TAB V372/1 Racking - Shielding Issues/ Humm

[Marceldarcel]

Hello dear all,


I am going crazy and can't make progress with my racking project for the v372 or V372/1. Everything works, except for a constant hum or buzz, which must be related to the grounding.
I had the V372/1 recapped under the supervision of an electrical engineer who is also into vintage German broadcast stuff. All capacitors are new.
I just received the v372 - it is actually the old version without any additional designation. I have the manual for it.

Both amps have a slightly different pin layout (for example, the "old" one needs a bridge between 5/6, which is NC for the V372/1. Also, 0V on the V372/1 is at PIN 13, 21, and 29, while on the V372 it's at PIN 7/8, 21, 29).

To avoid confusion, I want to focus here only on the V372/1 with the two blue NTPs.

Here is the pin layout:

1: Input (a) (+680 ohm resistor)

2: Input (b) (+680 ohm resistor)

3/4: NC / Bridge

5: NC

6: NC

7: NC

8: NC

9/10: Rg (220k Log)

11: NC

12: NC

13: 0V

14: NC

15: NC

16: NC

17: NC

18: Output (asym.)

19: Output (a)

20: Output (b)

21: 0V

22/23: Bridge

24: NC

25: NC

26: NC

27: NC

28: NC

29: 0V

30: Chassis

31: 24V


I have attached an image of the V372/1 board from which the pin layout can be seen. Where should I connect the shield? I have tried pretty much everything I found in forums and on the internet. I also attempted to follow the schematics of the 376a and 376.

There is a Gearslutz thread about humming on the V372/1, but unfortunately, the resolution is not revealed there. There is also the article "v372/1 Racking Completed" here. Looking at it, it can be seen that the shields of input and output are connected to Pin 30 and Pin 29. Even if I omit Pin 29 or 30, nothing changes for me.

My V376 is up and running smoothly. It doesn't cause any problems.

I have already swapped the lab power supply, all cables and XLR jacks, as well as pots. I also took my test lab to another house...
I am extremely grateful for any help!

Best regards,
Marcel

 

[dmp]

I would guess that the 0V pins 13, 21, and 29 are all connected on the unit, but you could confirm that with a multimeter.
The power supply should have +24v to 31 and the 0V(gnd) to 29 (& 13,21)
The chassis ground 30 should go to the enclosure and the 0v ground from the power supply should also connect to the chassis (&30) at one point.
The input and output twisted pair shields should connect to chassis.
If you connect it this way and it still has hum, start trying to diagnose where the hum is originating. I.e. bridge the input, does it still hum. etc.

 

[Marceldarcel]

Hi dmp,

thanks for your reply!
What do you mean with „twisted pair“ shields?

So I connect Pin 30 with i.e. a shoe to a screw of the V372‘s back?

Cheers
Marcel

 

[ccaudle]

What do you mean with „twisted pair“ shields?

I think dmp assumed you are using balanced connections in and out, and balanced audio connections use twisted pair for signal, typically with an overall shield. Just a generic way to describe standard Belden, Mogami, Canare, etc. wiring you would use for balanced connections.

So I connect Pin 30 with i.e. a shoe

A shoe? I think something is missing in translation there.

To what does this module connect?
Anyone know the history of why there is even a "chassis" pin on a module like this? Typically the power supply 0V to chassis connection would be once for the entire chassis, not once per module if you have an enclosure with multiple modules.

 

[Marceldarcel]

Hey fellas,
thanks again very much for your help! The V372/1 is now running well & sounds amazing!

It’s still a prototype…

Thank you!!!!

 

[sonolink]

Hey fellas,
thanks again very much for your help! The V372/1 is now running well & sounds amazing!

It’s still a prototype…

Thank you!!!!View attachment 121831

So what was the problem and how did you fix it in the end, Marcel?
Cheers
Sono

 

[Marceldarcel]

So what was the problem and how did you fix it in the end, Marcel?
Cheers
Sono

Hi Sono,
I connected Pin 30 & 29. And I racked a Siemens N224b… Now it‘s working just fine That Tab/Telefunken stuff doesn‘t get along with my lab power supplies.

Thanks again everyone!