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Old 21st June 2010, 06:13 PM   #1
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Question Quick scope probe question

I just bought myself a Russian analog scope model C1-71, for $117. The seller ran it thru the full set of tests with a function generator and it seems it can handle up to 200MHz. But he did not give me a probe along with the scope.

Taking a quick look online i spotted a 200MHz probe for $20 which seems fine to me and i thought about ordering that, since in my local stores the prices are higher. That probe is going to take about a week to arrive and out of curiosity i tried using a plain piece of 75-ohm TV coax about a meter long. The self-calibration wave looks perfectly square, but its frequency is just 2kHz so it could be a bit off in the MHz range... but i don't really need that, i bought the thing mainly for amplifier and SMPS work. I do have a certain project involving 133MHz but that's to come in the next months.

So, the question is, can i use the plain coax for basic audio frequency measurements? The scope itself can handle up to 60 volts input so i am gonna need the 10x probe for offline SMPS work anyway, but till it arrives can i just use this cable for low voltage testing? It's designed to carry UHF TV signal anyway.
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Old 21st June 2010, 06:24 PM   #2
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Yes you can make measurments and check signals with a piece of coax. You might consider using a high voltage cap in line with the coax. There is a specification for the maximum input voltage for the scope. If you excede that while measuring the plate voltage for instance, it could be very bad. Like a 400-500v in the low microfarad range.
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Old 21st June 2010, 06:43 PM   #3
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Thanks for the heads up. I'll buy a BNC connector and an alligator clip tomorrow to go with the cable and make this a "proper" hack. I already have a probe tip from my old multimeter.

The scope does have an AC coupled mode so the input capacitor is built-in, but i do not know its voltage rating so it's best to play safe (its voltage rating isn't mentioned in the scope's schematics either), i'll do like you say. 4.7uF at 400v sounds fine?
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Old 21st June 2010, 07:22 PM   #4
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Sorry for double posting, i can't edit anymore. I just did my first SMPS ripple/noise test. I used my trusty old 200W Seasonic ATX power supply, which i use as a general purpose/bench supply because it's very stable. 5v ripple clocks in at about 30mV, with 12v at 60mV. I checked Hardware Secrets and those are quite respectable numbers especially for a supply this old, never recapped, never even opened.

I'm still getting used to reading the divisions but the frequency the supply is operating at looks to be roughly 90kHz. The sync circuitry of the scope is a little bit unstable, but maybe it's because some higher frequency noise is also present in this power supply. In any case it's usable, and WAY better than nothing.
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Old 21st June 2010, 07:43 PM   #5
Mooly is offline Mooly  United Kingdom
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Divider probes (or times 10 probes as everyone call them) offer a low input capacitance of only a few PF. Using one of these is where you use the low freq "cal" signal to adjust the trimmer on the probe for a flat squarewave with no "tilt" on the tops/bottom.

A coax lead will load the circuit capacitively, which for most low impedance audio stuff isn't to important. It also loads the circuit with the input impedance of the 'scope usually 1 meg ohm, the probe raises this to 10 meg ohm.
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Old 21st June 2010, 07:45 PM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Th3 uN1Qu3 View Post

I'm still getting used to reading the divisions but the frequency the supply is operating at looks to be roughly 90kHz. The sync circuitry of the scope is a little bit unstable, but maybe it's because some higher frequency noise is also present in this power supply. In any case it's usable, and WAY better than nothing.
Look for an "hf reject" in the triggering options. Post a picture of the front panel
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Old 21st June 2010, 08:18 PM   #7
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Quote:
Originally Posted by Mooly View Post
Look for an "hf reject" in the triggering options. Post a picture of the front panel
Here's the pic... And here's for hoping that you can read Russian. I can only read like half the stuff myself. The very bright camera flash drowned out both that and the trace, but the screen is actually backlit by two glorious yellow bulbs. And it's adjustable too, from "nothing" to "barely enough"

The little knob on top of the volts/div selector (they do like dual pots don't they), has a rather major effect on sync stability. Turning it to the left = better sync and much clearer trace, but nothing comes for free and it happens to also attenuate the signal, thus the volts/div ratio changes and it's not possible to estimate the signal voltage anymore. I have no idea what that knob is actually for, but when turned fully to the right it also clicks, it seems to have a switch built-in.

Edit: There's a similar one on the time section and i just noticed they both read "Calib." which is definitely short for calibration, and their behavior confirms that they are fine tuning for the main switch. I still don't understand why the one on the volts/div switch makes the trace more stable.
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Last edited by Th3 uN1Qu3; 21st June 2010 at 08:22 PM.
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Old 21st June 2010, 08:25 PM   #8
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Sounds like that knob with the detent is the timebase veriner, to measure frequency this needs to be in the "clicked" position, otherwise adjust to best result.
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Old 21st June 2010, 08:34 PM   #9
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Quote:
Originally Posted by firechief View Post
Sounds like that knob with the detent is the timebase veriner, to measure frequency this needs to be in the "clicked" position, otherwise adjust to best result.
Bob
Ah, vernier. This term sounds familiar. So defeat them for measuring frequency/voltage, or tweak if all i need is a good look at the waveform. Gotcha.
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Old 21st June 2010, 08:45 PM   #10
AndrewT is online now AndrewT  Scotland
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the AC/DC cap is feeding 1M input impedance.
You don't need 4uF as it would have F-3dB~0.03Hz.
Even 470nF would be F-3dB ~ 0.3Hz and F-1dB ~ 0.6Hz.
That's still very wide for an AC input.

Look for a 630Vdc (400Vac) or 1000Vdc (600Vac) input cap in a value getting up around 100nF if you intend replacing the existing input capacitor. 100nF & 1M0, F-1dB ~ 3Hz.
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