Powered by Invision Power Board


Forum Rules Forum Rules (Please read before posting)
  Reply to this topicStart new topicStart Poll

> Practical Electronics Oscilloscope, Info needed on troubleshooting
Puzzled
  Posted: January 22, 2017 11:28 am
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 8
Member No.: 40,343
Joined: January 22, 2017




Hi all,
I built a valve (tube) oscilloscope in 1968 from a design that was in a UK electronics magazine (Practical Electronics). The scope was called "The Investigator".
The circuit etc was in the 1967 July and August issues.

I couldn't get it work properly after I built it and stuck it in the corner due to just starting a family and soon after that I emigrated to Australia and forgot about it. I recently found it amongst all my stuff and wondered if it could be fixed.

I still have the original (much tattered) magazine issues and after looking carefully I think it was wired up correctly.
I took a deep breath and powered it up and wonders of wonders there wasn't any magic smoke. I was surprised it didn't pop a capacitor or two!

It still has the exact same symptoms as when I put it away, nearly 50 years ago!

The problem is (with no input to the scope) I'm getting a wavy line on the CRT when it should be a straight line, the line also has a sharp 'rise time' sort of beginning to the trace at the start of the sweep that varies in height according to the Y amplifier switch settings. I think the problem is that the Y section might be picking up something from some other part of the circuit - or maybe I have a bad capacitor somewhere. I intend to eventually replace all the caps but in the meantime I wonder if anyone has ever had any experience with that (or similar) type of tube oscilloscope.
It uses an ECC81 for Y amplifier, an EF80 for Y driver, an ECC82 for Y phase splitter, an ECC81 for Sync, an EF80 for Timebase generator and an ECC82 for X phase splitter. They drive a VCR139A CRT.

I do not have any test equipment, apart from a reasonably good multimeter, so I don't have anything I can plug into it to display a 1V P-P square wave for instance to see if the display is showing somewhere near what it should.

If anyone has any experience with old (I mean OLD) tube scopes like this and can help get it working correctly I'd be much obliged.
I have scanned all the relevant pages from the magazines and can take a photo or two of the trace if anyone would like me to post or send via personal mail.

Hoping to hear from some genius (or two) that can help. thumbsup.gif

Many thanks,
Dave.


--------------------
Don't worry about the world ending tomorrow, it's already tomorrow in Australia.
PMEmail Poster
Top
kellys_eye
Posted: January 22, 2017 02:45 pm
Reply to this postQuote Post


Forum Addict ++
*******

Group: Spamminator Taskforce
Posts: 11,418
Member No.: 2,735
Joined: June 21, 2005




Is the 'wavy line' of a known periodicity? Commonly it is as a result of poor regulation that many errors such as you describe that result in peculiarities of trace shape.

The sharp rise time of the trace is linked to the flyback (retrace) and non-linearity of the timebase waveform.

Both are potential symptoms of capacitor leakage/failure and other component tolerances.

You can upload schematics and pictures to this site via Photobucket or other image-hosting services and I'm sure there'll be much interest and input to be had from our regular contributors.

thumbsup.gif


--------------------
May contain nuts
PMEmail PosterYahoo
Top
gremlinsa
Posted: January 22, 2017 07:43 pm
Reply to this postQuote Post


Forum Addict
******

Group: Trusted Members
Posts: 828
Member No.: 3,112
Joined: August 25, 2005




I have and use a old Tube scope (picked it up at the swop shop many years ago).. It's twitchy, difficult to lock on signals and picks up mains hum very quickly if the probe is not attached to anything..

Check it in action in this Youtube linky...

Upload what you can to Photobucket / Youtube / imagur.. link it here, and some of us here will look through it..


--------------------
Give me a Break... I'm dik-lic-sic, dus-kik-lic.... Gawd ... F'ed in the Head...
PMEmail PosterUsers Website
Top
Puzzled
Posted: January 23, 2017 08:48 am
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 8
Member No.: 40,343
Joined: January 22, 2017




Thanks kellys_eye and gremlinsa for your quick replies.
I've ordered a couple of scope probes and as soon as I get those I'll try looking at some stepper motor waveforms.
What I really need is a 1KHz 1V P-P signal that I can use as a reference, there are several trimpots that need to be tweaked to set the scope up correctly.

I've looked online for a suitable circuit but haven't found one yet that's cheap and easy to build and doesn't have pots on it to 'tune' the frequency and voltage. Chicken and egg situation; I need a scope to set up such a signal generator but before I do that I need a stable and accurate signal. LOL! doh.gif

I'll see about setting up an account with Photobucket or similar and let you guys know when pics are available.

Many TIA.

Cheers,
Dave.


