Oscilloscope woes

[Tony Moore]

Hi all,

This is probably not the right section for this post but it's the closest I could find.

I've just bought an old scope from someone on Ebay
and it seems to have a fault. I didn't pay much for it so I'm not too gutted but I wonder if anyone could look at this trace of the test square wave and comment on if this is likely to be a simple problem to fix or if it's going to be too hard to locate.



I have found someone who might sell the service manual so it could be a little side project to fix it.


Thanks,
Tony

[Little Mick]

Have you adjusted the probe

[Tony Moore]

Hi Mick,

Yes, I should've said I had tried adjusting probe, different probes that I can see work fine on another scope and also on a separate signal generator. It's most certainly the scope itself that is doing this.

The effect of adjusting the probes just affects the front edge of that ramp. The rest of the ramp stays at the same slope. It's as if a capacitor is charging on constant current. I'm puzzled!

Cheers,
Tony

[soulminer]

It might not be the 'scope Tony, it may be a faulty square wave generator inside the 'scope. Have you got an independent signal generator to test it with ??

[Tony Moore]

Hi Lee,

Yes, I've also tried it on my signal generator and it does the same thing. Sine waves look ok though. It's weird. It's as if there is something limiting the deflection of the beam. If I connect both channels to the same signal, invert channel 2 and add them then I get very nearly a zero line so the inputs seem to be working ok. It's the deflection of the beam at fault I think....

Cheers,
Tony

[Global]

Not an expert on this but if John Caswell is reading, I am sure he will be able to tell us exactly what is wrong with this scope.

John???

Mark

[Richard]

It looks like it's (very) AC coupled Tony. Does switching from AC to DC input have any effect? Is it a Telequipment; the selector switches break inside sometimes but can be fixed if you're careful.

Rich

[Tony Moore]

Hi Richard,

It's an Hitachi V550B 50Mhz dual beam and although the switches are all a bit old and tired I think with a few operations they'll all be less noisy. No, it's certainly on DC, switching to AC just takes off the offset as expected but the ramp still remains.

I have the use of another scope, I'll put the same signal into both and see if there's anything different with the levels of the signal being shown on each.

It's a weird fault, or perhaps it's very simple and I'm just missing the obvious?

I hope it's not the tube! The thing has plenty of brightness and the focus works a treat. I hope it's not too serious as this is otherwise a nice scope and would do fine for me.

Thanks,
Tony

[Tony Moore]

Some more piccies!



This is on AC on both scopes with the setting all the same on each. It looks like the scale would be correct if only the beam didn't go off on it's own after reaching the correct position.

Here is the same thing on DC



As you can see there's something very wrong with the DC offset!

Sorry to be a pain! It _is_ quite a challenge though.

Cheers,
Tony

[John Caswell]

Hi Tony,
As it is a dual beam scope are the results identical on both channels? If so this eliminates the attenuators and confines it to the main Y amplifier and beam switching. It does look as though a capacitor or possibly one of the balanced pairs of transistors in the Y amp is faulty. The DC shift could be because the front panel balance controls are not set correctly
It is certainly not the tube, this would show up as low brightness, non focussing etc.
As you appear to have two scopes you can probably trace the signal path using the good one and a suitable square wave/pulse generator. Having the manual will help considerably. Whatever you do don't adjust any internal pots or trimmers willy nilly. If you are suspicious the mark the position of the control knb using pen or pencil and try adjusting it, if no effect return to original position.
If still lost give me a bell

Regards

John