HP 1727A no display - Page 1
Forum section: electronics / repair

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: August 23, 2024, 08:40:17 pm
content: HelloI have an HP 1727A oscilloscope that needs repair. There actually was a very long thread on this unit two years ago, but I would like to start fresh to limit confusion.The symptom I have is that there is no trace on the display, I have spend some time troubleshooting and I seem to have a problem with A13, the gate amplifier. I checked the inputs to this board based on the provided waveforms and they were good, but I have no gate output. I tested a fair number of points but the big thing that I found is that the op-amp U1 is railed, its output is high all the time. I tried adjusting both front panel controls and internal pots with no luck. The two inputs to the amp are always 0.6V apart.From what I understand U1 is an auto intensity controller, getting feedback from a mesh in the crt, so that if beam current gets too high it will automatically reduce the gate pulse that controls the main beam. Since U1's output is always maximum the gate signal is decreased (or gone completely) as it should be. So essentially something is making U1 think that beam current is too high, and I dont know what it is.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: August 25, 2024, 05:59:12 pm
content: The annoying thing is that I had a display when I put this piece of crap away a couple of years ago, it still had issues then, but no display was not one of them.Also bumping this topic

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: January 07, 2025, 12:10:20 am
content: I am picking this up after a few months.Up to this point I was assuming that there was a problem with U1 or its associated components, However following the troubleshooting section I lifted R50, which should be able to bring back the trace if U1 was a problem. Lifting this resistor did not have any effect, which makes me think that there is another problem somewhere else.Both the "Dlyd Gate" and "Comp Gate" signals are ok.I have attached the Gate amplifier section from the manual.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: January 13, 2025, 05:32:15 pm
content: I went around the board with a multimeter measuring voltage potentials on the pins of all the transistors, making the assumption that Si transistors tend to have about 0.6v base to emitter. With this method I found that Q14 was bad, it had become a resistor.For the time being I replaced it with a generic npn I had in stock. This fixed the gate amplifier(I think) but we still have problems. I began by checking CRT voltages but I cant get very far into the adjustment procedure before I run into problems.I did find that when holding the beam find button under certain circumstances I can get a very faint cloud to appear and move around on the screen to some degree, however I cannot move it very much or bring it into focus at all. Perhaps somebody that knows far more about CRTs would know why this may be.

author: m k
author_rank: Super Contributor
timestamp: January 19, 2025, 07:09:03 am
content: So deflection is operational, but focus is far away.Is the scanned manual available somewhere?U1 power should be between 53V and 30.1V and its +input no more than 18.2V below 53V.18.2V from R22 would be far inside R23.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: January 19, 2025, 03:50:47 pm
content: Thanks for the reply M KI actually have an original print of an operating and service manual. I also have the scanned one from Agilent since my original is missing some stuff.U1 power is good, the zener diodes are new, and both its inputs are measuring 19.66V below the +53V rail. Since the two inputs are the same I would believe that U1 is working, but it could not be adjusted correctly.

author: m k
author_rank: Super Contributor
timestamp: January 19, 2025, 08:04:03 pm
content: You should upload the scan somewhere if you can, it seems to be missing.U1 is an integrator, means that input change is slowed to output, you should be able to check that.Around 0.6V apart is what those input side diodes allow, I guess less than that is what is expected.Maybe the current limit trimmer is not what it should be or ACCEL. input is incorrect.What kind of thing is guard ring?It seems to be connected to +input.But since lifting R50 didn't do anything maybe you should concentrate to focus.Manual should have a section for CRT voltage checks, older system has focus balanced between cathode and auto focus.Some high value resistors are usually also present and they are not best for keeping their values.Other thing is that despite the focus you should have a sort of normal intensity.Too much apart cathode and grid voltages can affect that, very far away focus also.Have you checked how beam intensity voltage adjusts?Q12 is pulling auto focus towards -15V.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: January 22, 2025, 01:25:32 am
content: I got the scan directly from Agilent, it is too big to attach here. I have attached a scan of the HVPS, Agilent's in nearly illegible.The beam current limit circuit seems to be working, current through R5 changes depending on front panel intensity setting.The guard ring is a trace that encloses that section of the circuit and has no solder mask on it. I believe this is to intercept leakage current that may upset the integrator, for this reason I have taken care to remove any solder flux from rework that has been done.I checked CRT voltages against the manual and tested some resistors on the HV board, although I don't honestly have an accurate way to measure 15+ megohm resistors.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: January 22, 2025, 01:30:44 am
content: Currently the service manual is available from Keysight, not Agilent, my fault!I think Keysight acquired Agilent, who was the test equipment division split off from HP.

