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chris ma 23rd May 2008 06:50 PM

How to measure JLH Class A with a Scope
This thread is intended for newbies like me not knowing what to do with a oscilloscope. What setting on the scope and where in the circuit to probe etc etc..This is only for the JLH High Power class A circuit. No need to be too detail. Just a rough idea as to.

1/ Checking for amp stablility. How to measure and understand what the scope is telling in a simple way.
Where to safely connect the probe, illustrated on the schematic diagram of various probe points.

Establish benchmark figures and interpretation at various point in the circuit with regard to output transistors. Driver transistor etc.

2/ Checking for noise. At the PSU. At the speaker output etc.

Simple task list with the scope to get a clean bill of health for the newly assembled JLH before daily use.

All pointers and helpful hints are welcome and appreciated.

Best Regards

Mooly 24th May 2008 07:39 AM

Before you can undertake any measurements you have to know the 'scope is calibrated correctly. From the pics in the other thread it looked like you were getting the overshoot with a X1 lead. The 1 Khz squarewave output-- have I got that right, is that what your scope puts out at it's cal socket -- should be square on all volts/div ranges when connected with a X1 probe. If it's not - compare also with channel 2- then the attenuators inside the 'scope require adjusting. Note, unless these have been twiddled in the past they should never need touching.
When you use a X10 probe you adjust the trimmer in the probe itself to obtain a "flat" squarewave and this adjustment should be correct then for all volts/div settings on the 'scope.
The squarewave response has to be right as it will affect all other measurements.
Remember the scope shows peak to peak voltages not RMS. So in the U.K. our 240 volt mains would be 680 pk/pk on the 'scope.
DO NOT try and measure the mains by the way.
Is the volts/div accurate ? The 'scope measures DC as well as AC so get some batteries 1.5v 9 v etc, read them on a DVM then read them on the scope and check the readings agree.
Safety, The 'scope should be earthed (mains earth) at all times.
This however means the probe is also connected to mains earth. Be aware therefore that you cannot connect the ground lead of the probe to say a positive rail in an amp that is also earthed. Big bang if you try that :hot: .
Do you have an audio signal generator etc or a test CD with various tones on it ?
Regards Karl

chris ma 25th May 2008 01:30 AM

Sand Box
Hi Karl,

When I connect the probe to the (4. Select CH1 VERTICAL MODE and insert the tip of the Channel 1 probe into the PROBE ADJUST output jack.) Cal socket I left the other clip of the probe lead not connected to anything. I assume this must be the (ground) as to the main tip of the probe be +ve am I correct?

The volts/div seems ok. It is just me not getting used to reading the scale on the screen sometimes. That is the only thing I dare to poke the probe, batteries for now :)

Are you trying to set me up using a working system as my sand box for using the scope?

Sorry, I have no signal generator.
I have a test CD with different frequency on different tracks. I also have a small DVD player that can be used to play that CD.

Once the new probe delivered to me I can start check to see if it is calibrate correctly. I wanted to buy 2 probes but the guy selling on e-bay only has 1 left. I will get this one sometime next week.

So the sand box consists of all working:
1/ DVD or CD player
2/ Power amp
3/ Speaker

Let me try and setup the sand box first this weekend...

Best Regards

Mooly 25th May 2008 12:04 PM

Having a measure with your test disc is as good a place to start. Just connect the 'scope to the line out sockets on the DVD, ground lead to the line out ( phono socket ? ) outer.
Try and calculate the frequencies you see on the 'scope. This will help confirm the timebase is accurate as well. Freq = 1/T So 1 millisecond for a complete cycle on the scope screen would be 1Khz. The test disc frequencies will be absolutely accurate to fractions of a percent.
I am still confused with the probe test thing on your 'scope.
Is this a 1Khz squarewave output. If you connect the channel 1 input here can you get a squarewave on the screen ? If you can, then with a X1 lead it should be square - no over/under shoot.
Regards Karl

And whats a sandbox ? Never heard that expression before :)

chris ma 25th May 2008 09:54 PM

2 Attachment(s)
Sandbox is the like playground for kids so to speak.

I have connected the dvd player to channel 2 with a interconnect (coax) with a RCA adaptor, see in picture in blue. The dvd player is set to play on repeat of a track that plays 1kHz square wave.

I will attach 2 pictures in the following posts.

Picture one showing the settings.
Picture two showing the result of those settings.

chris ma 25th May 2008 10:38 PM

Top Display

Channel 1 the black coax is connect to the scopes own CAL output at 1kHz 500mv P-P

Bottom Display

Channel 2 is the 1kHz square wave signal from the Sheffield test CD. But it does not look square no matter what settings are those dials are at.

Do they make any sense?

I tried to attach the second picture but for some reason I keep getting the picture size is too big although it is only 42kb

chris ma 25th May 2008 10:48 PM

2 Attachment(s)
Picture 2

chris ma 25th May 2008 11:25 PM

Bad news, while doing these pictures I fried the digital camera by not connect the USB to the camera last! When I plug the USB cable in my laptop with the cable already connected to the camera and I heard a crackle the camera became dead

KP11520 25th May 2008 11:56 PM

Ouch chris ma,

Sounds like not only the butcher but the fryer too! Yikes!

But Fried Chicken Thighs are delicious, not sure about a Digital Camera though.

Do they warn about this in the instructions? Maybe there is a fuse. If not, hopefully it is still under warranty!

Good luck!

BTW, I am a newbie too and I am interested about this as well for when I get a scope too.


Mooly 26th May 2008 07:54 AM

That's a pity about the camera. Isn't USB supposed to be plug and play, I think I would have a whinge about that !
Right --- I am still confused. The picture in post 7. Where has that squarewave come from. Is it the CD or the 'scope cal output.
Just confirm what we are looking at :)
When making any measurements you need all the "variable" controls on the 'scope ( Any pots in other words ) in the cal or calibrate position. If this isn't done the settings on the attenuator and the timebase are meaningless.
O.K. Amplitude of the sinewave on the screen. As an example if the 'scope is set to 1V/DIV and the sinewave is 4.25 squares from tip to tip this equals 4.25 Volts peak to peak. To find the RMS value ( only for sine waves ) divide the pk/pk by 2 to get the peak value. 2.125 volts peak in this case. Then divide this by the square root of 2. So the RMS value of 4.25 V pk/pk is 1.5 volts RMS. So working the other way a C.D. player with an output of 2Vrms will show on the scope as 2*1.414 *2 which is 5.65 volts pk/pk.
Regards Karl
Edit, just had another look at your pic, it's from the 'scope cal output isn't it. That looks fine, and when you get your X10 probe you adjust the trimmer in the probe so that the tops and bottoms of that squarewave are flat. When you try it it's obvious what you do.

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