Author Topic: Philips PM3208 CRO Intensity problem  (Read 188 times)

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Offline kermito

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Philips PM3208 CRO Intensity problem
« on: June 05, 2021, 02:57:37 pm »
First post, but long-time lurker, so thanks for the all the valuable information I've found here before.
I'm an electronics hobbyist, mostly into audio/recording studio stuff.

I'm trying to revive and clean up a Philips PM3208 scope and I'm at the point where everything works but I have a weird (to me) problem. When I turn the intensity up, the trace expands horizontally and is compressed vertically. See the two pics below: only difference is intensity setting. I've tried to adjust and calibrate everything, but to my limited knowledge, the intensity control is just not supposed to do that, so no wonder that didn't solve anything.

All power rails measure within spec (I cannot measure the HT), everything else works as it is supposed to. I realize this scope is of little value (and it was free to me), but I want to understand what's going on. I have schematics and service manual. Does anyone have a clue what might cause this? I can't seem to find any clues online as to this particular problem.

« Last Edit: June 12, 2021, 01:56:52 pm by kermito »

Offline kermito

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Re: Philips PM3208 CRO Intensity problem
« Reply #1 on: June 12, 2021, 02:27:13 pm »
I'm gonna try one more time. I've got this scope sort of working with mostly cleaning and changing a few components. My guess is my problem is either the CRT itself or its associated circuitry, both of which I'm not really familiar with.

What works:
Almost everything. Power supply rails are within spec. Knobs, switches & trimpots do what they're supposed; only exception described below.

The problem:
Turning up the intensity compresses the trace vertically and stretches it out horizontally (see pics in post 1). This is in both channels, in any setting (v/div and time/div), and with any signal at the input.
Since my first post, I have found out that the display is also distorted, best visible at the top and bottom edges of the screen. This happens to any signal, at any settings, and when I move it around, the distortion is worst at the top and the bottom of the display, i.e., this is not distortion on/of the signal, because whatever part of it I move to the middle of the screen looks fine). Pics show a signal applied to, only change is the volts/div.

My question:
My main question is whether I am looking at a CRT with problems (which I'm starting to think now) or if these could be the symptoms of a problem in the circuit driving the tube.

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