--------------------
Don't worry about the world ending tomorrow, it's already tomorrow in Australia.
PMEmail Poster
Top
deth502
Posted: January 23, 2017 09:14 am
Reply to this postQuote Post


Forum Addict ++
*******

Group: Trusted Members
Posts: 1,185
Member No.: 14,050
Joined: March 23, 2008




there are apps and programs out there to use your computer sound card as a signal generator. really, you probably dont even need that if you just play an accurate note and know what frequency it is. afa the voltage, use a battery. check its vltage with a dmm to verify.
PMEmail Poster
Top
Puzzled
Posted: January 23, 2017 11:08 am
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 8
Member No.: 40,343
Joined: January 22, 2017




Hi guys,
I couldn't sign up to Photobucket, it seems to want a US post code. I tried several times but no go.

So, try this;
Here's the link to all the pages of the two magazines: http://imgur.com/a/RPvpn
Contains description, construction details, circuit and setup procedure.
Scroll down to see all pages.

Let me know if they can't be accessed. nono.gif

Cheers,
Dave.


--------------------
Don't worry about the world ending tomorrow, it's already tomorrow in Australia.
PMEmail Poster
Top
gremlinsa
Posted: January 23, 2017 03:24 pm
Reply to this postQuote Post


Forum Addict
******

Group: Trusted Members
Posts: 828
Member No.: 3,112
Joined: August 25, 2005




Yup.. they are all visible.. I've read through most of it, and checked out the schematics..

A screen shot of the display's "Fault" will also help.


--------------------
Give me a Break... I'm dik-lic-sic, dus-kik-lic.... Gawd ... F'ed in the Head...
PMEmail PosterUsers Website
Top
MacFromOK
Posted: January 24, 2017 02:46 am
Reply to this postQuote Post


Forum Addict ++
*******

Group: Spamminator Taskforce
Posts: 15,184
Member No.: 5,314
Joined: June 04, 2006




Do you have access to subsequent issues?

Often there are errors in magazine articles that are corrected in later issues.

Just a thought. And thanks for posting it. beer.gif


--------------------
Mac *

"Basic research is what I'm doing when I don't know what I'm doing." [Wernher Von Braun]

* is not responsible for errors, consequential damage, or... anything.
PMEmail Poster
Top
Sch3mat1c
Posted: January 24, 2017 04:57 am
Reply to this postQuote Post


Forum Addict ++
Group Icon

Group: Moderators
Posts: 20,541
Member No.: 73
Joined: July 24, 2002




FWIW, combined,
http://i.imgur.com/Vz3vxOj.png
and cleaned up,
http://i.imgur.com/0hkEyRE.png

At least it has balanced deflection. I've seen more than a few scopes (of comparable complexity and functionality) that didn't... rolleyes.gif Astigmatism control, too!

It is unregulated, so expect "swell" as the line fluctuates. If you get this thing working well enough that you feel like improving it, consider, well the semiconductor diodes should maybe get replaced with modern ones, for reliability reasons, but then you could add a pass regulator to smooth the HT (and something for EHT as well, though that's a bit trickier -- by the way, be very careful with those 8uF 400V caps C31 and C32! wink.gif ), and linearity and bandwidth can be improved fairly easily by swapping out resistors here and there, in particular the "tail" resistors (R21, R42) with current sinks (use a BJT for a little cheating? smile.gif ), and the plate resistors for correctly matched values with peaking coils added.

Hmm, some other things I can spot too... and the elephant of DC coupling, which would be lovely, but oh well. Gotta get it working first!

Note that the "trigger" (it's not actually triggered, but an injection locked oscillator) taps directly off one of the vertical amp plates, so if the trigger amp circuit isn't behaving itself (trigger pulses coupling backwards through V4?), you'll get a vertical bump in the wave, consistent (I guess) with your description?

Tim


--------------------
Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
PMEmail PosterUsers Website
Top
Puzzled
Posted: February 09, 2017 12:53 pm
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 8
Member No.: 40,343
Joined: January 22, 2017




Hi guys,
Sorry, I haven't been checking posts for a while - I had subscribed to automatic e-mail notifications and got a couple but it apparently has stopped working.

Anyway, thanks to all who replied and for the suggestions.

I will post a pic of the display soon, been distracted with other things at the moment.
Interestingly, I fed in an AC 50Hz 9V signal and the display was quite good with no wobbling or 'shakiness', I will be building a square wave generator shortly to see what the display looks like with that - I need that anyway to calibrate the scope.

I have checked subsequent magazine issues and there are no updates for errors apart from one resistor that was incorrectly listed in the parts list - the circuit diagram and the resistor I put in is correct.

Thanks for cleaning up the circuit diagram, a bit easier to read now (I assume you only cleaned it up and didn't change any part of it?). I'm afraid the rest of your post is a bit beyond me though, I'm not that experienced in tube circuits.
I note your comment about the 8uF 400V caps, I had already bought a complete set of resistors and caps to begin replacing them all - with the exception of the 2% mica/polystyrene caps, I have been unable to find a complete set except on web sites where they are only sold in bulk (and very expensive).
Could I substitute ceramic caps for those? I can get those cheaply and my thinking is to buy a whole bunch of each and check the values with a multimeter to find those that are within spec.

