diyAudio (
-   Equipment & Tools (
-   -   Scope not Showing Correct Square Wave (

techbiker 24th December 2012 09:02 AM

Scope not Showing Correct Square Wave
2 Attachment(s)
I just finished replacing the dried out electrolytic power supply capacitors on my Philips PM3215. I turned on the unit, inserted a couple test tones, and ran some basic calibration procedures outlined in the service manual. One thing that keeps stumping me though is this problem with channel A. For some reason, channel A is not reading square waves correctly. It keeps rounding the corners. Fortunately, channel B is showing proper square waves so I could compare both. The channel does not appear to have any issue showing sine waves, but I haven't tested this extensively.

What do you think is causing this problem and where should I begin to find a fix? I have the service manual and can scan different parts of it if you would like to see it.

This is a frustrating issue because I cannot rely on measurements from channel A unless I know it can report a square wave properly.

I've attached a couple pictures of this problem. Top is B and bottom is A. Both are reading the same 1000hz square wave. Amplitude/v is .1v per division and time/div is .2ms

Thanks very much

Elvee 24th December 2012 10:07 AM

From my experience, this series of scopes is plagued by bad contacts in the input attenuator switch.
Could be that some compensation cap is never switched into circuit.

Is the problem identical at all sensitivities, or are some ranges spared?

JMFahey 24th December 2012 10:10 AM

None of them is working properly.
Both show a severe lack of high frequency response.
Both are showing what should be very "easy" for them, a 1KHz squarewave.
The rising and falling edges should be seen as perfectly vertical; which would imply frequency response flat to at least 100 KHz; your scope claims 50 MHz bandwidth after all so it's clearly very sick because it shows them quite tilted.
It's taking 1/3 div. , around 60 uS to switch high or low, which is *slooowwww*.
The bottom one is the same, only much worse.
Really can't imagine what caused this, but must be related to your job there.
Please recheck everything you did.

techbiker 24th December 2012 10:17 AM

Thanks for the quick replies. The square wave in these pictures was generated by my laptop (audacity) but channel B shows no slanted vertical lines using the CAL square wave on the scope itself. Just horizontal dashes (as i believe it should). I can post a picture in the morning if you want. A is still horrible.

The input attentuator for channel a does give me a poor connection between 50mv and 2mv (smallest setting). The trace gets very faint and garbled here. Should I replace the attentuator? If so, is the only option a used attenuator? It's late here but I'll run more tests in the morning!

marce 24th December 2012 10:56 AM

Just a check, you have swapped the probes to check it isn't a dodgy probe?

Elvee 24th December 2012 11:30 AM


Originally Posted by techbiker (Post 3297534)
The trace gets very faint and garbled here. Should I replace the attentuator? If so, is the only option a used attenuator? It's late here but I'll run more tests in the morning!

The problem is in the contacts; changing the whole attenuator assembly is not really an option, it is the hell of a job and you would have to realign it completely.
What you should do is to clean the contacts individually using for example a leather strip and a cleaning fluid. Very important: do not use Kontakt 60, CRC or similar fluid leaving an oily residue!

Use isopropanol, Kontakt WL or similar.

Be warned, the accessibility is nightmarish.

To prevent oxidation reappearing after cleaning, a vaseline spray is a solution, but it has to be used very sparingly, and not on the first stage of the attenuator, because it could promote arcing at high voltages.

techbiker 24th December 2012 10:11 PM


Thanks for the replies! I have a couple questions though. Can I only use a vaseline spray or is another inert oil acceptable (like silicone spray, etc.)? I've been looking around for vaseline spray and it appears to be somewhat difficult to find.

I also used a little bit of radioshack cleaner and lubricant on those switches so I will try to remove as much of the remaining oil as possible!

Finally, which part of the switch is the first stage?

Thanks very much

Merry Christmas!

P.S. I tried switching the probes in the hopes that one was bad, but both show the same square wave on channel A.

bear 25th December 2012 02:23 AM

See that little thingie to the upper right of the scope screen? Labeled "CAL"? That's ur source for square waves... :D

Jiggle the vertical amp switches... also try the square wave at different amplitudes and so different switch positions... I have a couple of these Phillips scopes and they seem to work pretty reliably...

What caused you to change caps in it??
Was this a non-working scope you got used, or one that stopped working?


gootee 25th December 2012 04:47 AM

Yeah. Please show results using one probe, same probe on one channel at a time, with the scope's CAL output, not something from your computer.

I have cleaned input attenuator switch contacts on a hundred or so used older Tektronix (usually 400-series) scopes.

If your scope has a Bandwidth Limit switch (or a Trigger View), or something similar, it might use similar switch contacts, and they might be part of the problem. (And those were MUCH easier to reach than the attenuator contacts, on the Tek 465 and 475 scopes.)

Here is a link to a procedure that I sent to a guy, years ago, for cleaning them. Maybe it will help you with your Philips.

Cleaning the Volts/Div Attenuator Switch Contacts in Tektronix 4XX Oscilloscopes

You can get CAIG products at many electronic supply shops, and can read about (and order) them on their website at:

Home of DeoxIT and HAND-E-GLOVE - CAIG Laboratories, Inc.

I have "fixed" many scopes, JUST using Caig's "DeOxit" and isopropyl alcohol. (I always made sure I had cans of the DeOxit spray and also the small bottle of the DeOxit concentrate, with the needle applicator.) They also have products for cleaning and treating potentiometer elements and different types of metal contacts.

[Please NOTE: Although I have left my webpages up because they are of interest to others, _NOTHING_ is still for sale, anywhere, there, even though that might appear to be the case, depending on exactly where you enter the pages.]

techbiker 25th December 2012 05:48 AM

5 Attachment(s)
I was fiddling with the controls on my scope (while using the "cal" hook) some more when I noticed that pushing the ac/dc switches in on both channel A and B triggers this problem with square waves intermittently. It seems that these push-button switches are oxidized and are making poor contact. If I wiggle them, I can get channel A to display a proper square wave as well!

It appears that this is the cause of the square wave problem. Do you think it would make more sense to source new push-button switches? I can imagine that these are simpler to replace since they are just solder-in pieces. I will also clean them out and possibly apply some dielectric grease.

Speaking of di-electric grease, I'm thinking about using a small amount on the ampl/div rotary switches to prevent oxidation since di-electric grease has been a insulating life-saver on the electrical components in my Nissan 300zx. I also have some silicone spray, teflon spray, and something called Noalox that the home depot guy recommended. Do you have any recommendations for an oxidation preventer? I believe that the rotary switch is the cause of problems on channel A since the trace becomes fuzzy when the switch is jiggled.

I've attached some pictures of what I'm referring to. I tried attaching a video as well but the forum threw up an error. Ampl/div is set to .1 v on both channels, bottom is channel A, and time/div is set to .2ms. Probe A wasn't compensated properly in the last picture.

Also, I was replacing the electrolytic capacitors on the power supply because they are from 1981 and are certainly bone-dry right now. There are also some electrolytics on the main board but I am planning to put off changing those until I have to since they are hard to reach.

Thanks again

All times are GMT. The time now is 08:22 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2