Author Topic: HP 1727A oscilloscope with multiple issues.  (Read 3919 times)

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Offline Alex_Baker

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HP 1727A oscilloscope with multiple issues.
« on: April 20, 2021, 09:01:38 pm »
Hello people of the EEVBlog forum, I am new here, this is my first post. I should start by saying that I am a beginner, or just above beginner level so bear with me here.

     The background here is that a few months ago I bought a used HP 1727A dual channel storage scope on ebay with unknown issues  :-BROKE , the seller had pictures of it powered on, but when I got it it the screen did not light up, long story short it arrived in the box with more issues than when it was put in the box. It turn out that the no screen issue was caused by power supply issues and a secondary issue on the "storage board" I fixed those problems, there were several shorted transistors, but a couple of major problems remain, both of which I cannot trouble shoot om my own, that where the forum comes in,  :)


      The first thing I would like to tackle is the channel inputs, channel A works kind of (although V/dv is way off), and channel B does not work at all. The puzzling thing is that while inserting the test signal from the scope into channel A, a trace shows up as normal but then there is a second smaller trace in the middle of the screen in a arch shape. I attached a picture of it, I don't know how to insert pictures in here yet.

      I have done some probing around with my USB scope and I have some ideas on what the issue could be but I don't know enough to make any conclusions. The other problem I think has to do with the CRT, or at least it shows symptoms of another problem, I will explain it more once I get some pictures of it but for now I want to focus on the channel issue.

      All you more experienced people out there feel free to point out any mistakes I make, I am here to learn and to hopefully fix my scope along the way. I am a freshman in college, and I hope to major in electrical engineering.

  Thanks,

 Alex Baker
 

Offline Wallace Gasiewicz

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #1 on: April 20, 2021, 11:04:44 pm »
Common problem when one channel is not working is that the attenuators or the input transistor on the dead channel, probably a dual FET, is blown by someone putting too much power into the scope. Happens all the time.
I would check that.
Don't know what the goofy trace is. Maybe the "Delayed" trace??
« Last Edit: April 20, 2021, 11:06:56 pm by Wallace Gasiewicz »
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #2 on: April 21, 2021, 12:17:57 pm »
Congrats, I think you got yourself a very nice scope! You can read a bit about it here: https://www.hpmemoryproject.org/wb_pages/wall_b_page_02.htm
Reading the service manual will also be a great learning exercise. Read it thoroughly! Those engineers knew quite well what they were doing back in the days.

Disclaimer: I don't own this scope, so I'm just giving some generic advice / guesses about what I would do.

The scope is an analog storage scope so you have potentially 20+ signals and voltages coming into the CRT which is a challenge by itself.
Also the usual stuff: This appliance has potentially lethal voltages in it. Before touching metal parts in it which are not connected to ground already, you should always discharge the capacitors and the CRT. Document yourself about how to safely do these things without damaging the instrument.


I think you should focus first on getting channel A to work properly end-to-end so that you have something working to compare to when fixing channel B.
The signal showing up multiple times could be as simple as an issue in the time-base or in the blanking circuit or could be as complicated as an issue with the storage mesh.
 I'd suggest to leave all the storage features turned off for the initial troubleshooting.

The vertical stage in this scope is also somewhat complicated because the CRT features a transmission-line deflection plate.

So in short: after ruling out all possible power supply issues I would look in this order at:
- Try triggering from an external source, follow trigger signals inside the scope and try to compare with expected signals in the service manual
- Look at the vertical output signal on the back on the instrument with another scope
- Look at the signals before the final vertical and horizontal final stages and try to follow the signal back to its source
- Look at the vertical delay line which drives the transmission-line vertical plate
- CRT blanking signals (Page 8-16 and 8-17 on the service manual)
- CRT storage meshes signals (even if turned off there may be faulty components / switches / whatnot which play a role in that area) after you ruled out other problems
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #3 on: April 21, 2021, 04:19:18 pm »
Update:  This morning I spent about 20min fiddling with stuff on the front panel and I discovered that channel B works fine, though it still has the extra funky trace. I just had to change the vertical display and trigger source buttons, i'm stupid.

Also I just realized that the user manual that came with my scope is missing section 4, which has a bunch of trigger information in it, oh well, I can get it from online. I need to look up how to use the external trigger since I have not used a scope like this before.

