Cgs synth

On Wed, May 16, 2018 at 4:11 AM, richs19@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

I'm troubleshooting a CGS85. I was going through the calibration procedure and the first two channels worked fine. I was using an input of 4.2V and was able to calibrate the outputs to match the inputs.

When I got to the third (the bottom on the wiring diagram - furthest from the power connector) the output voltage could not be raised above 1.2V. Even more strangely is that when I measure the input voltage when the 1/4" jack is inserted in the input socket, the voltage drops to .38 Volts.

It's like there's something in that section of the circuit causing a problem with the actual input signal. All the other channels work fine. I'm comparing the module to a working CGS85 built earlier and all of its inputs match its outputs.

I've re-flowed the solder. No connections look bad and I can't find any obvious bridge solders or wiring faults.

Any suggestions would be greatly appreciated!

--

Ken Stone otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>

On Thu, May 17, 2018 at 5:17 AM, richs19@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

Thanks for your reply Ken. I’ve triple checked the orientation of the diodes against the PCB and against the working module. Ditto for the other components. I swapped the LM3900 with the same results. I can’t see any obvious error although it’s obvious I’ve made one.

I took some measurements;

4.2 vDC (C3) IN

No Input

4.2V Input

#001

#004

#001

#004

IN Top

0.088

0.100

4.2

4.2

OUT DC Top

0.792

0.786

4.2

4.2

IN Middle

0.100

0.040

4.2

4.2

OUT DC Middle

0.803

0.794

4.2

4.2

IN Bottom

0.060

0.006

4.2

0.37

OUT DC Bottom

0.786

0.780

4.2

1.44

LM3900

No Input

4.2V Input

Pin

Function

#001

#004

#001

#004

1

1IN +

0.409

0.402

0.403

0.402

2

2IN +

0.408

0.403

0.403

0.400

3

2IN -

0.558

0.549

0.554

0.543

4

2OUT

0.768

0.790

0.775

0.789

5

1OUT

0.772

0.801

0.781

0.798

6

1IN -

0.554

0.549

0.551

0.544

7

GND

0.228

0.218

0.234

0.231

8

3IN -

Jumping

Jumping

Jumping

Jumping

9

3OUT

14.202

14.200

14.200

14.169

10

4OUT

0.763

0.787

4.160

1.456

11

4IN -

0.545

0.545

0.541

0.538

12

4IN +

0.395

0.402

0.474

0.414

13

3IN+

Jumping

Jumping

Jumping

Jumping

14

Vcc

15.000

15.000

15.000

15.000

#001 is the working module. #004 is the non-functional module.

The values in black are Volts. Blue are Millivolts. The value in Red is the suspect output. I tested the jack signal tabs and the pins on the LM3900 both with nothing plugged into the modules and then with a 4.2 Volt square wave coming out of an MFOS oscillator (C3). The Jumping values seemed to be in the millivolt range and appeared the same on each module.

I’m not sure if this info sheds light on where the problem may be. I’m guessing it’s a short somewhere as the value of the input drops from 4.2V to .37V as soon as the jack is plugged in.

Rich

--

Ken Stone otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>