Salvaged PCS500 needs multiple calibrations

Instruments PC Oscilloscopes and Recorders
1

I’ve salvaged a PCS500 literally from the bottom of a dumpster, full of water and sand/dirt inside. My guess it had been lying outside all winter (60cm of snow and -25 celsius), but after a rinse under warm water, some cleaning with a toothbrush, electronics cleaner and a nice warm and dry position for a week I started the scope up. Eureka, it came to life!

Calibration failed, but I tried a few times and finally it passed calibration, and I could start measuring. Now every time I start the scope it needs a few calibrations before it passes, and during the calibration I see high amplitude noise (sometimes 18V). Once the calibration has passed in PC-LAB 2000SE it seems to work quite OK with little noise. Is it normal that it needs to warm up?
Also it seems there is a user mod with a capacitor across one of the IC:s.

I am at work right now so can not supply any calibration logs or screenshots, but I’ll leave you with a few clickable pictures of the state of the scope when I found it. You can also see the cap mod (?).

2

I should probably add that I am using a power supply I found at home, rated 9V 1000mA, could this be the culprit?

3

Too low power supply voltage may be reason to the calibration fail.
Also the PCB seems to be damaged. A cleaning under distilled water may help or not.

4

The images were before cleaning, it is near clinically clean now, so I suspect that that is not the problem. On what do you base your statement that the PCB is damaged? It certainly is a possibility due to the nature of where I found it.

I’ll dig out another power supply to test with, I guess I need to keep close to the 9-10v specified? I have plenty of powerful supplies at higher voltages. Anyway, it does not bother me that much that I need to recalibrate a few times.

Calibration 1

[code]Calibration results:

CH1 offset at 15V/div : 275 FAIL
CH2 offset at 15V/div : 250 FAIL
CH1 offset at 5V/div : 51 FAIL
CH2 offset at 5V/div : 47 OK
CH1 offset at 15mV/div : 51 FAIL
CH2 offset at 15mV/div : 51 FAIL
CH1 offset at 5mV/div : 102 FAIL
CH2 offset at 5mV/div : 102 FAIL
CH1 offset at 20us/div : 197 FAIL
CH2 offset at 20us/div : 168 FAIL
CH1 offset at 10us/div : 201 FAIL
CH2 offset at 10us/div : 162 FAIL
CH1 offset at 5us/div : 193 FAIL
CH2 offset at 5us/div : 76 FAIL
CH1 Trigger low : 2 FAIL
CH1 Trigger middle : 2 FAIL
CH1 Trigger high : 2 FAIL
CH2 Trigger low : 4 FAIL
CH2 Trigger middle : 3 FAIL
CH2 Trigger high : 2 FAIL
EXT Trigger middle : 2 FAIL
[/code]
Round #2

Calibration results:

CH1 offset at 15V/div      :	190	FAIL
CH2 offset at 15V/div      :	192	FAIL
CH1 offset at 5V/div   :	5	OK
CH2 offset at 5V/div   :	11	OK
CH1 offset at 15mV/div :	25	OK
CH2 offset at 15mV/div :	36	OK
CH1 offset at 5mV/div  :	94	FAIL
CH2 offset at 5mV/div  :	44	OK
CH1 offset at 20us/div :	-13	OK
CH2 offset at 20us/div :	-9	OK
CH1 offset at 10us/div :	-19	OK
CH2 offset at 10us/div :	-15	OK
CH1 offset at 5us/div  :	-19	OK
CH2 offset at 5us/div  :	-16	OK
CH1 Trigger low            :	87	OK
CH1 Trigger middle         :	110	OK
CH1 Trigger high           :	136	OK
CH2 Trigger low            :	86	OK
CH2 Trigger middle         :	114	OK
CH2 Trigger high           :	66	FAIL
EXT Trigger middle         :	38	FAIL[/code]
Round #3
[code]Calibration results:

CH1 offset at 15V/div      :	120	OK
CH2 offset at 15V/div      :	126	OK
CH1 offset at 5V/div   :	1	OK
CH2 offset at 5V/div   :	2	OK
CH1 offset at 15mV/div :	0	OK
CH2 offset at 15mV/div :	7	OK
CH1 offset at 5mV/div  :	0	OK
CH2 offset at 5mV/div  :	22	OK
CH1 offset at 20us/div :	0	OK
CH2 offset at 20us/div :	0	OK
CH1 offset at 10us/div :	-2	OK
CH2 offset at 10us/div :	-2	OK
CH1 offset at 5us/div  :	-3	OK
CH2 offset at 5us/div  :	-4	OK
CH1 Trigger low            :	48	FAIL
CH1 Trigger middle         :	123	OK
CH1 Trigger high           :	147	OK
CH2 Trigger low            :	96	OK
CH2 Trigger middle         :	122	OK
CH2 Trigger high           :	147	OK
EXT Trigger middle         :	108	OK

