HP1740A Cal 1V test point using x10 hp probes - Page 1
Forum section: electronics / repair

author: Applemac
author_rank: Regular Contributor
timestamp: January 10, 2017, 05:56:14 pm
content: Hi,Was wondering if anyone could help me. Ive bought a HP1740A oscilloscope using HP 10006D X10 probes attached to Channel A and the 1V cal test point, with Volts DiV at 0.02 and in cal position and the Y axis Time/Div at .1 I'm measuring Y as 1.4 volts instead of 1 volt. Also the X axis is 5.2 divisions.thanks for any advice

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 10, 2017, 06:26:00 pm
content: Check the input selector is not at 50ohms, then try with a simple cable from the BNC connector to the cal test point just in case the probe is wrong. If you can't measure 1V here, then somethings wrong with that scope.Anyway, post a photo of the scope with the signal on.

author: Applemac
author_rank: Regular Contributor
timestamp: January 10, 2017, 07:29:36 pm
content: Hi Mastertech,Attached a picture and I made a mistake about the X axis its two divisions I did not have the sweep verner in the middle position, as show in IMG 0491 attached picture, with the sweep verner in the CAL position its 3 divisions as shown in attached pic IMG 0493.thks.

author: tautech
author_rank: Super Contributor
timestamp: January 10, 2017, 08:02:50 pm
content: On older analog CRO's the Cal output is often not precise for either frequency or amplitude. Some have adjustments to correct this, some don't. What really matters is probe compensation to adjust out undershoot and overshoot at the top of the rising edge.For now you can check a few things against a DMM, frequency and amplitude to see where the apparent error is.Even a DC source like a battery measured on the display vs DMM measurement should indicate where any issue might be.Do you have the Vertical fine set to Cal ?

author: tggzzz
author_rank: Super Contributor
timestamp: January 11, 2017, 03:02:03 pm
content: It is possible that the calibrator's amplitude needs adjusting. See the service manual service manual section 5-47 and potentiometer A3R116.But don't go blindly twiddling things inside the scope; that is likely to cause more problems than it cures. Slowly and steadily follow the information in the operations and service manuals.

author: Applemac
author_rank: Regular Contributor
timestamp: January 12, 2017, 08:21:56 am
content: Hi,Thanks to all, I will try the test from Tautech first to see if actually the volts are correct using a dmm to test a battery and then check that battery against the reading from the oscilloscope, if thats fails will look at the manual regarding the calibration amplitude adjustment of A3R116 as mentioned by tggzzz. Will report my finding on Friday as I'm a little busy today.thanks.

author: Applemac
author_rank: Regular Contributor
timestamp: January 20, 2017, 01:09:07 pm
content: Hi,OK check the measurement against my DMM and the voltage read by the oscilloscope is correct.Will run the calibration amplitude adjustment of A3R116 next as mentioned by tggzzz.Thanks all.

author: Applemac
author_rank: Regular Contributor
timestamp: February 05, 2017, 02:06:37 pm
content: Hi ,Checked the voltage using a DMM on the cal point it was 0.448 volts using A3R116 adjusted it to o.5 volts as shown in the manual. Seems to have solved my problem.Thank everyone for the great help.

author: Applemac
author_rank: Regular Contributor
timestamp: February 06, 2017, 01:01:41 pm
content: Hi all,Need some help again, just wanted to ask a question, whilst making the adjustment I accidentally adjusted A3C2 a .5v comp. I turned the pot a lot whilst my dmm was attached to the cal 1v which at the time was reading 0.448 volts and whilst moving the pot nothing changed on the dmm, then I realised that I should of been adjusting pot A3R116 to get the 0.5 volts which a I did. Now my question is what effect would of adjusting A3C2 have made, im worried as having checked the manual this is for attenuator compensation and to calibrate this pot you need an 10khz sine generator which i do not have.Any advice pls thks.

author: tggzzz
author_rank: Super Contributor
timestamp: February 06, 2017, 04:15:28 pm
content: Top tip: before moving any internal control, mark its original position so that you can always get back to where you started. Marking is difficult on 10-turn trimpots and some trimcaps.

author: Applemac
author_rank: Regular Contributor
timestamp: February 06, 2017, 08:08:17 pm
content: Yeah thats a great tip wish id thought of that before, but any thoughts on what I can do now?

author: tggzzz
author_rank: Super Contributor
timestamp: February 06, 2017, 10:21:09 pm
content: Service manual section 5-60 paragraph d, on page 5-14.

author: Applemac
author_rank: Regular Contributor
timestamp: February 07, 2017, 07:19:15 am
content: Tggzzz, yeah ive seen that but I dont have an lcr meter or a square wave sound generator...

author: tggzzz
author_rank: Super Contributor
timestamp: February 07, 2017, 08:35:44 am
content: Firstly, to avoid wasting our time, please stateallof your problem/constraints.You'll have to build one; they aren't difficult. Note that a "sound generator" is unlikely to be adequate, since you need sharp edges significantly less than 3ns transition time.Use a 74lvc1g14 as an oscillator, driving another as the output. Make sure they are well decoupled.

author: Applemac
author_rank: Regular Contributor
timestamp: February 07, 2017, 10:29:58 am
content: Tggzz,Firstly I did list all my problems, if you look 6 posting above this one you will see that I mentioned that I didnt have a sine wave generator or an lcr, which would lead a normally intelligent person to understand therefore id obtained that information from the service manual.Secondly I only mentioned about this issue 6 postings ago because it occurred when I made a mistake making the adjustment. Therefore was a follow on issue to the original.Thirdly I appologise if you feel ive wasted your time, but its responses like yours that put people off of asking for advice, so please in future read peoples posts carefully before posting answers which could lead them to be put off from seeking it in the first place.And last but not least thank you for your help.

author: tggzzz
author_rank: Super Contributor
timestamp: February 07, 2017, 11:09:15 am
content: Quote from: Applemac on February 07, 2017, 10:29:58 amYou and your problems aren't the only things on other people's minds. It is optimistic to expect everybody to remember everything you have said.You would be surprised at what can euphemistically be called "surprises" lurk behind questions asked on the forums. It is very unwise for anyone to assumeanythingabout a new poster. You would be surprised at the number of problems I've "cured" by asking "is it turned on?".QuoteJust trying to help you help yourself, and encourage you to write questions that get responses from more people.

author: Applemac
author_rank: Regular Contributor
timestamp: February 07, 2017, 01:38:04 pm
content: Tggzzz,Point taken, Now I need to look for a reasonable price sine wave generator and fix the problem. Thanks very much for your help.