Repairing an oscilloscope

IWATSU 350MHz 4 Channel Scope

A Green Splodge

Just over 10 years ago someone brought me a scope to look at for repair. They had bought it as non working, and was hoping that I could have a quick look at it, to see if it could be easily repaired.

When I say non working, it powered up, but all that could be seen was a non focused display, or as we prefer to say in the trade it was best described as a green splodge in the centre of the screen.

The scope was a 350 MHz bandwidth, 4 channel, with digital readout, so was well worth examining. A quick inspection showed that the electrolytic capacitors had dried out in the PSU, and they would all need changing. I priced the parts up, (about £75 at the time), and quoted for 3 hours labour, which would have been another £75, giving a total of £150, plus a caveat that there could be other faults which would also need to be attended to.

The Inspection

The first thing to do is look for the obvious, and start with the basics. Nothing is going to work without the correct voltages, so the PSU was the first point of call.

A quick inspection showed that the electrolytic capacitors had dried out in the PSU, with evidence that they had leaked all over the printed circuit board, so they would all need changing. I priced the parts up, (about £75 at the time), and quoted for 3 hours labour, which would have been another £75, giving a total of £150, plus a caveat that there could be other faults which would also need to be attended to. The customer said that they would think about it. After several repeated phone calls to the customer, which resulted by me saying that if he didn’t want it repaired, please could they come and collect it. The scope then got pushed to the back of the workshop and did not see the light of day until this month. I had a few days slack in my schedule, so I decided to repair it for myself.

An Entire Workshop Manual for Download

One of the first challenges was to try and obtain some circuit diagrams to help with any fault finding. This is one area that a lot of customers do not appreciate, which is before any serious work can be done on equipment, you need the circuit diagrams. I have lost count of the number of hours I have spent chasing up odd ball circuits for customers equipment, which is difficult to charge to customers.

Being as this scope was over 10 years old, I really thought that this was going to be a daunting challenge. I found a web site for the manufacturer of the scope (Iwatsu – based in Japan), and sent a request via the contacts web page for any information that they might have. In less than 24 hours I had a reply, which contained a link to their server, where upon I could download the entire workshop manual, including all the circuit diagrams and much more, in a pdf format file, which was just over 15MB in size. Boy was I impressed! How many other companies keep those sort of documents for outdated equipment, over 10 years old.

Rock and Roll

I was now ready to rock and roll, and start working on the scope. I carefully removed the PSU board and set about changing all the electrolytic capacitors. Once they were all removed the board had to be tediously cleaned, as the leaking capacitors had left their excretions al over the pcb. As the pcb was double sided, I had to make sure that the solder would flow well on all the top joints as well as the bottom. New capacitors were ordered (cost just under £100). They were some oddball values and voltages, and as with a lot of suppliers, you can not simply order just one, you have to buy a pack of 5 or more.

 

Now was the time to test it.

Mains powered up, everything hummed, and hey presto – green splodge in middle of screen! It is at times like this that you take a strong coffee break. OK time to check everything again. After checking all the voltages, I fond that the +130V rail is not working. This is the one that is required to bring focus to the display, that figures! The culprit for this was a sneaky little diode hiding on the final output rail of the 130V line. It measured perfectly normal on the meter with no power being applied, but was breaking down and going short when it had a voltage applied to it. It received a well deserved sending off ceremony after removal, along with an engineers epitaph, as it was sent to the great semiconductor graveyard in the corner of the workshop.

Time to try again. More success this time, and with a few adjustments on some selected trimpots following the guidance from the excellent manual from Iwatsu, the scope now has a beautiful clear display. Judge for yourself. Quick clean and polish on the casing, and one smart piece of equipment to decorate the corner of the work bench.

And voila

 

 

 

Pictures showing display with digital readout, and 1 KHz and 100 KHz waveforms. Sorted!

If you have an oscilloscope that needs to be fixed, get in touch with us at CJS-Services to get it repaired.