Hitachi V-212 Oscilloscope Adjustment

[max.wwwang]

I got a 2nd hand V-212 and after a while I noticed its 2nd channel is not correct on its vertical scale. With its own calibration 0.5V square wave signal, with Volts/Div set to 0.1V, the peak-to-peak amplitude of the signal is only about 4 Divs. Ch.1 is fine. I only found this out when comparing two signals from both channels when I though should be the same amplitudes but not which I struggled to understand.

With the service manual, it seems this could be adjusted by the RV 162 variable resistor (for vertical gain of Ch.2). It worked but it's only 4.5 Divs (should be 5 Divs) when I turned it to its extreme. What can I do to fix this? Thanks.

The service manual could be downloaded here:
https://www.manualslib.com/products/Hitachi-V-212-4946652.html

[tautech]

Is the front panel Var V/div control set to Cal and clicked right home ?

[max.wwwang]

I set it to 0.1V because I didn't have a signal generator to calibrate but had to rely on its own 0.5V signal for a rough calibration (so the signal won't go beyond the screen - 5 Divs). I only just noticed that there is a 'Cal' at the right end of the Var V/Div knob which is presumably for standard calibration signal to achieve the highest possible accuracy (corresponding to 5mV/Div).

The central smaller knob was in its clicked in position (without 5x gain). Actually this knob of my scope is missing so it was never in the popped position with me.

In my circumstance, do you think is there anything I can do to make it correct? Thanks a lot.

[tautech]

I only just noticed that there is a 'Cal' at the right end of the Var V/Div knob which is presumably for standard calibration signal to achieve the highest possible accuracy (corresponding to 5mV/Div).

The Cal indicates the position the control is locked where amplitude can be accurately measured against the graticules. If not locked in the Cal position graticule measurement don't mean a thing therefore any adjustment to amplitude accuracy must be made with the control in the Cal position.

In my circumstance, do you think is there anything I can do to make it correct?

First both channels must be in the Cal setting before channel amplitude can be compared and a quick check can be made with just a clean common DC source while inputs are set for DC coupling.
If there's a disparity then the fun begins, first which channel is right and then where is the error in the bad channel signal path ?
Much can be done just measuring points along the input signal path to where the channel mismatch appears then carefully check that part of the circuitry. Some manuals have DC measurement points to check while the scope is configured in a certain way.
Some go about this another way by starting at the CRT plates and working backwards along the signal path and both methods have their merits and also depends on the quality of guidance in the service manual.

Check of the Repair docs thread in the Repair board and grab the Troubleshooting Tek scopes pdf as it will offer more tips and understanding. Read, study, rinse and repeat ! 

[Brumby]

If not locked in the Cal position graticule measurement don't mean a thing

Read this again  ^   ^   ^   ^   ^
If it isn't obvious, "Cal" is short for "Calibrated".  Your graticule is only "calibrated" to the signal when the control is rotated and (usually) clicked to that position.

therefore any adjustment to amplitude accuracy must be made with the control in the Cal position.

Make sure this is done for both channels.  If any channel is NOT is the Cal position, then you will not be getting accurate measurement for that channel!!!!!

[max.wwwang]

Thanks both but I don't quite understand. I thought if I set V/Div to 0.1V and using the internal 0.5V signal, can I ROUGHLY adjust it when I adjust the amplitude of display to 5 Divs? I don't have an external calibration signal source.

[tautech]

Thanks both but I don't quite understand. I thought if I set V/Div to 0.1V and using the internal 0.5V signal, can I ROUGHLY adjust it when I adjust the amplitude of display to 5 Divs? I don't have an external calibration signal source.

The probe Cal source should never be used for internal adjustment, period !
It's only real attribute is a fast rising edge and both frequency and amplitude are typically 'near enough' for sanity checks.

Again, the Cal setting on the V/div must be OFF (Cal position) and you can check the amplitude accuracy with just a clean DC source.
To see what I mean use 1x probe or just some wire and a battery and note the 0V trace position then connect the battery and watch the trace jump up to display the battery voltage and measure against the graticules.

In practice you need some reasonably accurate and stable voltage and adjust the V/div until it makes sence and all comes together for you. 
Confirm what you see with a DMM then make adjustments or seek faults as necessary.

[Brumby]

These red knobs need to be turned all the way to the right, ending with a click.

ONLY THEN can you attempt to measure amplitude using the graticule.

[max.wwwang]

I was ignorant of this only because these two small red knobs were missing on my scope (good excuse!) The scope I bought second hand was not only in very poor aesthetic condition but also with these two knobs missing. I can now manage to pull out the one for Ch.2 with a pliers but still unable to get hold of the one for Ch.1!

After another good reading of the manual, I realised that even in the calibrated condition, the cope allows you, with the red knobs, to fine tune the gain of either of the two channel, or both, to perfectly match their vertical scales for a same signal level to facilitate an accurate signal level comparison. I did my adjustment of Ch.2 with the red knob pushed in but without knowing its rotational position. Now I'm equipped with the knowledge to set it back to its correct position!

Actually it was only by chance by rotating it randomly noticed I could manage to get both channels match on the vertical scales with the same gain.

And Yes, it is very easy to understand I don't have to have a fancy signal source to calibrate both channels' vertical gains! (I have a multimeter and of course AA batteries.)

Your help is much appreciated!!

[tautech]

Cool, you're getting there. 

Still, when both channels are set to the Cal position and the V/div settings match, the same input level must be displayed at the same amplitude for both channels. This is when you need to adjust the internal vertical gain with a known input level/signal and both internal gains adjusted so graticule measurement matches the known input value.

The overall concept is quite simple once fully understood. 

Gain/amplitude graticule measurements are only 'so so' accurate on a CRO however it's important they are reasonably close to actual input levels so one can see if waveforms are reaching their required amplitude levels, whatever they might be in real use.
Happy tweaking ! 

[max.wwwang]

Cool, you're getting there. 

Yeah!

Happy tweaking ! 

Yes!

[tautech]

Oh and some more for further understanding of what the Cal knob is mostly used for.

Rise/fall time measurements where the graticule vertical layout has 2 addition lines at 20 and 80% OR 10 and 90% and to use them the Cal knobs are taken from the Cal position and adjusted for the waveform to vertically fill the display.

At the points where the waveform crosses the lower and upper rise/fall time graticules the horizontal difference between these 2 points is counted on the graticules and multiplied by the timebase setting for the actual rise/fall time measurement.
You should be able to find YT vids on CRO rise time measurements.