HP 1727A strange beam reflections - Page 1
Forum section: electronics / testgear

author: silver34
author_rank: Newbie
timestamp: April 04, 2014, 11:18:18 am
content: HelloI have this fantastic old oscilloscope, but when the beam retrace (return to origin), the beam dont turn off.Of course, I would like to repair this problem, I have try to find where is this function (turn off the beam when retrace) in the "service sheets" of the manual, but my analog electronic knowledges, are not sufficient (like my English).So, can you help me ?Here is the complete "operating and service manual" with, at the end, all the circuits :http://cp.literature.agilent.com/litweb/pdf/01727-90903.pdfThanks in advance for your help.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 04, 2014, 12:02:05 pm
content: I would check the board that is "Service Sheet 17 gate control Schmitt".If you can check what goes in and out of that board you will be near the problem.That said opening it up and running it may be very hard. You may have to solder something on that you can attach a probe to.Lol assuming you have another Oscilloscope.I did manage once to use a scope to diagnose itself, but you would need to be lucky, in this case you may be able to do it as it is only the blanking that is a problem.

author: silver34
author_rank: Newbie
timestamp: April 04, 2014, 12:58:39 pm
content: Quote from: HackedFridgeMagnet on April 04, 2014, 12:02:05 pmFirst, many thanks for your fast answer.Yes I have others scopesSure it would not be easy to test this "gate control Schmitt" circuit card, so some questions :What, "in signal" and what "out signal" control the retrace beam who is not off?What do you think I will get in and out ?

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2014, 03:59:56 pm
content: Does the beam intensity control work correctly?When displaying two traces in chop mode, does the chop blanking work?Service sheet 5 shows how all of the blanking signals are combined. I would take a close look at the Comp Gate signal coming from service sheet 17 and the output of the z-axis amplifier at test point 1.

author: oldway
author_rank: Super Contributor
timestamp: April 04, 2014, 05:34:07 pm
content: Check transistors Q1, Q2, Q3, Q4, Q5, Q6, Q17, Q13, Q14, Q15 on the gate amplifier A13, sheet 5Check also with another scope if you have blanking pulses on TP1 (output of the blanking amplifier) of A13 (also sheet 5)

author: silver34
author_rank: Newbie
timestamp: April 04, 2014, 06:28:39 pm
content: Quote from: David Hess on April 04, 2014, 03:59:56 pmThe finger god !I have some doubt.I have replaced the intensity potentiometer many years ago (and dont use my laboratory since many years... hard to "reconnect")The freak is when I increase the intensity.QuoteFinaly, perhaps, its not a blanking problem during retrace.I will try to post photos (bingo, its seam to work !)One with just the intensity necessary and the other with a little more (and with the problem).

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 04, 2014, 11:29:49 pm
content: Wow that is a bit weird.A second copy of the 3 traces but with smaller gains, less intensity ( or less triggering), also some non linearity in the vertical.Would it be a power supply issue? Only becomes apparent at high power.

author: c4757p
author_rank: Super Contributor
timestamp: April 04, 2014, 11:36:14 pm
content: It's a storage scope, so not that unusual, and I would suspect the storage control circuits. It almost looks as though it's simultaneously writing the storage mesh and displaying it.Storage circuit is on pp. 165 and 167 of the manual.Also, just check the storage controls.

author: silver34
author_rank: Newbie
timestamp: April 05, 2014, 12:51:56 am
content: Quote from: HackedFridgeMagnet on April 04, 2014, 11:29:49 pmSure, strange small tracesQuote"Only" 2 traces (+ "mirror")Would it be a power supply issue? Only becomes apparent at high power.If you think it can be a power supply problem, this Saturday I will use my ESR70 to control/test power supply capacity and so on.Now,In my country its late to go to sleep, I must goThanks all for your help.

author: David Hess
author_rank: Super Contributor
timestamp: April 05, 2014, 01:48:06 am
content: Quote from: silver34 on April 04, 2014, 06:28:39 pmI do not see the problem. The second photo shows what looks like a reflection from the camera lens.

author: Sailor
author_rank: Regular Contributor
timestamp: April 05, 2014, 02:37:16 am
content: Quote from: David Hess on April 05, 2014, 01:48:06 amYes, especially considering the curvature of the 'reflections'.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 05, 2014, 03:11:32 am
content: Quote from: Sailor on April 05, 2014, 02:37:16 ambut surely when he noticed the problem, he wasn't looking at photographs of the screen.

author: miguelvp
author_rank: Super Contributor
timestamp: April 05, 2014, 03:22:49 am
content: Looks like some mesh inside the tube is creating the ghost image by deflecting the electrons at a different angle than the rest and the main deflectors are amplifying that sub image.Weird, the effect seems like if the electrons are bouncing back into the tube and back into the screen, but that is impossible, right?

author: David Hess
author_rank: Super Contributor
timestamp: April 05, 2014, 07:31:01 am
content: Quote from: miguelvp on April 05, 2014, 03:22:49 amAt high intensities or when the beam is deflected off of the screen into other tube elements this can happen and storage CRTs have more elements so tend to suffer more from it but this usually takes and form of shadows.Secondary emission from the screen itself will produce halos around the traces.

author: silver34
author_rank: Newbie
timestamp: April 05, 2014, 08:42:42 am
content: Many thanks to all for your interest.No, I am sure its not a photo/camera, problem, my eyes see the same strange "reflection"Yes I thinks its not a beam/retrace problem.Its surely a reflection, but whyIf I use the memory function, this reflection is not stored, and... when I display the memory the "ghosts" traces dont display, no "reflections" (using button number 15 in the manual.) !?!?And, if I turn a little the Brightness (button 13 in the manual) the screen is "flooded" (illuminated)Edit :Add 2 photos showing this (other ?) problemAnd more photos showing oscilloscope adjustment (buttons position)[Sorry, my English is very bad]Nota : I have changed the tittle of the thread.

author: David Hess
author_rank: Super Contributor
timestamp: April 05, 2014, 03:27:28 pm
content: So it is not a camera reflection and internal CRT reflections are usually smeared.If that is caused by the retrace, then I would expect the horizontal scale but not the phase of the mirage to remain the same as the sweep speed is changed because the retrace occurs at roughly a constant speed. If there is a sweep length adjustment, then changing it should shift the phase of the mirage.If I suspected a blanking problem, then I would do the z-axis and storage calibrations with an eye to either fixing it or revealing more. Storage CRTs can be finicky in this regard.

author: silver34
author_rank: Newbie
timestamp: April 06, 2014, 06:27:27 pm
content: Sorry, very sorry for my absence.During this time, I am "talking" by M.P., in French (its easier for me) with oldway (well know in these forums) and his precious advices.To resume :Yes, you are right, but before to do anything, I must resolve the "potentiometer beam intensity" (too much/abnormal "sensitivity").Again, many thanks to all.