Regarding the waveform I have, it occurs to me that I had deviated a little from the original build plan. I used two triple 50uF caps instead of three twin 50uF. The triples are higher than the twins so the aluminium plate holding the CRT doesn't go all the way across in front of the caps because it would foul the caps - maybe there is some hum coming from those caps and associated resistors that should be screened by the extended plate? I shall be rectifying that when I install the three new twins I now have.
Also, I shall be checking all other new caps to see which end should go to ground (the outer layer of foil), correct installation apparently avoids unwanted hum.
See Mr Carlson's Lab: https://www.youtube.com/watch?v=BnR_DLd1PDI

Cheers,
Dave.


--------------------
Don't worry about the world ending tomorrow, it's already tomorrow in Australia.
PMEmail Poster
Top
Sch3mat1c
Posted: February 09, 2017 07:23 pm
Reply to this postQuote Post


Forum Addict ++
Group Icon

Group: Moderators
Posts: 20,541
Member No.: 73
Joined: July 24, 2002




Ah, replace bad mica or polystyrene with C0G/NP0 ceramic or polypropylene. 2% may be hard to find. 5% and 10% tolerance is cheap and common, but will need some adjustment (ideally, there would be another deck or two on S4, that switches in a slightly different timing resistor, so they can be calibrated appropriately).

Mica and poly caps don't usually go bad. Beware that some mica caps fail from silver migration, and others ("Micamold") look like they're okay, but are actually paper-in-oil that have absorbed moisture (the molded body may or may not be cracked, as well) and become leaky or shorted. Wax paper caps are generally crap and should be replaced.

Old electrolytics aren't really a problem, once reformed. They may be unusually leaky and high ESR. Recommended procedure is to bring them up to voltage slowly, then check leakage at voltage, then bring down to 0V and check with an ESR meter.

Tim


--------------------
Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
PMEmail PosterUsers Website
Top
Puzzled
Posted: March 13, 2017 07:25 am
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 8
Member No.: 40,343
Joined: January 22, 2017




Greetings all,
I finally got around to posting pics of the trace display. See here: http://imgur.com/a/RPvpn

I have replaced a number of components with no improvement; among the components are the three 50uF caps and associated shielding I referred to in an earlier post.
I suspect the problem might be 50Hz hum from the transformer, I don't know if there is enough room to add a shielding, earthed 'cage' around it.

Interestingly, all the components I have so far removed were all within spec and showed no signs of anything untoward. Not bad for 50 year old components!

Anyone got any ideas? blink.gif

Note that although the posted pics are with no input to the scope I get the same problem overlaid on any signals I do input.

Cheers,
Dave.

This post has been edited by Puzzled on March 13, 2017 07:29 am


--------------------
Don't worry about the world ending tomorrow, it's already tomorrow in Australia.
PMEmail Poster
Top
kellys_eye
Posted: March 13, 2017 11:26 am
Reply to this postQuote Post


Forum Addict ++
*******

Group: Spamminator Taskforce
Posts: 11,418
Member No.: 2,735
Joined: June 21, 2005




Assuming the timebase and amplifier are reasonably calibrated the first image shows a 0.6V peak at 40kHz.... which could be instability in the amplifier section or pickup from a modern SMPS in the vicinity (more likely).

Can you short the input to ground and see if this waveform remains? If it does then it's definitely internally generated.

The other waveforms are only distortions caused by poor timebase design - the flyback suppression not being very effective at all but what you'd expect from such a simple design.

The 'interference' causes the multiple horizontal scans and wouldn't show if the oscillations above were dealt with.

When you say the components were 'within spec' did the tests include leakage in the capacitors (i.e. did you perform an ESR test on them)?


--------------------
May contain nuts
PMEmail PosterYahoo
Top
Puzzled
Posted: March 13, 2017 12:22 pm
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 8
Member No.: 40,343
Joined: January 22, 2017




QUOTE (kellys_eye @ March 13, 2017 09:26 pm)
Assuming the timebase and amplifier are reasonably calibrated the first image shows a 0.6V peak at 40kHz.... which could be instability in the amplifier section or pickup from a modern SMPS in the vicinity (more likely).

Can you short the input to ground and see if this waveform remains?  If it does then it's definitely internally generated.

The other waveforms are only distortions caused by poor timebase design - the flyback suppression not being very effective at all but what you'd expect from such a simple design.

The 'interference' causes the multiple horizontal scans and wouldn't show if the oscillations above were dealt with.

When you say the components were 'within spec' did the tests include leakage in the capacitors (i.e. did you perform an ESR test on them)?


