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Uncased oscilloscope and somehow broke it

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I opened up my old Philips PM3256 scope to try and fix the power switch. Now when I turn the scope on, the trace only takes up a third of the screen, and I think that everything is compressed now. When I measure a 60Hz signal, it shows up as about 180Hz.

This is different from the issue I was having before, where the trace stopped showing at the edges of the screen, because in this situation, I can still use the x pos knob to move the trace around to most of the screen.

I undid the change I made (soldered the contacts of the switch so it's always on), but that didn't fix anything. I don't think I disconnected any wires or broke anything when I did the soldering, so I'm not sure what went wrong here.

I'm guessing it's an X deflection problem, but I was looking through the service manual and I couldn't find any obvious thing to adjust that would solve this. I can put up a copy of this service manual if anyone wants to look.


Please download and read this tektronix training material.

I have followed and used this manual on all my repair projects.


Agreed on the recommendation about the above link (the fact that Tek wrote it doesn't preclude you to using it on other scopes, they almost all work the same as long as they're analog).

The most likely problem is a connection in my opinion, you might have accidentally have touched a wire, and loosened a connector that just barely made contact. Verify all connections, remove and re-insert them in case they're oxidized (make sure to use correct orientation). Also check transistors/IC's in sockets. Suspect would be power supply or something in the horizontal chain, including the deflection wires to the CRT.

Ahah, I took a second look at it and very obviously, one of the five wires going into the CRT was disconnected. That fixed it!

Also, I thought this was pretty curious: the front panel knobs are connected to a plastic tube which turn pots that are all the way in the back of the scope:

I wonder why that's done instead of the pots being right where the knobs are at? I guess just for space reasons?


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