Looking for help for circuit analysis. Troubleshooting a DBX 1066 compressor/gate/limiter

Thread Starter

Kesson

Joined Nov 7, 2019
5
Hi everybody! Hope you're good! :)

I'm strugglin since some days on that nice muscial tool. I've replaced some faulty switches. Replaced them with new one from Mouser.
Sadly I've managed to hurt some VIAS connecting both side of the board. Luckilly enough this is just 2 layer board (top/bottom) so I can manage to restore the connection easily.
I already found one problem quite faste by signal tracing with my oscilloscope and "beeping" some traces and fixed it. So sound is back on both channels. There still some problems and yesterdy, after overloading the input by accident, I apparently killed one of it's channel.

So now I need to understand exactly what went wrong and I wanna understand the schematic, but for this I need some help. I've learned much about eletricity but It's some years ago and I lack experience.
So I drawed this for you: (the second unmarked "+4/-10" switch is SW2C)!

EDIT: I also have forgotten to mention +/- of the opamp. So this is an inverting one, so the upper pin is - and the lower +

20191116_082238.jpg

So I'd like to know what my output will be if at Tx input is 1V. What resistors to take into account for the gain calculation?
As this is balanced audio input IN1- will always be the opposite of IN1+! So for 1V IN1- = -1V
Why are there to different grounds? What's the effet they're looking for?
So my analysis made me thinking this is also a band-pass filter because of C52 and C51 Right?

By the way, original schematic can be found under goolgle with "dbx1066 schem" query if you're interested.

Any help very much appreciated!

Cheers,
David
 

Thread Starter

Kesson

Joined Nov 7, 2019
5
I must apologize for the errors!

First this is apparently not in inverter but a substractor! I've looked more closely into my reference and turned that page wich showed more opAmps configurations!

Actually I'am taking measurements on the channel that seems to work fine. I feed a 1KHz 1Vp-p signal into the DUT.
I measure 0.4VACpeak at the inputs and 1.26VACpeak at the output of the first stage in the DUT wich is U7A.

I sadly messed that thing a bit up but hopefully I will learn even more! Any assistance would be greatly appreciated!

I will post more infos with pictures of the oscilloscope and detailled infos about the steps I take to troubleshoot this device. But I think I will need further assistance at some point soon.

Best regards,
David
 

Thread Starter

Kesson

Joined Nov 7, 2019
5
Thank you Eric!

Yes not the clearest. I try to avoid mistakes but I have to try harder :)

Here are some picture of the waveforms at in and outs from U7A (refer to schematic attached). There is also attached a pictures of my reference. So I now I think I'm not so bad in the understanding. I'm just a bit in trouble for taking measurments for Ue1 and Ue2 because at this point I read 0V at both points (to GND). I have to investigate more to be sure measuring the right resistor because it's not clearly marked... (see image, markings sometimes under resistor so I have to figure out by beeping and reading out color coding)

I will soon come to the next stage where things get much more complex.
At this point what I can say is that there is no output on channel 2 to be more precise signal is no more present at U14B aka U7B in channel 1. I didnt investigate much more at this point because I don't really understand the circuit and don't know where and what exact measeurements to take.

Actually I'm looking only at channel 1 an try to understand what's going on and verify that everything is where it should be.
I try to verify the input stage to see if everything work as expected. Sadly I'm yet not able to calculate down the signal path. For example Ue1 and Ue2 (refer to "theory.jpg") I try to measure but at this point I only measure 0V I'm not sure why?

Sorry for the mess, my errors, I'm willing to learn!


See ya
 

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Thread Starter

Kesson

Joined Nov 7, 2019
5
If you look on the PDF schematic you see U7B(channel1) aka U14B(channel2) this is the output stage on same page but all to the right. This is where my signal disappears on channel 2 (not working).
What I've attached is U7A on channel 1. This part is also working on channel 2!

So I made the measurments. I've already measured power and it looks fine but strangely, when I've on "DC" with my scope and measure Pin 1 I get the same as in "AC" mode. Is this normal?

How could I measure Ue1 and Ue2? And make the calculation and compare that to the schematic using the formulas given by "theory.jpg"?
 

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ericgibbs

Joined Jan 29, 2010
8,865
hi,
OK, a little confusion, you did say Chan1 .
Look at this Chan 2 clip and mark on it the points at which the 1kHz test is present and where it is missing, then post the marked up drawing.
E
 

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Thread Starter

Kesson

Joined Nov 7, 2019
5
Yes sorry it's a bit confusing. So I got further signal tracing and I've updated you're image. I'm not able to mesure anything at U12 only pin 4 says -2.5VDC. Strange. But actually channel 1 is the same and is working so maybe it's ok...

Any suggestions?
 

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ericgibbs

Joined Jan 29, 2010
8,865
hi,
If the 1kHz test signal is on U15B input pin #6 , there should be a signal on pin #7 of that OPA.
U12 is Voltage Gain controlled amplifier, for pin #2, check the setting of pot P4 for the VCA and compare the voltage with Chan1 setting.
E
 
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