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Blurred trace on Gould/Advance OS260

Can anyone help me repair an old Gould OS260 oscilloscope?

Really, I need a circuit diagram or service manual. This is the first 'scope I have tried to repair, I could really do with a second one so please help me get it going :-)

The trace will not focus, in XY mode the 'dot' is about 5mm square! The focus control has no effect, neither does the internal focus preset or any other control. Everything seems to work fine other than the bad focus problem, which persists on every setting.

I haven't replaced any electrolytic capacitors yet, most of them (4.7uF 500V) read from 4 to 6 ohms ESR - not sure if this is really bad or not.

Could it be a gassy tube? I hope not :-(

Any suggestions gratefully accepted - can anyone give me a hint of where to look for this problem?

One final query - there are four cement-coated resistors that are in line with some of the leads going to the tube. These are too hot to touch without some pain - is this normal?

Thanks, Rick.

Reply to
Rick
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This points to a lot of leakage through the caps! They are usually the part that dies anyway. Try to identify the caps on the focus circuit and replace one at a time.

-Dan Barlow

Reply to
Dan Barlow

Thanks for that, I'll try tomorrow.

Rick.

Reply to
Rick

Have you measured the negative EHT voltage at the CRT cathode (Don't use an ordinary meter, it should be about 2 to 3 kV!) Use a high voltage meter or properly designed probe.

Does it have an astigmatism control? If so have you tried that?

Some Gould 'scopes I have crossed swords with, supply the CRT heater from the mains transformer. That's a poor idea, the transformer insulation has to cope with several kV, and *does* break down, pulling the EHT down with it. Very common where the instrument has been brought back into service after long storage without desiccant. The square spot (plate shadow) seems to indicate you're short on EHT, is the brightness poor, too?

The cure for the above is: fit a separate heater transformer.

--
Then there's duct tape ... 
              (Garrison Keillor)
nofr@sbhevre.pbzchyvax.pb.hx
Reply to
Fred Abse

I've just spotted that some previous owner has removed and not replaced a few components! A couple I can guesstimate (supply section) but a couple are in the focus circuit. D'oh!

Now I really need a schematic. There are part numbers on the board but not values.

Can anyone help?

Rick.

Reply to
Rick

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