Oscilloscope repair help!

[carebare47]

Hello all =),

I managed to get myself a free oscilloscope. It was sent down from a friend in London who hadn't used it for some time and wasn't sure how well it was (or wasn't) working. It is an iwatsu ss-5702 20MHz dual-channel oscilloscope. He said that the second channel didn't work properly as it was damaged by a mover when he last moved house (possibly dropped). He said that the line bent a bit at the end but in my experience it doesn't detect input at all and the volts/div knob gets stuck if you try to turn it fully in either direction (picture: 2010-10-21 20-58-21.269.jpg). The scope has slightly bent casing at the bottom front left corner. Also, the plastic border around the display is slightly cracked. The oscilloscope was packaged in a large box with about 3-4 inches of foam packing peanuts around it and delivered by parcleforce.

The first channel, when the scope arrived this morning, worked okay. It took a while to calibrate, but once set properly it correctly showed a voltage of 3v (from my power supply) in a straight-ish line that went right across the screen (but only when I had the scope set on a voltage/div of .2, I think it showed no voltage when set to .5). It also happily displayed the audio output from my ipod (just did this to see if everything was working).

I went to school, and when I got home I managed to get the same results. Then it changed and now many strange things are happening ^_^. When I connect the scope to nothing with a time/div of 2 milliseconds and a volt/div of .1 I get this: 2010-10-21 20-26-25.306.jpg. If I decrease the time/div to .1 milliseconds then the line flattens a bit (2010-10-21 20-26-49.909.jpg). I then connected the probe form channel 1 with a volt/div of .5 to the calibration pin (0.3v and I think its meant to produce a square wave). It gave me a flat line of the same length as the last picture in the same place but with a slight fuzz around it (looked a bit like a triangular wave around the line but difficult to see). I then lowered the volt/div to .2 and got this: 2010-10-21 20-28-33.820.jpg

When I lowered the volt/div even more to 20mV I got this: 2010-10-21 20-28-45.980.jpg This upset me a bit, as it looked like my oscilloscope had a rather sad face =(. Decreasing the time/div to 20 µsecs produced this: 2010-10-21 20-29-12.459.jpg

Increasing the time/div from 50 µsecs to 2 milliseconds gave these images:
2010-10-21 20-29-19.288.jpg
2010-10-21 20-29-26.325.jpg
2010-10-21 20-29-32.470.jpg
2010-10-21 20-29-38.252.jpg
2010-10-21 20-29-47.525.jpg

At this point (1 millisecond time/div, 20mV volt/div) I got this... interesting picture: 2010-10-21 21-30-26.633.jpg Upon testing the horizontal position of the scope starting at the left and twisting to the right I got these images (.1 volts/div, 2 milliseconds time/div):
2010-10-21 20-31-00.077.jpg
2010-10-21 20-31-07.373.jpg
2010-10-21 20-31-20.303.jpg
2010-10-21 20-31-26.103.jpg

Throughout all this the other scope settings were: Input: Channel 1, AC-GND-DC was set to DC, power: on, intensity and focus at a comfortable level, scale was set in the middle (I don't know what this does), source for EXT was channel 1 (but nothing was connected to ext), sweep mode: auto, Coupling: AC (ext DC) (the other option was TV-V). There was an option for 10x magnification, that was off and the red knob on top of the volt/div and time/div knobs was set all the way to the right so that it clicked in place.

I know that its an old oscilloscope, but I think I would gain a fair big of experience in attempting to repair it, and I have always wanted an oscilloscope because they are invaluable in fault-finding with home made circuits (and look awesome in my room ^_^). I tried to find a copy on the manual but without success. I am relativity good solder-er and have worked with high voltage appliances before (well, looked inside a couple of CRT TVs and know the safety procedures for high voltages). I have equipment for soldering, a multimeter and a power supply cannibalised from an old computer PSU. Sorry for the bad quality pics, if higher def ones are really needed then I think I can borrow a friends high-def camera and re-upload. Any help at all would be massively appreciated.

Many thanks,

Tom


EDIT: after opening and briefly visually examining the insides of the scope, I discovered the cause of the stuck second channel http://flic.kr/p/8LVZuv & http://flic.kr/p/8LVZte. Those gaps are cracks in the part of the PCB that connects to the volt/div adjuster on channel 2.

P.S. All photos now uploaded to flickr with very brief description. http://www.flickr.com/photos/30423719@N06/

I guess the thing I want to get fixed most is making the line longer (and slightly thinner) and getting the scope to display the test signal from the calibration output properly instead of just displaying two lines.

 

[(*steve*)]

I'm not sure if your intensity is set way too high, the focus is poor, but the trace should be a thin line, and it should go all the way from the left to the right of the screen.

The unit has had one hell of a thump as evidenced by the bent case. I guess you're lucky that the tube wasn't smashed.

Since you've found obvious damage, repairing that should be the first step.

Please be careful because there are very high and dangerous voltages inside!

Oooh, just looked at that switch. It may be very difficult to fix that.

It's one sick CRO

 

[carebare47]

I'm not sure if your intensity is set way too high, the focus is poor, but the trace should be a thin line, and it should go all the way from the left to the right of the screen.

The unit has had one hell of a thump as evidenced by the bent case. I guess you're lucky that the tube wasn't smashed.

Since you've found obvious damage, repairing that should be the first step.

Please be careful because there are very high and dangerous voltages inside!

Oooh, just looked at that switch. It may be very difficult to fix that.

It's one sick CRO

I can actually get an output from channel 2 now that I have removed the case, but only on .2 volts and below. And I cant seem to make the line any thinner by adjusting the focus or the intensity (and they both actually move the line about 2cm up and to the right a bit when turned)

 

[carebare47]

The really frustrating thing is it worked enough to be usable when I got it!!

 

[(*steve*)]

It is really badly damaged.

Have you seen the display from an oscilloscope that isn't broken?

As I said, the damage on the switch is really serious. The more you move that switch the worse you'll make it.

 

[carebare47]

It is really badly damaged.

Have you seen the display from an oscilloscope that isn't broken?

As I said, the damage on the switch is really serious. The more you move that switch the worse you'll make it.

*Stops playing with switch*

Yes, I use oscilloscopes as a part of my AS electronics course. That damaged switch is only for the voltage/div switch on the second channel, but all the pictures that I took (apart from the pics of the broken second channel) were of inputs on the first channel. I still need to figure out why things are not going well on that.

 

[carebare47]

Ok, I gave it a hit and poked the insides a bit and now its working ^_^ Thanks for the help