Scope not Showing Correct Square Wave - Page 2 - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 25th December 2012, 07:38 AM   #11
diyAudio Member
 
Join Date: Sep 2011
Location: Texas
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!
__________________
2X Com-tech 400, Improved Headphone Amplifier
  Reply With Quote
Old 25th December 2012, 09:21 AM   #12
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally Posted by techbiker View Post
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
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!
  Reply With Quote
Old 25th December 2012, 06:00 PM   #13
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
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
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 27th December 2012, 05:34 AM   #14
diyAudio Member
 
Join Date: Sep 2011
Location: Texas
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!
__________________
2X Com-tech 400, Improved Headphone Amplifier
  Reply With Quote
Old 28th December 2012, 03:52 AM   #15
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
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...
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 28th December 2012, 06:42 AM   #16
diyAudio Member
 
Join Date: Sep 2011
Location: Texas
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
__________________
2X Com-tech 400, Improved Headphone Amplifier
  Reply With Quote
Old 28th December 2012, 08:22 AM   #17
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Quote:
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
  Reply With Quote
Old 28th December 2012, 01:02 PM   #18
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
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
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 28th December 2012, 02:14 PM   #19
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Quote:
Originally Posted by bear View Post

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)
Attached Images
File Type: png Calibrator.png (104.7 KB, 69 views)
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
  Reply With Quote
Old 28th December 2012, 06:45 PM   #20
diyAudio Member
 
Join Date: Sep 2011
Location: Texas
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.
Attached Images
File Type: jpg 2012-12-28 13.30.55.jpg (929.1 KB, 60 views)
__________________
2X Com-tech 400, Improved Headphone Amplifier

Last edited by techbiker; 28th December 2012 at 06:48 PM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Linn Lingo vs. Dr. Fuß or Square-Wave vs. Sine Wave Oscillator for Motor Control tiefbassuebertr Analogue Source 62 15th December 2013 10:23 PM
Understanding Scope Trace Square Wave Hearinspace Tubes / Valves 5 3rd August 2010 05:23 AM
Sine wave - Square & Triangle wave generator using Transistors / OP-Amps lineup Solid State 20 9th October 2006 12:15 AM


New To Site? Need Help?

All times are GMT. The time now is 11:30 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