Scope not Showing Correct Square Wave

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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
 

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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.
 
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!
 
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.
 
Elvee,

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.
 
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?

_-_-bear
 
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.]
 
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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
 

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Update:

I tried removing the front plate to get access to the rotary switches but it was to no avail. The front rotary plastic switch knobs are held on by bolts which are covered by stupid plastic protectors. I tried to pry one off and nearly broke the front plastic knob. I then tried to destroy it and remove it in chunks but I didn't make much progress in an hour. I guess the only solution is to get the right size drill bit with a round cut-out to remove the plastic bolt protector remnants. As I don't have one now, I cleaned the ac/dc switches and sprayed a ton of evaporating cleaning solution at the rotary switches. I will wait a bit and let you know what happens when I power it up!
 
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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).

That's the give-away that there is nothing wrong with the scope.
A square wave from a PC with 44.1 sampling can be expected to have non-ideal rise- and fall times; add to that interlinks that are probably not very good on HF and there you go.
As bear noted, check with a short cable from the scope CAL terminal. If that shows sharp squares, the problem is the signal going into the scope and the scope shows the rounded corners truthfully.

jan
 
The little blue things - the caps with the indicators - other colors on other knobs - do snap off. However they are brittle as they can get after aging 25+ years and will crack. What you can try is to put a drop of silicone lube on the border/edge of the cap and knob, wait a few hours for it to migrate in. Add a bit of heat from a hair dryer (this softens the plastic a bit), then use a razor knife to go parallel to the surface of the indicator cap at that edge and pry gently... they may snap out... or break.

These caps can be bought new from the mfrs of knobs of this type... or stolen from other gear that has the same knobs.

Under the cap you need a metric thin wall nut driver or a thin long nose pliers to unscrew the nut about a turn - they are collet nuts.

I'd exercise the switches, and hit them with the aforementioned contact lube before trying to replace them... they will likely "come back".

_-_-bear
 
Howdy!

Bear, I removed those blue caps without a problem but my collet nuts have thick plastic blocker pieces either glued or molded to them (inside the outer plastic switch knob). The only way I can think of to remove them would involve smashing them and removing them in bits or using heat (but this might damage the outer knobs).

Instead, I used some Fry's branded contact cleaner (completely evaporating) in a large can and dumped about half of it on to the contacts over about a 2 hour period while twirling the knobs. The room was practically flooded with cleaner. I've gotten every amplitude switch down to 20mv clean with 10mv and 5mv still intermittent but adequate for now.

Cleaning the push button switches, then filling them with di-electric grease seems to have fixed the square-wave problem! Hopefully the grease will keep moisture and dust out for now.

If you have a PM3215 and have the same problems as myself, there is a small opening in the back of the faraday cage around the attenuators that can be opened enough to spray contact cleaner through.

Honestly though, if this is a major problem I will consider cutting a hole in the cage somewhere and installing hinges or something for easier access. Removing the front faceplate is an exercise in frustration in my opinion.

Thanks for all of your help!
 
I have a different but similar Phillips scope... slightly upscale version.

I can't say for sure, since silly stuff has been done, but usually the parts of these scopes comes apart pretty ok... but as long as you have it working, then that's good enough.

Can you post a closeup of the inside of the knobs, so we can see the collet? I'd suggest putting a magnifying glass on the lens, using the closeup setting, and moving the camera to focus with the shutter pushed slightly (that's the focus/light meter position) and then shoot... something is odd that there is anything in there on all the knobs...
 
Sure! I'll grab some good shots tomorrow when I get up. It seemed strange to me too since this was not in the manual...

By the way, if I choose to calibrate my scope completely, are there any modern substitutes for a time mark generator and square wave calibration generator? I'm considering buying a new function generator but I hope to find out whether I can acquire all of the requisite test equipment in a calibrated state (i.e., not from 1975). lol
 
Are there any modern substitutes for a time mark generator and square wave calibration generator?

Sure--just check out the Fluke 9500B...

Seriously: even if you buy a recently calibrated Tek PG506 and TG501, this will be much cheaper than any modern equivalent. Furthermore, IIRC, the basic important calibration procedures for these two units are not particularly difficult.

Samuel
 
With all due respect to the previous poster, there's absolutely no good reason to "fully calibrate" this scope. Unless it is way off, nothing you can do will really make it stable or accurate enough to use it as a precision measurement instrument. Save your effort, just check that it is close now - the internal time base of the calibrator is good enough to check the timebase accuracy. That can be checked against a decent freq counter. Good enough.

The amplitude (gains) have tweaks for the main and the high sensitivity ranges. It might be worth tweaking those - they tend to be off a bit... but I use a HP AC voltmeter for accurate sinewave testing, or another more modern scope...

The cost of a calibrated Tek or Fluke will exceed the value of that scope by a significant amount - you could buy a higher bandwidth more modern scope or one of the DSP based import scopes for equal or less money, I expect.

_-_-bear
 
The cost of a calibrated Tek or Fluke will exceed the value of that scope by a significant amount - you could buy a higher bandwidth more modern scope or one of the DSP based import scopes for equal or less money, I expect.
Quite right and sensible, and anyway an excellent calibrator can be made for very little money:
Start with some 10MHz xtal oscillator (or any similar value, a round number is more convenient) and add a string of frequency dividers (74HC390) down to 10Hz or so.
Add a selector, a buffer (8 operators of an octal HC or AC buffer in //), a precisely adjusted 5V regulator and a string of dividers, see example.
Adjust the supply voltage with a static level, monitoring the first 5V out with a DMM.
The resistors can be selected accurately, also with a DMM.
A 0.1% total accuracy can be reached easily, more than sufficient for a +/-2.5% calibration.

More division ratios can be added if desired (1-2-5 for example)
 

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Cool! I suppose that I can build this calibrator to produce the frequencies that I need to calibrate my scope. Also, when it asks for a time marker voltage with a repetition time of 1 us and an amplitude of 80mv p-p, do I just need to convert the repetition time to a frequency for the oscillator?

Thanks very much

By the way, I have attached a picture of the front knobs with the blue tips removed. Notice the gray plastic pieces on top of the collet nuts. I cannot seem to remove these easily without destroying the knobs themselves. Let me know if you need more pictures.
 

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