Blurred trace on Gould/Advance OS260

[Rick]

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.

 

[Dan Barlow]

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.

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?

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

 

[Rick]

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

Thanks for that, I'll try tomorrow.

Rick.

 

[Fred Abse]

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?

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.

 

[Rick]

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.

 

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OS 260 Ciruit diagram

I have a circuit. Tell me exactly what you want and I will try and help.
Colin