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 Using a CRO
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 Return to top of page · Post #: 1 · Written at 6:00:57 PM on 20 April 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

For those tyros with a CRO like me this might be useful

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjDnrTD97f-AhVAt1YBHeWSDYoQFnoECDoQAQ...

What I was after in particular was typical waveforms and this has some.


 
 Return to top of page · Post #: 2 · Written at 5:39:58 PM on 22 May 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

I was boning up on probes and come across this which is very comprehensive, if at times rather esoteric.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjk5eOmv4X_AhXZmlYBHVh3BCwQFnoECAgQAQ...

But also the Application Note No 47,Appendix J at the end of the above is on Murphy's Law, entitled "The Contributions of Edsel Murphy to the Understanding of the Behaviour of Inanimate Objects". Particular attention is paid to electronics engineering and the design, construction and servicing thereof. Written as a paper published in EEE magazine 1967, and is a very thorough documentation of the subject, complete with references.

No doubt extra corollaries could be added for valve technology as the author's thesis is that the Law is universally applicable.


 
 Return to top of page · Post #: 3 · Written at 8:48:21 PM on 22 May 2023.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6720

"The Contributions of Edsel Murphy to the Understanding of the Behaviour of Inanimate Objects".

https://worldradiohistory.com/hd2/IDX-AUSTRALIA/IDX/Archive-ETI-AU-IDX/IDX/ETI_1971_12-OCR-Page-0086.pdf

III.3 All constants are variables.


 
 Return to top of page · Post #: 4 · Written at 9:01:00 AM on 23 May 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

"III.3 All constants are variables."

As a CRO will tell you - or will it?


 
 Return to top of page · Post #: 5 · Written at 10:45:28 AM on 24 May 2023.
Tinkera123's Gravatar
 Location: Melbourne, VIC
 Member since 5 October 2009
 Member #: 555
 Postcount: 465

Hi STC830 et al,

Some very good info in those documents. The Web is a great source of info on Scopes .... and Youtube has many good videos. The important thing is to find information which is relevant to your Scope and the type of work you wish to do. Analog vs Digital, Low or high frequency etc ... very easy to get sidetracked. Operator and Service Manuals are another good source. The early part of the ABC of Probes by Tektronix (included in your second web link) is excellent. Learning how, and why, to compensate Probes is important.

I have several Scopes, mainly Tektronix 7000 series ... analog, 1970's and 80's ... I haven't as yet used a modern digital scope, although there is the allure of 4 traces, colour, automated functions, cursors, real-time calculations, mixed domains etc ... but they also have their downsides, cost, you get what you pay for, disposable unless you really know what you are doing.

Cheers,
Ian


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Cheers, Ian

 
 Return to top of page · Post #: 6 · Written at 5:47:42 PM on 24 May 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

My CRO is a 20MHz , 1MOhm, 30pF which has quite a good manual but I was looking for something for valve applications in general. Probes are 60MHz, 10MOhm 23pF, operated at X10.

I have been measuring stage gains looking at the difference between points with the two channels. To decrease the effect of the CRO on the RF sections I have soldered low value mica caps to small alligator clips, and attached the probes and recompensated. This set up does effect RF stages a bit especially for grid connections, but have been making measurements for each stage after re-tuning to the output of a signal generator. In this way I get a rough stage gain which is all I want at this stage - is the stage working?


 
 Return to top of page · Post #: 7 · Written at 10:57:44 AM on 25 May 2023.
Tinkera123's Gravatar
 Location: Melbourne, VIC
 Member since 5 October 2009
 Member #: 555
 Postcount: 465

Hi STC380,

Yep, some good technique there ... I have a low value cap built into my Signal generator lead and always start with Probes at x10. Know the voltage rating of Probes and Scope in case of hookup to high voltages. I add electrical tap or thin tubing to my Probe tips if they are long enough to short out components (mmmm ... yes ... pop .... Sad ).

Tracing an audio, modulated IF and modulated RF signals through a Superhet radio with a Scope is very helpful in understanding how these radios function and in troubleshooting.


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Cheers, Ian

 
 Return to top of page · Post #: 8 · Written at 8:37:43 PM on 25 May 2023.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 1284

I will follow your lead on insulating the probe tips as best I can.

I found this video showing how to use a CRO on an American five.

https://www.youtube.com/watch?v=niBGkqlh2XY

I was mainly interested in probe matters, but enlightening stuff on the use of various settings. It is a video of a verbal presentation so you have to put up with background noise.

CRO probes will put the local oscillator especially off frequency. He was doing what I found you have to do - just retune your radio.

I refer to various caps as mufs, nufs and pufs - this is the first time that I have found someone else using pμF to refer to pF.

Edit: There appears to be an automatic editing process that replaces u-lower case f with with Greek mu-upper case f when Farads are in the offing.


