Help with Philips PM3211 - Page 1
Forum section: electronics / general

author: shebu18
author_rank: Frequent Contributor
timestamp: October 26, 2012, 04:16:40 pm
content: Hello all,I start this topic in the hope that someone could help me with my scope. I bought it from ebay and it is in a kind of working condition. The monitor displays the waveform but in somekind of distorted way.In the first picture you can see a pwm wave form from the arduino's digital pin 6. Value is set to 120 from the max 255. With the probe set to 1x i get 2 div and a bit with the V/div set at .1V. This means that the waveform has .22V, instead of 5V. If i activate the 10x on the probe i get a even smaller waveform.Pic 2 is what happens when i want to move the waveform up the screen and pic 3 is when i want to move the waveform down the screen. They get skewed.Could you recommend me what to do with the scope to have the waveform normal? If i set the pot to 5v/div and get almost a div would be great.Thanks.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 26, 2012, 05:20:26 pm
content: Check the voltages from the power supply and check the Y amplifier. See attached schematics.

author: Smokey
author_rank: Super Contributor
timestamp: October 26, 2012, 06:03:39 pm
content: I have a very similar model Philips scope I picked up on ebay as well.When I got it, all the rotary switches, both vscale and timebase, made terrible contact and the trace was all distorted and skipped around. If I tweaked them just right I could get a good trace but the scale was all jacked up until I took it apart and cleaned the contacts. You might have something like that. Or your scope may just be way out of cal.A couple questions first.1) Does chan2 have the same problem? Does the trace look the same on both when they are on the same vscale? Is the trace pretty stable on all scales, just attenuated?2) If you change the vscale knob, does the trace scale change in about the correct ratio? Since it looks like you have a good trace and the vscale is just wrong, the first thing I would check would be the actual knob position. When I got mine, someone had seriously cranked on the chan1 knob and rotated it so it didn't point at the correct number. Once I cleaned the contacts, it made it look like the scaling was wrong when it was just the knob position.Anyone else ever have problems with the rotary switch contacts being flaky on these old scopes? I don't know if mine was just owned by a chain smoker or lived in a machine shop or something, but it took a lot of q-tips and IPA to resurrect the rotarty switches.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 26, 2012, 06:09:02 pm
content: I agree that he should check CH2 behaviour first, but when I see how the amplitude drops by just turning the POS control I think there must be something wrong with the Y amp. Anyway, a good time to attach a PDF with adjusting instructions.

author: floris
author_rank: Newbie
timestamp: October 26, 2012, 06:47:59 pm
content: I had this too on an old Philips scope, the vertical ampswitch of ch1 was dirty.

author: Smokey
author_rank: Super Contributor
timestamp: October 26, 2012, 07:17:26 pm
content: when I cleaned up my contacts, I put a meter across from the input of the first plate of the switch to the output of the last plate and measured the resistance. It took a couple good scrubbing of each plate before the resistance at each switch position was constant and repeatable. And I had to wait for the IPA to evaporate each time because that messed up the reading too. I don't know if it got contaminated or just corroded a little. It's crazy how many discrete wires connect those boards together in these scopes.

author: shebu18
author_rank: Frequent Contributor
timestamp: October 27, 2012, 05:58:08 am
content: I will reproduce the wave and display it on ch2 and make some pics. I will also clean the contacts of the ampl/div switch. I still do not think that this is the problem, o rather think it is on the Y amplifier but for checking that i need a new meter, the old one got fried by 2kV. SO i will wait 3weeks for a 61E.