Electronics > Repair

HP 1727A oscilloscope with multiple issues.

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Alex_Baker:
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

Wallace Gasiewicz:
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??

cosenmarco:
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

Alex_Baker:
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.

cosenmarco:
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


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