Calibration failure PCSU1000 CH1

Instruments PC Oscilloscopes and Recorders
1

When I power up the oscilloscope, the blue CH1 trace is missing and on running calibration I get:
Calibration results:

CH1 offset at 2V/div : 174 FAIL
CH2 offset at 2V/div : 117 OK
CH1 offset at 1V/div : 51 FAIL
CH2 offset at 1V/div : 2 OK
CH1 offset at 0.5V/div : 51 FAIL
CH2 offset at 0.5V/div : 5 OK
CH1 offset at 5us/div : 41 OK
CH2 offset at 5us/div : 1 OK
CH1 Y-position low 31
CH1 Trigger : 50
CH1 Trigger : 47
CH1 Trigger low : 48 OK
CH1 Y-position mid 127
CH1 Trigger : 92
CH1 Trigger : 89
CH1 Trigger middle : 90 OK
CH1 Y-position high 163
CH1 Trigger : 108
CH1 Trigger : 103
CH1 Trigger high : 106 OK
CH2 Y-position low 31
CH2 Trigger : 49
CH2 Trigger : 46
CH2 Trigger low : 48 OK
CH2 Y-position mid 126
CH2 Trigger : 92
CH2 Trigger : 87
CH2 Trigger middle : 90 OK
CH2 Y-position high 223
CH2 Trigger : 135
CH2 Trigger : 130
CH2 Trigger high : 132 OK
EXT Trigger middle : 50 OK

I searched the forum and saw a similar topic but it mentions R48 as going high (should be 10 ohms). However
my R48 is reading 9 ohms in circuit. Also they replaced IC11. Help please! My s/n is 201108899 running on Windows XP Pro.

2

Indeed, maybe IC11 is damaged.
You may do following test:

  • Run the software.
  • Select couplings GND.
  • Measure the voltage on the output pin 6 of IC11.
    The voltage should be about zero.
    As a reference you can check voltage on the output pin 6 of IC4.
3

Do you mean that the reference on IC4 should also be zero volts?
Both IC4 and IC11 read 1.7v on pin 6 with GND coupling.

4

Yes, the voltage on pin 6 of IC4 and IC11 should be about zero.
I checked the voltages in my PCSU1000 and got following results:
IC11 pin 6 : 7.2mV
IC4 pin 6 : 1.5mV

5

How do I attach a screenshot after calibration failed. Both IC’s showing 1.7 volts on pin 6. The image is in my Downloads folder.

I read the pin 6 for a longer time and IC11 cycles between 1.7 and 1.3 volts while IC4 cycles 1.7 and 1.6 volts.

Sounds like a capacitor not holding charge?

Sorry I feel like a fool. I was measuring IC12 for IC4. Getting the same reading though on IC4. Cycling between 1.7 and 1.6 volts.

6

[quote]How do I attach a screenshot after calibration failed.[/quote]You can upload the picture to a free file hosting service and post the link here.

Please check the supply voltage on IC11 and IC4.
There should be about -2.1V on pin 4 and 3.3V on pin 7.

You can use the input BNC connector “shield” as the ground reference when doing the measurements.

7

Yes, those voltages are present on both ICs. Seems I was not using a proper ground hence the bad readings. On pin 6 I now get the mv readings you first mentioned. IC11 329 mv and IC4 2 mv.

This is the screenshot: dropbox.com/s/xsbduktoqy21u … t.png?dl=0

8

It seems that IC11 is defective.

9

The CH1 blue trace ends up at about 2.5 divisions above ground after the calibration attempt. While CH2 red trace is at ground as expected. This is shown on the screenshot. Are you able to access it?

10

From the markings IC11 is a TI op amp 354a. Where can I get it from?

11

Yes, I noticed the problem in the screenshot.
It seems that IC11 is “partly” defective.
You can use the oscilloscope, but there is quite a big offset on CH1.

12

The IC11 is OPA354AIDDA
Manufacturer is Texas Instruments.
It is available from many distributors.

13

Because of the bad readings I made, I rechecked the R48 resistor and it is now showing 33 Meg ohms in circuit, not 9 ohms as I previously reported. That reading is for the C40 cap not R48. I do get 10 ohms across R48, sorry for the waste of your time.

14

OK - no problem.

So, maybe it is IC11…

15

So first thing to do is change IC11, correct?

16

Yes, I think the problem is most probably there.
Alternatively diode D6 or D8 may be “leaking”.
This can cause similar symptoms.

You can try to check possible diode leakage by measuring the voltage on pin 3 of IC11.
There shouldn’t be no more than about 0.5mV.