Philips pm 3215 channel behavior

[Stoes]

Hello all,

I received a Philips pm 3215 repair which is acting kind of weird when hooking CH A & B up to the reference 1.2V square wave output of the scope.
Whenever I hook it up, I see that the sq wave is kind of differentiating (see att.).
When playing with the gain knob of that channel (which is also on the v/div knob) I manage to get a proper sq wave.
But when changing the v/div of that channel, the sq wave is once again differentiating.

This happens on CHA mostly, on CH B the dif behavoir is not happening all the time when turning the v/div knob.
Also when wiggling the knob for v/div a little bit changes the differentiating sq wave.

Anyone has an idea why I don't get the proper sq wave? Some kind of calibration?
I don't want to unscrew the whole scope and look at these knobs from the inside if it can just be some sort of calibration on the front panel.

This is the link to the service manual of the device:
http://elektrotanya.com/philips_pm3215_u_sm.pdf/download.html

Thanks in advance!

[tautech]

Welcome to the forum

First thoughts would be dirty contacts on the attenuation switch.
Are any of the problems seen with specific settings repeatable?
If so that might suggest a damaged divider resistor just at those settings but you do have the other channels to reference/check against. Open her up and clean the switch contacts first, then go from there.

[Kilrah]

+1, my 3214 has the same flaky contacts on the vertical settings.
I'll really need to take it out any clean it up one day.

[singapol]

The shape of the square wave indicates high frequency peaking on ch. A. Test again by setting coupling to DC.
If it's still the same then vertical amplifier AC gain needs to adjusted. See service manual for which trimpot to adjust and follow the procedures or steps to calibrate the vertical section/circuit. Yes, you need to open up the scope to get inside. Study the manual before doing this and good luck.

[mzacharias]

Philips scopes of that era are pretty notorious for dirty switches. Difficult to get to, but they clean up OK - until next time that is...

[Stoes]

Hello guys,

cleaned out the pot's for both channels (A & B) and also (while everything was layout dismantled) also cleaned out the one for time axis knob. The contacts inside the pot were very dirty.
The problem also was the disc that was mounted on the shaft of the knobs that weren't making contact any more with the little clips on the side.
WD40 was my hero.

mzacharias took me a few hours to dismantled everything..