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Old 26th June 2008, 05:43 AM   #1
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Default Questions about my oscilloscope...

Hi.

I've had an old Tektronix 561B oscilloscope for about two years now.

Unfortunately, I don't have any probes.
If anyone knows where I can get some probes for little money, or perhaps construct my own, I would be grateful.

I've been doing the best I can, however. I purchased some compatible connectors (gold-plated) from Radioshack and have been making due in the best ways I can. It works wonderfully!

However, I think the unit needs adjustment. The DC offset moves up or down whenever I turn the volts/div knob. I've taken the slide-in unit out, and there are a bunch of trimPOTs which I am assuming are for correcting this/calibrating the switch points. However, these POTs are soldered in their position, likely the position they were in when it was just built!. I'll have to draw a diagram and see what I can do.

I'm also not quite sure about the calibration unit. It produces a nice square wave, alright, but I don't know if it's really putting out the voltages it says it is. Either my calibrator needs some working or my volts/div switch needs some heavy maintenance.

As far as I can tell, nothing in this unit has been tampered with, and if it has, by a professional. There is a metal ID on one corner saying that it used to belong to some university.

Does anyone know where I might be able to get the internal diagrams and adjustment procedures for fine-tuning one of these? The manual isn't particularly helpful, as it only covers maintenance.

At any rate, all the tubes glow warmly on cold winter days... And threaten to burn the house down during the summers down here is Texas! I'm glad to know that if our household heater goes out, we can rely on this thing for steady, intense heat.

- keantoken
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Old 26th June 2008, 10:34 AM   #2
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Okay, I just got done tuning it. I was amazed by how much of an improvement was made just by reseating the tubes! A definite increase in high-frequency response.

There are still some weird problems with triggering, but that's to be expected when you're reading 60Hz AC ripple from the other end of a 10Mohm resistor...

Now I have to build a circuit, so that I have more to study than the EMI fields in my room.

For those who are interested, I can probably take pictures.

I still need probes though.

- keantoken
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Old 26th June 2008, 10:57 AM   #3
AndrewT is offline AndrewT  Scotland
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Don't adjust any of the internal pots, until you KNOW what you are doing.

The external adjustments should allow the scales to be set up accurately.
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regards Andrew T.
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Old 26th June 2008, 12:00 PM   #4
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This unit was probably out of use for quite a while, and in Texas weather, it's a good idea to tune things up regularly.

I found the manual online and did everything it said... There are certain internal POTs that are meant to be used for maintenance. There was a bit of work to be done, and me being me, I had all the time in the world to make it perfect.

I only regret that I don't have the expensive gear required to do the more in-depth adjustments, like phase correction etc.

But I don't really need all that extra fluff. All I really need is to see what my circuit is doing. That is the most useful feature, and trump card of the standard oscilloscope...

One of my friends said he'd ask a guy at work if he had some oldies like the probes I need. I need to check back with him...

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Old 30th June 2008, 06:02 PM   #5
Mooly is offline Mooly  United Kingdom
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You again
You can check the volts/div very accurately by just measuring a known DC voltage. Use a battery and 10k pot say to set some known voltages (measure with a DVM) and check the trace displaces the required amount. 'Scopes measure DC just as well as AC remember.
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Old 30th June 2008, 07:12 PM   #6
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Hi.

Hmm, I'll keep that in mind. What bugs me is that since I don't have probes I have to stick 10Mohm resistors into the connectors and use alligator clips.

Also, the DC offset is always wandering around. By the time it's stabilized, I'm already done with my experiments.

I've recently been thinking about making an accurate saw-wave generator and developing a curve tracer adapter. I think I've seen a few threads about such... I think Rod Elliot also has a project for it. I'll have to reread it sometime.

It's funny... I've never really felt the draw towards tube electronics until now. I've now been infected by the magic of tubes. I have a whole new level of respect. There's something weird about these little glass things that I can't put my finger on.

- keantoken
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Old 30th June 2008, 07:25 PM   #7
Mooly is offline Mooly  United Kingdom
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Why do you need 10Meg resistors ? The input impedance of most scopes is 1 meg and this is what a times 1 probe sees. A 10 to 1 probe will have a 9 meg resistor in it and a trimmer cap to adjust the input capacitance to match your scope, so you dont get under and overshoot on squarewaves. Use your scope with a direct lead with no resistor in series but keep to the max input rating of the Y amplifier of the scope. It usually says this next to the input socket.
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Old 30th June 2008, 07:27 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally posted by keantoken

There's something weird about these little glass things that I can't put my finger on.

- keantoken
Probably the heat
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Old 30th June 2008, 11:51 PM   #9
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Quote:
Why do you need 10Meg resistors?
My scope is weird. I was trying to use the built-in calibrator as it is, but I was getting 10X the readings I was getting on my DMM. It turns out that putting that 10Mohm resistor in place made everything a whole lot more accurate.

I don't know, maybe the people who used to work with this one modified it just for convenience. At any rate, they knew what they were doing, if they did it, because nothing seems out of place.

Quote:
Probably the heat
Yes, that may be it!

They're just so cool, though.

Really, though. These things are just strange. They're more mind-boggling to me than transistors are, that's for sure. I want to learn more about tubes. I'm sure they have some use, outside of audio preamps and such.

- keantoken
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Old 1st July 2008, 07:26 AM   #10
Mooly is offline Mooly  United Kingdom
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You have to remember that a 'scope measures (shows) the peak to peak value of a signal. A DVM on AC volts measures the RMS value. For example in the UK the mains is 240 volts RMS. The peak to peak value is 680 volts -- thats why it bites .
The error is even worse if you try and measure something like a squarewave. To convert from RMS to peak to peak (sine wave only) in the above example it's 240 times 1.414 (this is root 2) which is 340 volts. This is the peak value -- peak to peak is 340 times 2 = 680. So the DVM shows 240 and the 'scope 680 --- both are correct.
Try as I said and compare DC voltages on your DVM and scope. No resistors. That is absolutely conclusive on accuracy of the volts/div switch. Apply 1 volt DC with the switch on say 0.5 volts/div and the trace will move 2 divs ( set scope on DC coupling of course)
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