Tesla EKL 021 Need Help

Hi,
I have this old Tesla EKL 021 compressor, it comes from an old console, it seems to be a FET compressor (Germanium compressor I read on a forum) But I can't find much information.
I found the schematics.
On the schematics, the voltage is -24VDC on pin a12 and b12, the input is on pin a4 and ground is on b4, b6, b8, b1.
The output seems to be a6 but I'm not sure because the is a signal but It's bigger than the input ...
I also don't know what is a8 (side chain ?) and a10.

The settings on the front are a little bit strange, the first know is "KOMPR." it's a switch with 4 positions (lim, 6, 12, 18), I thought it was a ratio but the way I plugged it (output on a6) it just seems to act as a gain.
Next setting is "EXP." it's a pot, it goes from 45dB to 65dB, I don't know what it is for ???
The last setting is "T SEC.", it seems to be a time constant or just the release time of the compressor, it's a switch with 5 positions.
There are also 5 trim pots inside but I don't know what they do.
Does anyone know this compressor or could help me read the schematics ?
Thanks a lot.

Link to the Schematics

I did a few tests, I used a PC soundcard as a signal generator and oscilloscope.
I sent a signal to the input, when the signal becomes bigger, the needle of the meter get higher until it reaches the 0 (corresponding to 50mV on soundcard scope's generator) then it stays at 0 even with a much bigger signal.
When the input signal is below 50mv, let's say 20mV if I turn the "EXP" pot to the right (R75, 5K lin on the bottom of the schematics) the needle go higher but still don't go higher than 0.
I also measured the voltage at the FET's gate, it measured -4V when the signal is very low (or absent) then rise to 0V when the signal is bigger. The 0V on the gate correspond to the 0 on the front meter.
It seems to act upside down, isn't the gate supposed to have voltage when the signal is bigger ??
I measured the output signal on the b6 output, the signal is a little bit lower than the original signal when the KOMPR switch is on "lim", then it's double when it's on 6, then go higher and higher when the switch in on 12 and 18 ... it seems to just act as a 6, 12 and 18dB gain ...
And I still don't know what to do with a8 and a10 outputs ( or inputs ?) I measured no signal on them, no AC no DC, I tried to send a signal, didn't notice any change ...
Can anyone help me with this one ? give me a clue ?
Thanks

fjgaston said:

It seems to act upside down, isn't the gate supposed to have voltage when the signal is bigger ??

Click to expand...

0V is "bigger" than -4V

A N-channel fet needs a negative voltage compared to the source on its gate to switch off.

Well, after some time spent with an friend looking studying the schematics, it seems that the gain reduction is not done by the FET but by the second transistor (T2).
In this case, a8 shoulld be the sidechain (and "main chain" too)
We don't really understand the purpose of the FET.
The top board seems to be the main board with the amplifier and the VCA.
The middle board is the envelope conversion and the bottom board seems to be dedicated to the meter.
We measured that 2 capacitors acted weird, I now need to unsolder them and test them properly.

The FET resistance controls the gain of the opamp on the main pcb, so it indirectly is responsible for the the gain reduction.

T2 is responsible for the the input gain of the unit and changes gain with the setting of the input attenuator switch.

Majestic12 said:

The FET resistance controls the gain of the opamp on the main pcb, so it indirectly is responsible for the the gain reduction.

T2 is responsible for the the input gain of the unit and changes gain with the setting of the input attenuator switch.

Click to expand...

In fact the FET is in parallel with a capacitor and it acts only when the signal is very small or absent.
T2 is where the gain reduction happens so it's not a FET compressor.
I changed C25 and now when I inject a signal on a8 it drives T2 to reduce the signal.
It seems like a few electrolytics are bad.
I also need to calibrate the thing with the trimpots it seems.

To me it seems that the source of the FET is AC coupled through C9 to the inverting input of the opamp. The  resistance of the FET is parallel to R27 and changes the gain of the opamp according to the change in the FETs gate voltage.

For me it basically IS a FET compressor since the FET is the component that finally converts the sidechain CV voltage to a change in output gain ever though it is not directly strapped accross the audio path (like on an 1176) but in the feedback loop of an opamp.

T2 seems to  be more of an input gain control to me.

But of course I can be wrong. Maybe here's someone who has worked on this design before.

Yes, this is exactly what I though in the first place but I was not able to understand how the time variable (the switch on the middle board) was active in the gain reduction as the only connexion between the mid board and the upper boar is by the base of T2.
At first when testing it there was no voltage on the wire that link these two boards, and it was because C25 was leaking, now that I changed C25, there is a voltage going to the base of T2 and there is a gain reduction.

My problem now is that the gain reduction is too hard and I still don't understand how the "KOMP" setting work and I don't really understand how to calibrate this thing. But there might be some other bad capacitors as the unit is from the 70s.

Well, I spent some time away from the compressor and just got back to it.
I'm not familiar with Compressor schematics but I just tried to plug the output into what seems to be the sidechain input and it now seems to work much better.
Now that I have looked at some compressor circuits, I realise that's what is called feedback compression.
It works much better but seems to work just like a limiter more than a compressor and still don't understand how most of the settings work  ???

I saw these mixers in the early 90s.Tesla made the former Czechoslovakia and have been the best the post-Soviet space.
I dealt with this compressor two years ago and I still have some entries.I remember exactly what he has high sensitivity.If you add the input transformer and phantom power,he will work as a microphone amplifier.
I adapted it to line level +4dBu.my scheme with changes is in the attached .I can not find the changes for the side chain of amplifier.
But I'll try to remember....