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Need some help with Philips Scope

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david77:
Hi guys.

I've got an old analog Philips PM3212 scope that was sitting unused under the bench for years, it has some problems which I now decided to fix. The biggest problem were dirty switch contacts. I've cleaned the switches and they now seem to work fine.

There's still another problem and I'm not sure where to start looking.
The scope starts clipping waveforms if the input amplitude gets too high.
Here I'm measuring a 10kHz sinewave of 0,5Vpp vertical ampl. is set to 50mV/div and everything
is still fine.



Now if I switch to 20mV/div the clipping starts:



The funny thing is this is happening on both channels. So I somehow don't think the Y amplifier
is to blame.
I do have the servie manual for the scope but don't really know where to start looking.
Any suggestions would be much appreciated.

amspire:
It is odd behavior.

If you put a 0.5 Vp/p into a 20mv/dev input, you expect it to clip, but in both photos, the waveforms are too small in amplitude for the channel sensitivity settings..

The input channels will have a vernier control that should be turned to the CAL position. Is this correct?

Perhaps you have them turned fully in the opposite direction.

Richard.

Psi:
Yeah, first pic is showing a problem also, a 0.5V p2p waveform should take up 10 vertical divisions at 50mV/div.
First thing I'd do is check all the power rails are correct, especially for the analog front ends.

Does moving the entire waveform up/down effect the clipping at all?

Can you get a waveform to take up the entire screen on any setting?

alm:
My guess is that the 1 div amplitude at 50mV/div is because David uses a 10x probe. Does the vertical position control have any effect? Does the clipping point remain on the same place or move with the signal? This may tell you if it's in the front-end or in the amplifier driving the CRT.

I'm not familiar with the service manual of Philips scopes, does it have detailed troubleshooting instructions like for Tek scopes, or just schematics and parts lists? Tek has a document about troubleshooting your oscilloscope, this may be contain some helpful hints.

david77:
Yes, the first photo is also not correct. I'm not using a 10x probe, the generator is connected directly to the Y input. I missed that  :-[.

The vernier controls are in the CAL position.

And no, I can not get a full screen display. Also if I change the Y position the waveform clips on the
positive or negative side if I move it up or down.
The first pic shows Y position turned fully up, the second fully down.
If I put the input to zero I also can't move the trace over the whole screen. It only moves about
1 div up and 1 div down.

That would point to a faulty power rail I'd guess. Probably shorted caps or something.

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