GW GOS-522 OSCILLOSCOPE



Gw GOS-522 Oscilloscope

I bought this oscilloscope from a mate for very few dollars as it only had one beam working. He had bravely bought it on Ebay from another country. It is dual trace 20MHz and features a single shot sweep trigger mode.

A search of the web revealed no pictures or other information about it so I have given it a place on the web. If you have a problem that I may be able to help you with please contact me via my index page.

I have done a lot of fault finding on digital circuits but know very little about fixing CROs but at least I had one beam working so I had something to copy from. I downloaded a manual with a circuit from another brand of about the same vintage. This revealed the general principals of how a CRO worked and the sort of circuit that I might find..

The symptom was no beam 2 in normal operation but if both beams was selected 2 traces appeared. If a signal was put on the good beam it was reflected at half the size on the other beam. However no signal showed if a signal was put on the bad beam. My first idea that a switch was faulty was dashed when I found from the downloaded representative circuit that the switches only provided logic levels to a bunch of gates and the all important flipflop that switched  between beams. A heap of diodes do the actual switching and I suspected that one was shorted or o/c but no luck there.

I could get to the solder side of the vertical board and see most of the other side but I was having trouble tracing and remembering just what solder pad connected to what when I had the idea to take a picture and print it out. It would then be easy to pencil in the components and get an idea of the circuit.

CRO PCB

This worked like a charm and I  improved on the method by photographing both sides of my next  repair project  then fliping and combining the images and making the component picture semi transparent. It was like having Xray vision.

When checking the beam switching diodes I noticed that the voltage  on the faulty channel was near the +12 volt rail  so the hunt was on to find where this spurious voltage came from. Tracing backwards lead me to a 14 pin package  a CA3086 that thanks to Google I found was a 5 transistor array. Some metering proved that it no longer contained any transistors. Problem solved.

Using a digital camera to help trace a circuit.

A composite photo showing the Xray vision effect.
PCB showing both sides.

My next fix was a megohm insulation tester that had no high voltage. It was easy to photograph both sides of  the board and use my photo editing software (The GIMP) to flip the parts side picture, place it on to (with some stretching and rotating) the track side picture and adjust the transparency of the top layer to get a result, part of which is shown above. It could have been improved by making some of the darker components individually more transparent but it was OK for a first effort. My printer is not colour but the printout was useful.  If I added another layer I could have drawn resistor symbols and such over the top of the components then deleted the component layer and be left with a very useful and easy to read result.

The above method requires experience with a photo editor but simpler methods give good results.

To attempt to fault find a glow plug timer I photographed the solder side of the board and with IRFANVIEW ( a popular free simple photo editor) I flipped the picture horizontally, lightened it and printed it to fill an A4 sheet. All very easy with IRFANVIEW especially the printing.

The component symbols could then be drawn in rapidly by hand as their location agreed with the printout.  The hard work of understanding the circuit could then begin but made simpler by the easy to follow component connections. Fault finding can be done from the component side with no turning over the board to follow tracks.





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