I got some pictures of the odd CRT behavior I am talking about, If you look at the pics, you can see a dark region in the upper right hand corner. It slowly grows from the corner until I assume it would cover the whole screen, although I have never had it on long enough to do that. Additionally you can see that the dark area covers up the good trace, only leaving the weird trace behind. I will focus on the second trace issue first but I figured I would post some pictures of the screen problem when I had them.
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #4 on: April 21, 2021, 06:15:02 pm »
Good news with channel B!

What you captured in picture is most probably related to the storage mesh which is probably not driven correctly.
I would solve the problem with the phantom trace first before looking at the storage functionality. Just make sure storage is disabled for now. I think you have to press the button at bottom left corner of the screen so that "display" led is lit.

I also read the following from the manual: interesting


 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #5 on: April 21, 2021, 06:18:02 pm »
Could the phantom trace be also be a malfunction of the "beam find" functionality? What do you see when you press the "beam find" button ?
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #6 on: April 22, 2021, 02:01:39 pm »
When I press the beam find button it just scrunches everything down and in towards the center of the screen.

I agree with your idea that there is an issue with storage mesh. When I was repairing the power supply I traced a short on the +120V rail to the storage mesh output amp on board A17, where I replaced three transisors that tested as resistors on one of those cheap component testers. The ones I replaced were Q4, Q5, and Q6.

Through some more messing around, either I dint know how to use the storage function or the storage mesh is not working, both cases are quite likely.  ;D
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #7 on: April 22, 2021, 07:44:01 pm »
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When I press the beam find button it just scrunches everything down and in towards the center of the screen.

Ok this is expected and fine.

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I agree with your idea that there is an issue with storage mesh

Ok bear in mind that in general CRT are very hard to break. Provided one doesn't burn the filament or strips the cathode by applying excessive cathode current it's hard to cause a fault on the CRT (but not impossible).
I would check (when you get to fix storage) in case you didn't already C15, R52, CR7, CR9 and R57.

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Through some more messing around, either I dint know how to use the storage function or the storage mesh is not working, both cases are quite likely.

I think you have quite a lot to read and I as well... I will try fixing soon something similar: a HP 141T mainframe though way older than yours.
Anyway, does the phantom track go away when storage is turned off ? If it doesn't, I would try to look at the blanking circuit straight away.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #8 on: April 23, 2021, 02:23:54 pm »
I don't know what you mean by turning the storage mesh off, as far as I can tell, it's never off. If you mean "off" as in the normal operating mode of the scope, where it is not storing or displaying a stored trace, then my answer is that the phantom trace is always there, nothing I do can make it go away.

I also should note that yesterday when I was messing around with a function generator, (until now I was just using the CAL signal) at some higher frequencies the trace would ghost a little. Could this be related to the blanking circuit as well?

By "issue with the storage mesh" I meant the circuit that drives it, however with some of the erratic behavior the CRT has, physical damage is not out of the question in my opinion, but I will proceed as if there is not.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #9 on: April 24, 2021, 08:40:08 pm »
is it possible to learn anything by directly looking at the signals on the horizontal and vertical plates? Like if the second trace was being produced by the vertical circuitry or if there is a problem with the blanking circuitry or something?
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #10 on: April 25, 2021, 07:59:18 am »
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I don't know what you mean by turning the storage mesh off

OK as far as I know, the scope can work in 2 modes: storage on/off. When storage is on, even if the scope doesn't seem to store, it's driving the storage mesh and erasing it quickly and it's ready for you to freeze the trace.
When it's working with storage "off" then the storage mesh is not actively driven and behaves as if it's "transparent" to the electron beam.
This is the mode I would concentrate on at first. That said, maybe I'm also oversimplifying and the reality is more complicated. I'll have to read the details.

Quote
Like if the second trace was being produced by the vertical circuitry or if there is a problem with the blanking circuitry or something?

Yeah I'm thinking in the direction that you may have "re-traces" meaning that the blanking circuit doesn't work properly, generating the ghost trace during the re-trace cycle of the horizontal.
The horizontal should look like a tooth saw with some gaps between the teeth depending on the current horizontal settings. If the signal is not blanked correctly during re-traces, then you get artefacts on the screen.