author: m k
author_rank: Super Contributor
timestamp: January 22, 2025, 08:25:14 am
content: Cathode voltage is marked as -2700V, can you measure it?Checking those zeners wouldn't be bad either, intensity limit also.Old model has cathode as -2950V and its intensity limit down, new limit is up, both are a bit over 100V.Are you sure you have a correct tube?Disconnecting H and V deflections should bring a dot to the screen, then checking focus should be easier.X-Y scope should be fine with spot on screen, but it's not an optimal situation.T1 pins 9 and 10 has a heater voltage, its value should be mentioned somewhere.Inside the tube are also some "after market" parts, everything can be messing the ray.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: January 23, 2025, 12:03:15 am
content: I have an old Fluke 6kV probe that I bought for working on this, I set the cathode to -2700VThere is no doubt that I have the correct tube, I can't imagine that there were many other tube that would fit in here anyways.I can check heater voltage.The problem that I have to think about whenever I work on this scope is the fact that my old manual does not match the scan, and sometimes my scope does not match either documents. The serial number on my unit is 1947A 00075, which makes it the 75th one produced out of some, likely small, production total.

author: m k
author_rank: Super Contributor
timestamp: January 23, 2025, 08:00:54 am
content: If you've seen the machine being fine then fine, otherwise question everything.1725A and -2950V tube is 5083-5552 CRT, P31, anode seems to be 17.5kV, but picture is not very good.Elsewhere is approx. 20.5kV for anode, so 17.5 positive + 2.95 negative.Everywhere is also that 15% variation is fine, I guess that's for something else.E,wrong voltage.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 23, 2025, 04:22:55 pm
content: Hi AlexJust out of curiosity I read your posting on this forum and elsewhere. However I must say at this point that I have no intention of providing you with help via this forum due to the number of irrelevant and rubbish postings that are made. It is bad enough trying to provide remote help without the constant rubbish and nonsense posting that are made.To possibly clarify the situation regarding the serial number. The first four digits of the serial number give the date that the design or revision was released to manufacturing not the date it was manufactured on. I would suspect that HP had problems with the manufacture of the CRT hence the delay in the scope being sold, CRT’s can have horrendous problems in manufacture.As to a service manual, the copy on the Keysight web site is of a very poor quality and is almost unusable as are all of the copies on the internet. If you look at the 1982 HP catalogue (available on line) you will see an almost identical model to the 1727A, the 1743A, it appears the difference may just be the options fitted as standard. Artek Manuals have a high quality scan of the 1743A service manual available.I would not be too concerned about the differences between the scope that you have and the service manual, rarely is an oscilloscope exactly as in the service manual.The fact that the beam finder intermittently produces some form of trace is good news, at least most of the oscilloscope is basically working. What you need to do now is to check the oscilloscope section by section starting off with the low voltage power supplies and the voltages around the CRT only two of which are normally critical, the heater voltage and the pinch off voltage.PLEASE take no notice of the multiple postings by one individual regarding CRT’s, there is NO understanding of how CRT’s work in them.From what information you have provided I can see no reason that the oscilloscope cannot be repaired, my only caveat would be possible mechanical damage to the CRT internally, but this is unlikely and easily checked for..G Edmonds

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: January 23, 2025, 06:10:31 pm
content: Thanks for the reply Mr. EdmondsI would not be surprised if the 1727 CRT was difficult to manufacture, compared to a non-storage tube it has far more internal parts, soe of which can be damaged by tube itself if improperly adjusted. I am hopeful that there is no internal physical damage to the crt, since it would be nearly impossible to identify.Great tip on Artek manuals! I noticed that they do have a copy of the 1727a manual, would this be a better quality scan then the other online ones? They also have the 1722, which is perhaps a closer match to the 1727 since it has the same 275MHz bandwidth.When I get a chance I will try to go step by step checking critical components on the scope, I will report what I find.Alex

author: m k
author_rank: Super Contributor
timestamp: January 23, 2025, 08:42:48 pm
content: I missed the 27A version HV, it's only 9.5kV andnegative, withX3 multiplier.What is its part number?22A has 5083-4070, 17.5kV anode and X6 multiplier, just like 25A.43A is not clear, maybe ending 3558, se even older.Maybe 27A manual has how to strip CRT functions to minimum for basic testing.But for the type you're right, there are most likely only correct tubes available.E,nope, not that exotic.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 23, 2025, 09:07:37 pm
content: Hi AlexArtek Manuals scans are usually of a very high quality. I did look for HP1727A as my first go to search but did not find it, possibly my very poor rural internet connection was causing the search to fail.The Artek part number is the correct one for your serial number.If you do buy the Artek copy and we correspond by direct email I will help you. I have been designing and repairing CRT oscilloscopes for some sixty years now and know the short cuts.George Edmonds