Author Topic: HP 1727A no display  (Read 667 times)

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Offline Alex_BakerTopic starter

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HP 1727A no display
« on: August 23, 2024, 08:40:17 pm »
Hello

I 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.
« Last Edit: August 25, 2024, 02:33:51 pm by Alex_Baker »
 

Offline Alex_BakerTopic starter

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Re: HP 1727A no display
« Reply #1 on: August 25, 2024, 05:59:12 pm »
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  ;D

 

Offline Alex_BakerTopic starter

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Re: HP 1727A no display
« Reply #2 on: January 07, 2025, 12:10:20 am »
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.
 

Offline Alex_BakerTopic starter

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Re: HP 1727A no display
« Reply #3 on: January 13, 2025, 05:32:15 pm »
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.  :-//
 

Offline m k

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Re: HP 1727A no display
« Reply #4 on: Yesterday at 07:09:03 am »
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.
Advance-Aneng-Appa-AVO-Beckman-Danbridge-Data Tech-Fluke-General Radio-H. W. Sullivan-Heathkit-HP-Kaise-Kyoritsu-Leeds & Northrup-Mastech-OR-X-REO-Simpson-Sinclair-Tektronix-Tokyo Rikosha-Topward-Triplett-Tritron-YFE
(plus lesser brands from the work shop of the world)
 

Offline Alex_BakerTopic starter

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Re: HP 1727A no display
« Reply #5 on: Yesterday at 03:50:47 pm »
Thanks for the reply M K

I 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.
 

Offline m k

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Re: HP 1727A no display
« Reply #6 on: Yesterday at 08:04:03 pm »
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.
Advance-Aneng-Appa-AVO-Beckman-Danbridge-Data Tech-Fluke-General Radio-H. W. Sullivan-Heathkit-HP-Kaise-Kyoritsu-Leeds & Northrup-Mastech-OR-X-REO-Simpson-Sinclair-Tektronix-Tokyo Rikosha-Topward-Triplett-Tritron-YFE
(plus lesser brands from the work shop of the world)
 


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