There may also be issues with the horizontal stage itself, that's why you have to confirm triggering and horizontal work fine. Blanking circuit is usually slightly more complicated because drives some high voltages involving galvanic isolation.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #11 on: April 25, 2021, 03:20:05 pm »
Looking through the section of the manual I am missing in my hard copy I found the section called "service", It details the theory of operation for all the boards and functions. In there it shows some expected waveforms, yesterday I compared a bunch of the examples to mine. The horizontal output looks exactly as it should, and most of the others I compared on board A8 do as well.

If I remember right the CRT "blanks" during retrace by turning the electron gun off(or something to prevent a picture), correct?  I don't know if there is a specific section in the schematic for blanking or if its part of another circuit.

I just looked at your post saying to look at the blanking signals on page 8-16, I will do that now.
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #12 on: April 25, 2021, 04:22:10 pm »
Update: I quickly checked the three waveforms shown on page 8-16 with my USB scope and the one called delayed gate is missing. I also found a waveform called "chop blanking" on board A3 page 8-26 that goes to board A13, this signal was also missing.

I haven't had a chance to look more into this yet but hopefully I will later today.
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #13 on: April 25, 2021, 08:09:46 pm »
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Looking through the section of the manual I am missing in my hard copy I found the section called "service", It details the theory of operation for all the boards and functions
Yeah, you definitely need that  ;D

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I quickly checked the three waveforms shown on page 8-16 with my USB scope and the one called delayed gate is missing. I also found a waveform called "chop blanking" on board A3 page 8-26 that goes to board A13, this signal was also missing.

Delayed gate: I think this is a signal which is present only if you set "Detla-T STOP" at something different than 0. It allows to wait a certain time after a a trigger event before triggering again. Useful for slightly more complex signals where the trigger may go nuts. If you want to see that signal then set the scope as described in the "Waveform measurement conditions service sheet 5". If the signal is not there, then it may be some problem but maybe not the one which generates the re-traces.

The "chop blanking" I think is visible only when you select channels A and B simultaneously. The scope quickly alternates  (in some scopes is called "alt" or "chop") between the two channels at a certain frequency giving the impression that the two channels are visible at the same time even though the scope has only one electron gun in the CRT.  If you want to see that signal you have to set the scope as described on page 8-29 (EDIT: I mean page 8-26) on paragraph 5 of "waveform measurement conditions"
« Last Edit: April 25, 2021, 08:22:01 pm by cosenmarco »
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #14 on: April 25, 2021, 08:17:39 pm »
Ok, that makes sense, I always forget to look at the measurement conditions.

I'm still not finding a blanking circuit though.
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #15 on: April 25, 2021, 08:24:37 pm »
Quote
I'm still not finding a blanking circuit though.

Oh... sorry in these scopes it's called "gate" so it's the circuit on section 5
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #16 on: April 26, 2021, 01:32:42 pm »
ohh ok, yeah that makes sense, thanks.


I should mention that this week is finals week in college, so if I am not very active on here for at least the first half of this week you know why.
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #17 on: April 26, 2021, 06:16:52 pm »
No problem at all. Focus on the finals and you'll work on the scope with less accumulated stress (which is in general bad when operating with high voltage equipment).
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #18 on: April 28, 2021, 06:11:55 pm »
Whew, finished my finals this morning, now I have about two weeks before summer classes start so I should have all the time I want to work on this, It takes up soo much space on my little workbench!

I took a brief look at A13, the gate amplifier the other day, and everything lines up with what is in the service manual as far as waveforms go. I am not sure where to go from here unfortunately. I think I saw a block diagram somewhere in the manual, If I can find it I will take a better look at it and see if it gives me a direction to go.

The extra trace is an exact copy of the normal trace, but half the size, does that give any clues as to the cause?
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #19 on: April 28, 2021, 06:18:53 pm »
Update, I was fiddling with a couple of adjustment pots and I found that the HV colimator pot R42 on A16 LVPS does not affect the brightness of the extra trace, whereas the normal one is affected. Looking at the diagram of the CRT tube itself, this would suggest that whatever is producing the extra trace is past the HV collimator? is that possible. if it is true then that would point to an issue in either the lenses or the storage mesh, but please correct me If I am misunderstanding how these tubes work. :)
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #20 on: April 28, 2021, 08:20:40 pm »
Ok I read page 8-52 and it seems to me this CRT is even weirder than the usual storage CRTs  ;D
It seems to me you have to check all storage elements even though you're not using storage. If I get it right, the writing gun never writes directly onto the phosphor layer but rather writes on the storage mesh and then the flood guns will transfer the image onto the phosphor...