Thanks for the reply.
Not sure if anything is calibrated - I don't think so, I haven't checked that since firing it up recently and I can't remember if I did it 50 years ago, but anyhow it's a bit difficult to calibrate it with the trace like it is. :-)

The problem it has now is exactly the same as when I built it. What with raising a family and emigrating to Australia I didn't have time or resources to fix it then and it's been stored for around 49 years.

I had already tried shorting input to ground and got same results.

Yes, caps tested OK for ESR but I replaced them anyway - there is still a few to go yet though (but no more electrolytics).


--------------------
Don't worry about the world ending tomorrow, it's already tomorrow in Australia.
PMEmail Poster
Top
Puzzled
Posted: March 14, 2017 10:27 am
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 8
Member No.: 40,343
Joined: January 22, 2017




Here's an update:
I just tried inputting a 14V 50Hz signal from a mains 250V transformer and got a perfect waveform with no distortion and rock solid triggering, so what I am seeing with no input I'd say is definitely a 50Hz 'signal' coming from somewhere in the 'scope itself.
I think I can get an earthed screen across the 'scope transformer etc without too much trouble so I'll give that a go.
Stay tuned for further developments...


--------------------
Don't worry about the world ending tomorrow, it's already tomorrow in Australia.
PMEmail Poster
Top
kellys_eye
Posted: March 14, 2017 07:58 pm
Reply to this postQuote Post


Forum Addict ++
*******

Group: Spamminator Taskforce
Posts: 11,418
Member No.: 2,735
Joined: June 21, 2005




laugh.gif the waveform you showed has the timebase at 10uS/div - perhaps it should be 10mS??

Anyway the usual screening material used to be mu-metal around the tube but screening the transformer rather than the tube is obviously easier. Can it be relocated (even 'rotated')?


--------------------
May contain nuts
PMEmail PosterYahoo
Top
Puzzled
Posted: March 16, 2017 09:32 am
Reply to this postQuote Post


Newbie
*

Group: Members+
Posts: 8
Member No.: 40,343
Joined: January 22, 2017




QUOTE (kellys_eye @ March 15, 2017 05:58 am)
laugh.gif  the waveform you showed has the timebase at 10uS/div - perhaps it should be 10mS??

Anyway the usual screening material used to be mu-metal around the tube but screening the transformer rather than the tube is obviously easier.  Can it be relocated (even 'rotated')?

Hmmmm, well, I had tried calibrating the 'scope using the 14V 50Hz sine wave I mentioned but wasn't too successful. The 14V was measured with an el-cheapo multimeter, so not RMS Volts.

The magazine article says to use a square wave (at 1V p-p I think) but trying to use that is hopeless because of the 50Hz overlaid on it.
So I have no idea what the timebase calibration was or is now and I have to adjust the fine frequency pot to get a stable waveform anyway..
My maths is hopeless so I have no idea what 50Hz is in terms of milliseconds; but according to the WWW it is 20mS so I'll look into that next time I power it up and see if that coincides with any timebase settings.

Yes, some mu-metal would be good, the tube has an internal screening coat on the business end but the majority of the tube's neck is unscreened.

Can't relocate the transformer, not sure if I can rotate it - I'll check that out.
I'll try screening it first though.

EDIT:
I just fired it up and calibrated the timebase as close as I can get it using the 50Hz sine wave (2cm at 10mS per division). In doing so I discovered that I have the timebase switch labelled backwards! doh.gif
So you were correct in saying it should be 10mS. Well spotted!
I put that little stuff up down to the fact that the circuit diagram shows the timebase switch at position 4, whereas the V/cm switch is shown at position 1.
I confirmed that by inputting a 1kHz square wave and switching to the next timebase position (which would be 1mS) and it was as near as I could judge (given that it's very blurry with the 50Hz on top of it) 1cm at 1mS per division.
Sooo, that explains a lot of things! annoyed.gif
I also recalibrated as close as I can the Volts per cm using the 5V square wave, that also makes more sense as I ran out of adjustment trying to get it calibrated to 14V AC.

Now, if I can just get rid of that unwanted 50Hz interference I'd be somewhere near having a useful 'scope! thumbsup.gif

Edit again:
Now I know correctly what the timebase and voltage settings are supposed to be it seems that the waveform I get with no input is a 1KHz distorted sine wave. It looks more like a triangular waveform. Very odd. I can lock it with the trigger control. Very mysterious...
dunno.gif

This post has been edited by Puzzled on March 20, 2017 10:46 am


--------------------
Don't worry about the world ending tomorrow, it's already tomorrow in Australia.
PMEmail Poster
Top
1 User(s) are reading this topic (1 Guests and 0 Anonymous Users)
0 Members:

Topic Options Reply to this topicStart new topicStart Poll

 


:: support us ::




ElectronicsSkin by DutchDork & The-Force