And number 4 - pass

Calibration results:

CH1 offset at 15V/div      :	120	OK
CH2 offset at 15V/div      :	126	OK
CH1 offset at 5V/div   :	1	OK
CH2 offset at 5V/div   :	1	OK
CH1 offset at 15mV/div :	0	OK
CH2 offset at 15mV/div :	7	OK
CH1 offset at 5mV/div  :	-1	OK
CH2 offset at 5mV/div  :	22	OK
CH1 offset at 20us/div :	0	OK
CH2 offset at 20us/div :	0	OK
CH1 offset at 10us/div :	-2	OK
CH2 offset at 10us/div :	-2	OK
CH1 offset at 5us/div  :	-3	OK
CH2 offset at 5us/div  :	-4	OK
CH1 Trigger low            :	98	OK
CH1 Trigger middle         :	122	OK
CH1 Trigger high           :	147	OK
CH2 Trigger low            :	97	OK
CH2 Trigger middle         :	122	OK
CH2 Trigger high           :	148	OK
EXT Trigger middle         :	108	OK

I’ve done nothing between these calibrations, just gone to the hardware setup and restarted.

5

The last calibration is very good. It is strange why the warm up period is needed if there is no moisture on the PCB any more. Maybe some more cleaning of the PCB is anyhow needed…

6

I’ll see if I have another go at cleaning the board then, otherwise I can live with a couple of calibrations extra.

What about the capacitor mod?

7

[quote]What about the capacitor mod?[/quote]It may be good idea to change this capacitor. The C57 is 100nF.

Here is a link to download the complete circuit diagram and operation description:
box.net/shared/ybz9k9adq4

8

I was thinking about the cap soldered across IC14 in my pictures, looks like a home mod, but I don’t know why?

9

[quote]I was thinking about the cap soldered across IC14 in my pictures, looks like a home mod, but I don’t know why?[/quote]OK - Now I see… This is a factory made modification. The capacitor should be there.

I was looking the other picture where is the very bad looking C57.

10

Yes it did not look nice there… A bit better here once cleaned , but still not nice.

Anyways, I’ll dig out another power supply to see if that changes anything.

11

Looks much better!
Is there any difference if you keep the scope ON for some time (e.g. 1h) before trying the calibration?

12

I’ll try that the next time. Will post here.

13

I let it stand with power on for an hour or so, then did the calibration, and the result: PASS

[code]Calibration results:

CH1 offset at 15V/div : 120 OK
CH2 offset at 15V/div : 126 OK
CH1 offset at 5V/div : 0 OK
CH2 offset at 5V/div : 1 OK
CH1 offset at 15mV/div : -2 OK
CH2 offset at 15mV/div : 4 OK
CH1 offset at 5mV/div : -8 OK
CH2 offset at 5mV/div : 13 OK
CH1 offset at 20us/div : 0 OK
CH2 offset at 20us/div : 0 OK
CH1 offset at 10us/div : -2 OK
CH2 offset at 10us/div : -2 OK
CH1 offset at 5us/div : -4 OK
CH2 offset at 5us/div : -5 OK
CH1 Trigger low : 100 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 : 108 OK
[/code]

14

The calibration results are very good.

I think the reason to this is that there is some conductive material (salt) on the PCB. Normal room humidity makes it conductive when the power is turned OFF.
When turned ON, the unit heats and its internal humidity decreases and the “salt” is no longer as conductive.

I think your PCS500 is anyhow usable “as is”.

15

That is a plausible explanation. I think I’ll leave it as it is, the risk of ruining it with another cleaning is probably a bigger problem than calibrating a few times or waiting a bit longer.

16

Wow,

I think I want to hang out in the dumpsters in your area.

Good find.

17

[quote]I’ve salvaged a PCS500 literally from the bottom of a dumpster, full of water and sand/dirt inside. My guess it had been lying outside all winter (60cm of snow and -25 celsius)[/quote] I think this is the most severe environmental test ever made to the PCS500. - Still survived!

18

Yes, it is not exactly the normal storage conditions.