This means all storage elements must work properly in order to have correct image on the screen, so forget what I said about the "display" mode not making use of the storage elements...I should have done my homework better  |O

You have on page 8-53 a recap of the various signals / voltages you should have on the various elements of the CRT. I suppose you may want to check those for the various modes and make sure everything is ok (especially everything related to lenses, collimators and meshes).
I would also try to go through the "adjustments" section and see whether you can go through the adjustment procedure successfully for things related to your issue (everything storage).
 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #21 on: April 28, 2021, 09:05:30 pm »
I agree that now would probably be a good time for me to go through the adjustments section, especially after fiddling with a bunch of stuff for a while. The adjustment procedure is very long and takes a considerable amount of time, but what may be good for me to do is to slowly follow the procedure and take notes on what doesn't work or seems off. I am missing some of the recommended equipment for the adjustments procedure, the one I most likely can't substitute is the High voltage 1000:1 multimeter probe, so the CRT anode voltage will remain unknown until I either get one or find a different solution.

I an mot sure if you have ever looked at other people repairing the 17xx series scopes or not but I have seen at least two other instances of bridge rectifier failure on the LVPS, causing intermittent of weird faults, and I think my scope is no different. At one point when I was working on the LVPS it failed such that the unregulated 15V rail was a lot higher than it was supposed to be, which in turn affected the regulated side of the +15V supply, I cant remember hoe much it was affected but my fear is that if the rail was high enough it could have damaged several other circuits unrelated to the original issue that the scope had, which at this point I don't know what it was since the thing didn't work when I got it.

Long story short I would really like to replace the bridge rectifiers, but I don't know where to get any that would just drop in, the best I could do is somehow retrofit others. Anyways I'm rambling now,  :blah: I will start the adjustments procedure this evening.

About the scope CRT being weird, it seems like most of the devices I work on have something weird about them that makes fixing them harder, especially vacuum tube stuff, that's just my luck!
« Last Edit: April 28, 2021, 09:09:16 pm by Alex_Baker »
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #22 on: April 29, 2021, 12:14:14 pm »
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the CRT anode voltage will remain unknown until I either get one or find a different solution
You see stuff on the screen so don't worry about the high voltage: it's fine.

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I have seen at least two other instances of bridge rectifier failure on the LVPS, causing intermittent of weird faults
Yeah, that's why you always start from the power supply. You check the voltages, ripple, you check the capacitors and diodes one by one. Ideally for the caps you check ESR for all of them and leakage for the caps starting from 50V using an isolation tester or some sort of capacitor tester.

In general for regulated and unregulated voltages you want to measure DC value and AC ripple. Ideally you put the scope on it (using appropriate probing technique) and check the noise figure. Experience in knowing the design of power supplies usually will guide you for what's acceptable in your application and what's not if the manual doesn't provide information.

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Long story short I would really like to replace the bridge rectifiers
if you have a suitable replacement this shouldn't cause any harm but
a) if you just replace because others in other situations had issues with that I would suggest not to touch it, just test it in operation. Desoldering stuff is messy and you get a lot of chance to damage the PCB (...how many pads I lifted... :'( )
b) if you decide for replacing, check the performance of the supply before and after the replacement to see if the replacement brought you something

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that's just my luck!
I always do my research before acquiring some equipment on the chances I may have to fix it... of course then I ignore the findings and aquire irrepairable stuff anyway  :-DD

 

Offline Alex_Baker

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #23 on: April 29, 2021, 02:21:29 pm »
Thanks for the info, I did decide to go ahead and replace all the large filter caps on the LVPS board because one had white fluffy stuff oozing out of the end, so I figured it was just best to replace all of them if one was bad.

You may not hear much from me today because I got my second covid vaccine dose yesterday, and this morning I feel like I was ran over by a truck.  ;D
 

Offline cosenmarco

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Re: HP 1727A oscilloscope with multiple issues.
« Reply #24 on: April 29, 2021, 05:34:01 pm »
Caps are ok to replace, especially in 40+ years old equipment and especially if they show sign of deterioration.

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I got my second covid vaccine dose yesterday, and this morning I feel like I was ran over by a truck
Oh boy... get well soon!
 


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