Easy way to see if my oscilloscope is accurate?

I have this Protek model 200 mHz, dual trace scope.

I'm wondering if there's an easy way to see how accurate it is.

All I'm going to be doing w/ it is aligning the heads on my tape decks,
so it will be in x/y mode.

Anyway, how do I make sure it's accurate?

you could do an azmuth adjustment using a VU versus a scope and then check it with the scope.

IF my memory serves me correctly

Put on the 1k tone from your alignment tape at the speed you're going to record. Bring both output channels of the machine up on faders panned to the center of your stereo buss Set the output to read 0 VU. Throw one channel out of phase. (If your console doesn't have a phase button on each channel strip, it should!) The output should now be almost nothing. Test it by putting the channel back in phase.... should be back to 0 VU. This is good.

If your console doesn't have a reverse-phase button, you'll need to make a special out-of-phase cable to put in the signal path of the output of one track... doesn't matter what track. If you have a phase reverse button, check the output of that fader to be sure the gain of the left channel is the same as the output of the right channel. Sometimes a phase reverse button causes a gain loss.


Playback rough azimuth adjustment: Now play the 1k tone off the playback head, looking at the buss output. If you set the in-phase output to 0 vu, there should now be an output of... oh... minus 10 to minus 20 or less.

Carefully, get a non-magnitized screw driver (probably phillips or maybe hex wrench) and slowly turn the azimuth screw. . You should see the output of your buss meter change. It should go up, again, assuming you've' got one channel out of phase on the board.

Turn the screw until you see the buss output go as low as it will go. Look carefully. Don't do this by candle light. Rechecking is a good idea.


Playback fine azimuth adjustment: Now play the 10k tone and do the same thing, only slower. It may not go down as far as the 1k tone did, since often the 10k region isn't as stable as the 1k. It's smaller frequencies up there.... NOTE: This is called a negative peak, and there will be one major negative peak, and a couple minor negative peaks if you turn the screw a lot in one direction or another. This is when you tilt the head so far off, one channel is reading the next whole sine wave instead of the same one in line with the other track. Be sure you're adjusting the major negative peak which always dips the lowest on the buss.

Go back to 1k (in playback) and check to see that the 1k tone is still way low on the buss meter. If you need to do this a couple times to see these negative peaks, go for it. In time it will become second nature. Come back to the 10k tone and recheck to be sure it's showing the lowest possible buss output.

Record rough azimuth adjustment : Now put on some blank tapepreferably the kind that you'll be using for the session. On your record pad or some piece of tape FAR away from any recorded music, send a 1k tone from your oscillator or tone generator to your machine. Record 1k onto the tape. The console and machine should be set up the same way. You should still be in playback - you're seeing your setting via the playback head's output. Turn the comparable screw on the record head and adjust the record azimuth to give you the lowest negative peak reading.

Record fine azimuth adjustment: When it's set to that lowest point, turn your oscillator to 10k, roll back the tape and adjust the fine azimuth setting. Same stuff as above - record 10k to tape, and still reading playback, find the lowest negative peak.

easy enough but even easier with a minty fresh scope. :grin:

> see how accurate it is. All I'm going to be doing w/ it is aligning the heads on my tape decks,
so it will be in x/y mode.

How does that work? You put Left and Right into X and X, and trim for a 45 degree line?

Heck, 200MHz is 10,000 times better than you need for that.

In any case: feed the SAME signal to both X and Y. You should get the perfect 45 degree line. If it aint perfect, use grease-pencil to mark the actual squiggle you get. Then apply your Left and Right, and trim to the same perfect or marked-squiggle line.

That's how I would do it on my 1938 DuMont. It has been rebuilt with IDENTICAL 20KHz (KHz, not MHz) H and V channels. But the CRT itself is unbalanced drive and also pretty abused, so a raster is a trapezoid (not square) and phase-shift elipses are egg-shape. No matter: same signal in both inputs, whatever I get is "perfect", and then the deviation from that for different X and Y inputs is my error.

If you actually want absolute accuracy: first check a "1.5V" battery with a good voltmeter, then measure it on the scope. That tells the DC error. For AC, you should have got a square-wave output on the scope or a separate calibrator. This should be a particularly good square wave with zero droop across the top (cheap ones have slanty rise-times, but should be flat-top). Droop indicates the DC response is different from the AC response. The square wave output may also be marked "1 Volt" or some such, which is handy, but compare it to a battery and good voltmeter to be sure.

Calbrating the high frequency trimmers is hard. The trimmer on the X10 probe should be trimmable with the scope's own square-out. Adjust for minimal over/under-shoot on the square corners. The internal trimmers to tweak the 100MHz-200MHz response are NOT stuff for casual adjustment, leave that to specialists. Anyway I can't imagine why you'd care for audio (even most radio and video) uses.

Time base: musicians can get a good cross-check with a good A=440 reference. Techs like 1KHz and similar round numbers. For most audio purposes, the hum-wave from an open probe (16.66mSec or 20mSec period) is really good enough; a scope is a lousy frequency meter, but we have good cheap meters to read frequency.

Wow,

Thanks guys,

Both those posts are a big help.