Archive for June, 2013

It’s been some while….

June 13, 2013

since my last postings. Well one of the reasons was my professional work, and another was that my workhorse for measurements, my LeCroy 9314M scope, was broken. And my backup scope, a LeCroy 9450, was broken already from the day I rescued it from the scrapyard: the screen went dark after warmup.

So what to do – shelling out close to 1K for a decent 4-channel scope or trying a repair? This question kept me blocked for many months, till I kicked myself and started to see whether I can do something. Two of the 4 channels of the 9314M went bad already a few years before, and now the trigger stopped working, but sometimes it came back, apparently depending on the temperature. This is the worst kind of errors to have! I was afraid that one of the numerous custom ICs from LeCroy went bad, or that there would be some error in the logic board. Nevertheless I gave it a try, partly because the schematics are available in the service manual.

In fact, as silly as it may sound, looking for errors in a fantastically complicated circuitry has something to it. It is a bit like reading a criminal thriller…who is the culprit? How can we use logical reasoning combined with intuition and experience to corner it? And then, can we do something against it? Actually in the past I had successfully repaired quite a number of measuring equipment, from Tex to HP, and given up only on one thing, an intractable Schlumberger Stabilock 4040.

So I undertook this journey for a couple of evenings. First thing was to take everything apart and create extension cables for the front panel, otherwise one never would be able to come close to the live main board. And then finding the way through the SMD circuit with another scope, voltmeters, and most importantly, freeze spray and heat gun. Unfortunately the latter produced most of the time misleading and contradicting results, partly because there were two independent thermal errors. Finally the culprits were found, both very close to hot IC’s which again confirms the expectation that most likely a component fails due to thermal stress. The one responsible for dropping 2 of 4 channels was IC(*) A4/74HCT138 and the one for loosing trigger was diode(*) CR402/SM4004 which ran extremely hot, so that it had partly unsoldered itself, creating in intermittent contact (almost the worst thing to find…). Anyway, to make a long story short, after exchanging these parts the scope worked like new!

And since I was in the mood, I also took on the other scope, LC 9450. There again a thermal error, this time it was the mosfet(*) Q76/IRF830 responsible for emergency blanking the screen. It turned out to leak current at higher temperatures. After exchange all was well!

So now were things back to normal and I restarted activities by investigating a few new diodes for ECDL operation; among them the green 520nm Osram PL520. Stay tuned for results!

(*) refers to schematics in maintenance manuals