PCS500 not calibrating

Instruments PC Oscilloscopes and Recorders
1

We use your machines PCS100 and PCS500 for more than two years.
They worked fine before today.

PCS500 shows strange wave-form with frequency 2.98 kHz on both inputs and all “volts/div” buttons except “15mV” and “5mV”. “CH1” and “CH2” inputs are free and “Coupling” is “GND”.

Look at the pictures (they are done using “standart” calibration settings for PCS500, taken from here https://forum.vellemanprojects.eu/t/faulty-pcs500/433/1)



This is calibration.log:
Calibration results:

CH1 offset at 15V/div : -24 FAIL
CH2 offset at 15V/div : -24 FAIL
CH1 offset at 5V/div : -51 FAIL
CH2 offset at 5V/div : -51 FAIL
CH1 offset at 15mV/div : 51 FAIL
CH2 offset at 15mV/div : 12 OK
CH1 offset at 5mV/div : 102 FAIL
CH2 offset at 5mV/div : 102 FAIL
CH1 offset at 20us/div : -201 FAIL
CH2 offset at 20us/div : -201 FAIL
CH1 offset at 10us/div : -201 FAIL
CH2 offset at 10us/div : -201 FAIL
CH1 offset at 5us/div : -201 FAIL
CH2 offset at 5us/div : -201 FAIL
CH1 Trigger low : 128 OK
CH1 Trigger middle : 128 FAIL
CH1 Trigger high : 128 FAIL
CH2 Trigger low : 128 OK
CH2 Trigger middle : 128 FAIL
CH2 Trigger high : 128 FAIL
EXT Trigger middle : 128 OK

It seems, that triggering don’t work and Y-position scroll-bars don’t work too.

We checked +5V on PCB, it seems fine: +5.01V on the output of IC23, and +5.03V on the output of IC17.

Is it OK, that on the input (pin2) of IC1 (LM2909T) is -1.82V (with respect to main ground -pin1) and on the output (pin3) is +0.81V ?

Unfortunately, we can’t send PCS-500 to Velleman because of current location.
What else can we check and do (as an electronic engineers) to repair PCS500?

Some long ago there was a link to schematics “PCS500-9a_cir.pdf” here: https://forum.vellemanprojects.eu/t/velleman-cd-for-pcs500/1047/1 but it is not available now…

2

It seems that the -5V is not OK.
You noted that the output of IC1 is not -5V as it should be.
There should be stable and clean at the output of IC1.
Here some measured values:
Input:-7.9VDC ripple: 46mV rms
Output: -4.96VDC ripple: 1.2mV rms

Here is the charge pump waveform at the collector of transistor T14:

The frequency you see on the screen is the frequency of the -5V charge pump.

You can now download the PCS500 documentation from this link:
vel255.diinoweb.com/files/PCS500_operation.zip

3

Great thank you for incredible support!
Replacing charge-pump capacitors C13, C16, C24, C19 helped a lot.
Old ones were good-looking, but their ESR was > 50 Ohm.

Here is new calibration.log:
Calibration results:

CH1 offset at 15V/div : 110 OK
CH2 offset at 15V/div : 111 OK
CH1 offset at 5V/div : 2 OK
CH2 offset at 5V/div : 1 OK
CH1 offset at 15mV/div : 10 OK
CH2 offset at 15mV/div : 3 OK
CH1 offset at 5mV/div : 33 OK
CH2 offset at 5mV/div : 10 OK
CH1 offset at 20us/div : -1 OK
CH2 offset at 20us/div : -1 OK
CH1 offset at 10us/div : -3 OK
CH2 offset at 10us/div : -3 OK
CH1 offset at 5us/div : -4 OK
CH2 offset at 5us/div : -5 OK
CH1 Trigger low : 98 OK
CH1 Trigger middle : 124 OK
CH1 Trigger high : 150 OK
CH2 Trigger low : 98 OK
CH2 Trigger middle : 124 OK
CH2 Trigger high : 150 OK
EXT Trigger middle : 112 OK