Spectrasonics 110A Large DC offset problem

[Biasrocks]

I'm in the process of racking up a few 110A cards.

Balanced 600:600 input through a A67J transformer, unbalanced output.

I have a bi-polar +/-24 volt supply and have followed the pinout hookup directions in the 110 manual.

Pin
1 (A)
2 (B)
3 (C) -24v (B-)
4 (D) Signal Input + (Input transformer secondary +)
5 (E) Signal Input - (Input transformer secondary -)
6 (F) Signal Output - (Power supply common) (Grounded to the power supply, shields of input transformer and shield + Pin 3 of output XLR)
7 (H) Signal Output + (Pin 2 of output XLR)
8 (J) +24v (B+)
9 (K) DC Signal Output + (One leg of 10K FB resistor)
10 (L) Feedback (other leg of 10K FB resistor)

What's happening is I'm getting around 22Vdc on Pins 4,5 & 7.

I have no idea where this offset is coming from, but it's present on either side of both capacitors which should be stripping it off. I recaped, and tried 3 different cards, all with the same results.

What the hell am I doing wrong?

Mark

 

[thermionic]

Have you looked at the 'DC' with a 'scope? According to Klett, Spectrasonics cards need a lot of attention to detail or they will oscillate.

Justin

 

[Biasrocks]

Good point. No, I haven't done that.

Is the oscillation problem generally related to grounding issues?

Update: It turned out to be a ground issue, I hadn't grounded the Input -. As soon as I did that the oscillation went bye bye and no more offset on the output.

Many thanks for pointing me in the right direction.

Mark

 

[Di_Sousa]

Hi Mark,

I will start racking my SS 110A cards but I'm a bit confused on what transformers to use at the input.

Theres seems to be some confusion around these cards, 101 , 110 and 110A.

I know the transformers used at the input of 101 and 110 cards were Triad A-67J since the input impedance of these boards is 600 ohms, but the input impedance of the 110A is different it's 2,5K,so my question is,
should another input transformer be used for the 110A card since the A-67J  is 600 : 600  or is it fine?

thanks,

Tiago

 

[Biasrocks]

I definitely used the 67J in 1:1 configuration. I may have modified the input impedance of the card to 600 ohms, which is one resistor iirc. I think the only difference between the 110a and 110 is that resistor.

Regards,
Mark

I know the transformers used at the input of 101 and 110 cards were Triad A-67J since the input impedance of these boards is 600 ohms, but the input impedance of the 110A is different it's 2,5K,so my question is,
shouldn't another input transformer be used for the 110A card since the A-67J  is 600 : 600  ?

thanks,

Tiago