DBX 160x always shows Gain Reduction

[dandeurloo]

Hey guys, I have a 160x that recently started to show 6db of gain reduction even when nothing is plugged into it.  It still passes audio but the meter bugs me.  Have any of you ever had this issue, any suggestions on what may cause it.

It goes away if I switch the unit to slave mode so it must be something in the sidechain meter path before the slave mode is switched in. 

Thanks

 

[pucho812]

Schematic.... 


https://3e7777c294b9bcaa5486-bc95634e606bab3d0a267a5a7901c44d.ssl.cf2.rackcdn.com/product_documents/documents/602_1324058489/160X%20Schematic_original.pdf

 

[dandeurloo]

Yeah, I have been working my way through it.  Basically working my way back from the Slave switch in the sidechain. 

So far no luck.  I am still poking around in it trying to get to the bottom of it.

 

[JohnRoberts]

Is the meter indicating actual gain? Is it actually running -6dB?

That could suggest where to look... either meter error, or  side chain control voltage error.

JR

 

[Bo Deadly]

If that schem matches your particular model then it should be fairly straight forward to trace backwards from the LED ladder using a basic volt meter. Start with pin 3 of P101 / J112. Voltage should be near zero in steady state. If it's not (positive a few volts), then you know the problem is up-stream. Try grounding pin 3 (but maybe through a 100R resistor since it seems it can connected directly to the output of U108B so limit the current with 100R just as a precaution. If it corrects the issue, then the LED ladder circuit is ok. Then just trace backward to what looks like switch S102B. Locate and test each point and check to see if it's zero volts with no signal. If not, keep tracing upstream. It could some noise is getting into the sidechain. Or some trimmer lost contact. Or a cap failed short. Divide and conquer.

 

[dandeurloo]

Been working through it.  Something goofy is going on.  I am thinking one of the trimmer pots has opened up or lost its adjustment.  My unit is really old and doesn't follow the schematic exactly but I do think overall it is working mostly right. 

I'll keep poking at it.

 

[Monte McGuire]

So if the trimer opened up or otherwise failed, then there is actual GR voltage applied to the VCA, which means that the GR display is working properly. Replace the trimpot, or just remove it for the time being and forego the THD trim?

 

[dandeurloo]

The question is which trimmer could be bad?  The circuit has a number of them that I believe all interact with each other.  More sleuthing to come!

 

[dandeurloo]

The meter PCB had a few faulty IC's.  Swapped those out and it is all working again!