dbx 160A with noisy bypass switch

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pvision

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Joined
Feb 1, 2014
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Location
Brighton, UK
I have a dbx 160A with a noisy bypass switch. I had a snoop around and found 8 VDC on pin 2 of the output

I can't find a schematic for the 160A but believe it's broadly similar to the 160X & XT. The XT has a pair of 5534s to drive the output, the two opamps being used for the two halves of the balanced output

There's no 5534 in the 160A but there is a pair of 5532s. Assuming one of those to be the output drivers I checked them for DC on the outputs. IC6 is the driver for the balanced output and has 8 VDC on pin1. It has 4 VDC on the input

The 160XT is all DC-coupled apart from a blocking cap between the input buffer and the VCA. Assuming the 160A is similar I need to find where the DC is getting on to the audio path

Finding the fault is tough without a schematic and/or a layout. A schematic or suggestions how to proceed gratefully received!

Nick Froome
 
I think for the outupt the schematic is more similar to the DBX166A, it has  a 5532 for the balanced output.
 
pvision said:
I have a dbx 160A with a noisy bypass switch. I had a snoop around and found 8 VDC on pin 2 of the output

I can't find a schematic for the 160A but believe it's broadly similar to the 160X & XT. The XT has a pair of 5534s to drive the output, the two opamps being used for the two halves of the balanced output

There's no 5534 in the 160A but there is a pair of 5532s. Assuming one of those to be the output drivers I checked them for DC on the outputs. IC6 is the driver for the balanced output and has 8 VDC on pin1. It has 4 VDC on the input

The 160XT is all DC-coupled apart from a blocking cap between the input buffer and the VCA. Assuming the 160A is similar I need to find where the DC is getting on to the audio path

Finding the fault is tough without a schematic and/or a layout. A schematic or suggestions how to proceed gratefully received!

Nick Froome

By noisy bypass switch, can I ASSume you mean that it clicks or thumps when you switch it, not that it causes noise in the signal?

A click or thump when switched suggest an excessive DC when in one mode or the other. A DC problem is probably something simple and perhaps look at one of the similar models. Simple but could be major like a bad VCA.

JR
 
Thanks for the replies. I have a schem for the 166 and will compare it with what I see

The problem is definitely the DC on the output and it thumps like crazy, as you'd expect

Good call re the unbalanced output. There's an opamp buffer feeding the output transistors. I'll check both sides of that to see what I find. The input to the buffer and the input to the 5532 for the balanced output come from the same point which is also TP9 on the 160XT. I'll check all the 160A test points
 
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