 
 Return to top of page · Post #: 9 · Written at 8:56:40 PM on 25 May 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

I have managed to acquire three oscilloscopes. A Heathkit Os-1 with its original paper work; Same deal with a BWD 539C, at an auction for $30. It was filthy and took around 2hrs to clean. I am sure it would have gone for considerably more had it been presented in a better manner; lastly a Jaycar, QC 1910.

The later is amusing as it gets just past its quoted maximum frequency & dies, whilst the Heathkit with its TV valves loses resolution but does not actually die. It puts out signal at varying frequency from its X amp which is of great benefit in audio work.

I use one for calibrating principally valve sets, as that gives a better indication as to what's going on and distortion is easily spotted.

I would point out that a Signal Generator and Oscilloscope are a formidable combination, when signal gets lost in transit. I have a semi permanent attenuator box that allows the Signal Generator and Frequency Counter to be coupled and the Sig Gen audio is used to sync the oscilloscope giving more stability.

A trap with many counters is to calibrate the signal generator with modulation / tone, turned on.

As with all instruments you need to understand them and more so with the oscilloscope, where you need to understand and interpret what it is showing you. I do have "Rider" book and another, specifically dedicated to oscilloscopes.

Marc


 
 Return to top of page · Post #: 10 · Written at 7:20:51 AM on 27 May 2023.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 1267

Ian, the main good feature about a modern digital CRO is the measurement readouts while you are viewing the picture.
you get a check on your separate instruments like frequency counter and DVM, if they all give the same value you are reassured.
All the comments about the probes are spot on, NEVER use a standard probe on anything but x10 setting near a valve radio.
A digital CRO may seem overkill for AM radio work but it is the facilities not the performance.
The cheapest Digital CRO like Jaycar serve up is fine for valve radio.
I use two of them to see 4 things at once if needed, plus I have an old Jaycar CRT CRO when I don't believe what the digitals are telling me!
Digital CROs get a bit confused if you present them with unclean signals, they can lock on nothing and measure wrongly.
Thats when I grab the QC1920 and clip that on, it may give a bit of a blurred picture but you can see what is happening and then re adjust the digital guy to lock up.
For radio if any body is thinking of buying a digital CRO just get the cheapest Altronics or Jaycar.
the other advantage is you can HALT the image and look later and also SAVE to a flash stick.
Having said that I keep forgetting how to do SAVE and instead just blaze away with my mobile camera, as you see in my articles.
Fred.


 
 Return to top of page · Post #: 11 · Written at 9:27:44 AM on 27 May 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

Their are traps with all instrumentation and you have to know them. A lot of modern equipment will not withstand the voltages of tube radio.

Many have no appreciation of the theory of measurement.

When there has been the decency to actually present voltages & currents, like many German circuits; We get many with no realisation that as resistances get higher, an analogue meter will load the circuit substantially. This means that you have to know what was used to measure the volts. The digital meter and other "high impedance" meters will not "load" to anything like the same extent. That means the voltage presented will be considerably different and you can easily draw the wrong conclusions.

An RCA quote on a schematic, is to the effect "That the voltage measured is not that which the Radiotron is running at" a clear reference to the analogue meter loading.

Some early digital meters and semi digitals were allergic to RF which they either rectified and added to the reading (Reflexed sets interesting) alternatively it "flashed over', destroying it. The way a digital works can be its undoing. They have a "sample rate".

Sample rate translates to:- Reading - Averaging - Then presenting to the display, in a cyclical fashion. That read write frequency and averaging is where things like dirty fluctuating input, can make what is being presented: Meaningless.

The analogue meter movement has "moments of inertia" and will not see RF, or high frequency AC. That is why I check DC automotive generators with a 500 ohm per volt meter. As there is no sample rate these and the more traditional Oscilloscopes with no read write; Read in real time.

Marc


 
 Return to top of page · Post #: 12 · Written at 11:55:26 AM on 28 May 2023.
Tinkera123's Gravatar
 Location: Melbourne, VIC
 Member since 5 October 2009
 Member #: 555
 Postcount: 465

Hi Fred et al,

Agree with you comments about the advantages of modern digital scopes ... also the size and weight ... extremely small and light compared to my old Tek scopes ... which is starting to become more important as I have trouble lifting some of the Tek scopes due to a crook back etc.. The Sample rate that Marcc referred to is what I had in mind when I wrote 'you get what you pay for'. A digital scope is definitely on my Bucket List.

That said, I get a lot of fun out of fixing and calibrating my Tek stuff ... I remember using a Scope in 1st Year Uni which had the SYNC function mentioned in the article posted by STC830 (now 'Triggering') ... I had no idea what it did, except that rotating it suddenly 'froze' images on the small screen.


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Cheers, Ian

 
 Return to top of page · Post #: 13 · Written at 10:31:49 PM on 28 May 2023.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5303

On fixing Tech stuff: comes a warning for all.

It does pay to check meters etc. regularly. On top of that, much of the old stuff has the same discrete components as the radios and they suffer the same fate. I have been obliged to replace caps in particular in several bits of test gear.

There is no place for leaking non-polarised caps (to any extent) in test equipment.

Marc


 
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