look a the Block Diagram, I see it should not affect it at all.
What could cause this?

Thanks,
Tilmann

[walrus]

Dodgy ground connection somewhere?

[Edit] What input board do you have?

[walrus]

Also, do you have signal going into the line input at the same time?
If so Switch RC2 maybe faulty.

[tlmaen]

Thanks Walrus!

I have only one input with signal patched to the line in (Converter Out Line Signal, -40db patched into Mic in)
Its also not on all Channels...

The Board is the 82E01 J1079
Very old without the green solder mask.
Jensen Transformer Mic Input

[walrus]

Hmm....
RC2 is the only place where Line and Mic meet for Line to affect Mic so looks like it's out with the scope.
Possible short somewhere in the front panel wiring?
Look for signal on R47 100 Ohm resistor to see if RC2 is slightly short circuit.

[tlmaen]

There is a little level at R47.
Also on Edge Card Pin 55/57 Line In.
Even when I dead patch Line In and Mon In.

When I route the Pro Tools Osc to Out 1 and Patch it to CH 13, I still have level at R47,
but not at Pins 55/57 anymore.

(Im at Chanel 13 all the time).

Could be the RC2, but on almost all channels?

Ist app. 27dbu less level at R27 compared to Edge Card Pin 23 (other side of RC2).

Edge Card Pin 24 (to switch off RC2) gets -17,18vdc
Datasheet of J112 says -5vdc is enough to switch off the fet completely.

[walrus]

What level are you feeding into the mic input, and what is the level of signal you are seeing at R47 and Pins 55/57?

Sorry, I don't what you mean when you say "When I route the Pro Tools Osc to Out 1".
Also not understanding "Ist app. 27dbu less level at R27".
Where is R27.

Signal at line input pins when there shouldn't be does make me suspect wiring problems.

[tlmaen]

Sorry for my confusing reply!

When I route the osc in pro tools to output 13, patch it to SSL Mic in CH 13, dead patch SSL Line In 13 and SSL tape Mon In 13,
then there is a signal present at R47 (R27 was a typo in my last post).
There is also a Signal present at Edge Card Pin 55/57 (Line In) of the same Channel.

When I route the OSC in PT to Out 1, patch PT out 1 into SSL Mic in 13, there is Signal at R47, but not on Pins 55/57 anymore.
So this could be a Patchbay Crosstalk thing (The Console is very old and has the old Switchcraft patch bays with a common Gnd).
But Signal is always present at R47.

The OSC is set to -10db.
PT Out is set to -40db (because I patch into the Mic amp).
Converter Headroom is -18dbfs.

I don't know exactly how much gain the Mic amp has, it was three clicks up from the lowest settings (my console has a Stepped mic gain, was requested by Broadcast).

With my NTi Minilyzer I probed roughly -65dbu at R47 and about -38dbu on Edge Card Pin 23 (going to the Pol Switch)
(It was connected with only the middle pin of the RCA cable, no gnd connection, so the reading is probably not correct, but the difference probably is).

Hope this makes more sense now.

Thanks!

[tlmaen]

Hey Walrus,
maybe this thread explains it:

https://forum.sslmixed.com/index.php?topic=1965.0

[walrus]

Yes, that would certainly explain it! 

Doesn't explain why you were seeing signal on pins 55/57 though....

[tlmaen]

Yes!
I will check why there is a signal on Pins 55/57.
I Suspect the Patchbay somehow...

Anyways, thanks for your help!