Hitachi V-352 callibration / odd waveforms

[georges shed]

Hi, New forum user here.
I recently bought a HITACHI V-352 in unknown condition. It powers on and shows waveforms and the trigger and all functions seem to work okay. Ive tested the powersupply voltages and they are all within a volt of their specification so seems to be okay. There is no adjustment to these voltages anyway. Im trying to follow the calibration manual. When looking at a 1 khz square wave i am greeted with an oddly shaped waveform (see images). Ive tried adjusting some of the internal adjustment pots but so far no luck. It seems to be the same on both cahnnels both AC and DC Any ideas?

[t1d]

Caution, I am not a scope expert...

My guess is that you are looking at a square wave, but that your probes have not been attenuated. Also, you need to change the intensity of the picture to see the vertical legs of the traces. Depending on lots of variables, you may not get a great picture of the verticals on square waves. Sometime you just have to know that they are there. Did you start by using the on-board test signal and truing your probe leads? The probe calibration output connection appears to be in the top-right corner of the front panel.

How to calibrate your probes

[t1d]

Oh, just as encouragement, because you bought it secondhand and are wondering if it will work... The trace you have is encouraging that 1) it may work okay as-is and/or 2) it may need very little repair/adjustment.

[georges shed]

Thanks for the reply!

I have hooked up the 1khz test point on the scope itself and a 1khz test signal from another scope and view the same result. Only one of my probes has an adjustment on it when i adjust it the shape changes slightly but still doesnt produce a nice wave. Here is the test sigal on another scope that is lovley and crisp.

[t1d]

Are the probes that came with the unit the original probes, or something else? Did you use a proper tuning screwdriver, or one out of your toolbox?

[t1d]

Just any-old-screwdriver may effect the adjustment.

If the probes are not original, make sure that they are the proper Hertz to match your scope... 35MHz. I take it that this is your first scope, for learning. If so, and you need new ones, the cheap Chinese ones may do for a start. That's what I did.

[georges shed]

Thanks for the video, it was very useful. The one that can be adjusted came with the unit when i bought it. I dont think it is a probe issue as i can only adjust it so the line is straight annd looks like a slanted square wave. The other probes i have only have a x1-ref-x10 switch. Im just using a regular screwdriver.

[t1d]

Okay, let's make sure that I understand correctly... When you put your probe on the other scope, your probe can achieve a proper square wave. When you then put your probe on your scope. you can get a non-skewed top on the square wave, but that top line is at an angle, correct?

[georges shed]

yes that's right. Odd

[t1d]

No, not odd... It just means that some adjustment needs to be done. Do you have the operators manual? Not the service manual, we will get to that later.

[georges shed]

here is the same signal on 2 scopes for clarity. I have this manual: http://ftb.ko4bb.com/manuals/5.198.31.235/Hitachi_V202_V352_Oscilloscope_Service_Manual.pdf.

Thanks a lot for the help!

[t1d]

That link did not open directly to the manual... But, it appears to be the service manual, not the operator manual. You don't want to start with trying to calibrate the scope inside the case. We need to make sure that we have done all the setup stuff, on the front panel, first. To do that, you need to find a copy of the operators manual.

It would help to know what tools, skills and experience you have, so I know how to communicate with you.

[georges shed]

i cannot find an operators manual. The service manual contains a setup procedure to get a wave which is what i used to setup. I have basic ocilloscope knowlwdge and not much in the way of test equipment. I have another 20MHZ scope, a multimeter and a variable psu and the usual arduino's for an electronics tinkerer.

[t1d]

Good base of skills... We can work with that... What about soldering skills and tools?

Okay, I want to clarify... Your probe connected to your scopes cal signal produces the skewed trace. The other scopes probe attached to your scopes cal signal produces a true enough trace. Correct?

[georges shed]

fairly good soldering skills and a mediocre selection of resistors and components. And yes. That proves the cal output is good. Ive swaped all the probes to yield the same result.

[t1d]

That proves the cal output is good.

Agreed.

You do need to find the operator's manual, at some point. It out there, somewhere.

Okay let's talk about one more front panel adjustment... I think this will not be the cure, but we can talk about it... On your scope, below the Focus knob, there is a Trace Rotation Screw. I believe this to be the horizontal (Like horizontal hold on an old TV) adjustment.

If you were using a straight voltage, say from a 9V battery, you would get a straight trace, offset 9V above the baseline. If that straight line were tilted off of true horizontal, you would use this adjustment to level it. I am rather sure that it will not true the square wave. Meaning...

I think that you are on the correct track by proceeding with an internal calibration. However, this will take a good bit of knowledge. You have the manual, but it may be outside of your skills, even with the book. You will have to decide. If you did not pay much for the scope, then its a good learning opportunity.

The skew on the square wave is a common adjustment. I suggest that you watch several YouTube videos, before you start on yours. "W2eaw" does a good job on walking you through it, but you will be working on a different scope, of course.

Read the service manual, completely, first. Follow its directions step by step. Don't skip anything. If you can not true it with the adjustments provided, that will mean that there are parts issues.

You are going to need a set of proper electricians tuning tools. A regular screwdriver may influence the adjustment and you may be chancing its effects and never be able to hit the sweet spot.

I hope you do well with it. I'm glad you are saving a good piece of equipment from the trash bin. You will want to hang onto this scope, even if you buy a fancy new digital scope. Analog scopes are better at some jobs, so its always good to have one around.

Any other questions? If not, I'm going to log off.

[t1d]

One last word... Analog oscilloscopes use dangerously high voltages to drive the display tube. And, their capacitors can hold the big charge for extended periods, after the unit has been unplugged. You will need to discharge the caps, before doing anything else. There is a significant amount of genuine high voltage shock risk.

So, be super cautious... Use the "One-hand" probing method. If you don't know what that is, it would be a good indication that you may not be ready to be working inside a scope. Professional assistance might be worth the price. Ask your wife what she thinks, if she still loves you and isn't after your life insurance. Lol...

[georges shed]

Thanks a lot for the help!

[GreyWoolfe]

The other thing to look at is if the probe compensation range will match that of your scope.  For example my Tek 2465 has an input capacitance of 15 pF and my GW-Instek GOS-6112 has an input capacitance of 25 pF.  Your scope will list it right by the probe connector.  Your probes capacitance range must include what your scope shows or you won't be able to properly adjust the probes for the scope and you won't see a proper waveform.

[georges shed]

i dont think its the probes. The waveforms on my other scope look fine and adjusting them does not solve the problem.