Author Topic: Very nice HP1745A with little problems.  (Read 18507 times)

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Offline TGyuri

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Very nice HP1745A with little problems.
« on: October 07, 2014, 10:39:22 am »
Hey Dear All,

I'm new on this forum.
My name is Gyuri.
My english is crappy, please forgive it.
Just as my knowledge in electronics.
But that does not stopping me to build amplifiers,  DACs etc.
Indeed I have repaired four TEK scopes.(2445, 2465, 2467B and 2440.)
Of course I have got some help from very nice people at TekScopes.
I have had four scopes, two of them from above, 2467B and 2440, and 2225 and a Velleman PCS500.
I have acquired a beautiful HP1745A now.
It is in very good cosmetical state inside and outside also.
Wonderful, that screen filter is glass, not some crappy plastic.
But I have some problems:
First, scale illumination, it is works only after a hour or so, after switching on.
It is uniform and stable otherwise.
Second, although waveforms are quite O.K., at all frequencies with stable trigger,
there is a strange problem.
I have built a Jim Williams pulse generator for fast rise signal, HP can not be able displayed of it's waveform.
If I press Beam Find button, the signal appears at the begining or ending of trace.
I don't know, where I can start.
I have checked LVPS voltages, seems O.K. to me.

Thanks for any help in advance!

Kind regards,

Gyuri



 

Offline tggzzz

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Re: Very nice HP1745A with little problems.
« Reply #1 on: October 07, 2014, 02:18:33 pm »
Can you see the pulse on your other scopes? What's the amplitude and duration? The HP scope is "only" 100MHz.

If you drive the HP with a simple digital signal, what is the observed risetime? The scope should have a 3.5ns risetime, but that is often limited by an inappropriate probe.

Do you have the BW limit on?

Do you have the 50ohm termination in/out?
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #2 on: October 07, 2014, 06:54:04 pm »
Thank you for dealing with me!
I'm trying to answer your questions:
1.,
Yes of course! See picture, it was taken from 2467B.
It has a frequency about 130 kHz, amplitude about 7V into 50 ohm and 8.5V into 1 Mohm.
Even 2225 which is only 50 mHz scope, possible to view this signal, only amplitude and rising time degraded.
2.,
I have a good old Bradley 192 oscilloscope calibrator.
It has a fast rising output 200-250 mV.
According to 2467B it has a rising time 1.25 ns.
HP1745A can show this signal, I've measured rising time about 3 ns it's quite good, isn't it?
But there are some strange thing, which I discovered now.
It shows falling edge, and if I pressed down trigger slope, then it shows leading edge.
Moreover, if I playing with trigger level knob in the range, then I get twice stable trigger. One at position 9, other one at position 15.
I think there is a problem.
I have read something about it in the SM, chapter 5-21 TRIGGER SENSITIVITY ADJUSTMENT. If I understand it correctly.
3.,
Of course not.
4.,
It only changes amplitude slightly, and I get a better waveform in 50 ohm, because impedance matching.
Generators has 50 ohm output impedance.

Kind regards,

Gyuri
« Last Edit: October 07, 2014, 06:58:16 pm by TGyuri »
 

Offline c4757p

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Re: Very nice HP1745A with little problems.
« Reply #3 on: October 07, 2014, 07:13:16 pm »
I probably wouldn't worry about it. Based on my experience with the scopes I've worked on, I generally consider the ability to trigger on an avalanche pulser nothing more than a nice bonus for anything under 150 MHz. It's not a nice waveform at all...
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Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #4 on: October 07, 2014, 07:56:26 pm »
I know it's not so nice, probably I need to do some homework on my pulse generator.
So Tek 2225 a very kind scope, because it displays stable trigger with this!
Despite of it is a 50 MHz scope.
There is an other picture, it shows pulse generator output, while I pushing Beam Find button permanently.
It is quite stable, although not so nice. :)
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #5 on: October 08, 2014, 04:46:05 pm »
There are some photographs about oscilloscope:

https://onedrive.live.com/?cid=E894CB7D50B65FBF&id=E894CB7D50B65FBF%21857

Gyuri
 

Offline MarkPalmer

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Re: Very nice HP1745A with little problems.
« Reply #6 on: October 08, 2014, 05:27:26 pm »
I have one of the 1741A storage scopes as a daily driver at my service bench.  The 1745/46A’s came out in 1983 and were right at the end of the 10 year run of the HP 1700 series scopes.   With this series in the late ‘70’s HP finally had scopes that could match Tektronix head to head in performance and reliability, and they eventually passed Tek in scope sales once digital scopes started becoming popular.   Despite the Tek 2225 being a few years newer than it, the 1745A when working right should be able to trigger on any signal a Tek 2225 can trigger on, so I suspect there is an issue with your 1745A somewhere. 
 
-Mark-
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #7 on: October 08, 2014, 07:31:43 pm »
Yes, I think it so! :)

Gyuri
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #8 on: October 08, 2014, 11:29:27 pm »
If your Tektronix 2225 can display the pulse, then your HP 1745A should not have any problem.

Maybe the HP 1745A is not unblanking quickly enough or the CRT is too dim.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #9 on: October 09, 2014, 06:38:08 pm »
Well, I have solved first problem.
I wrote it, LVPS voltages are O.K. but there was an odd-one-out:
That one, which is built around CR3-C7-Q11-Q12....
As there was no output voltage, when scale illumn. doesn't work.
Turned out, that connector between tranny and six diode bridges, has some ugly cold soldering.
It is clear that there was some repairing earlier.
So I removed LVPS board, and changed all those bridges pre-emptively.
Also I removed that connector, cleaned and soldered back its pins
And voila, scale illumination works, as new. :)
I will be unable to solve the second problem so simply.


Well, I killed one of those little blue bugs, I have pulled out one of its legs.
Doesn't matter, I put back a red one.



« Last Edit: October 09, 2014, 06:45:18 pm by TGyuri »
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #10 on: October 10, 2014, 06:21:13 am »
I think, needed a little bit more help, what direction I going to with second problem. :)
I think there is only some settings need to do.
But I don't want to turn the trimmer potmeters and caps at random.
I suspect it is not a trigger problem, because on the picture above, where beamfind button was pushed, there is a stable signal.
Also trigger led shows, signal is triggered.
Since I do not understand it, just guessing.

Gyuri
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #11 on: October 10, 2014, 07:09:10 am »
To me it looks like the CRT is dim which will create problems displaying fast low repetition rate signals.  The vertical and horizontal deflection looks good so the acceleration voltage at the cathode is accurate but maybe the post deflection acceleration voltage which is like 19 kilovolts is missing.  Can you disconnect the CRT anode or the voltage multiplier and see if that makes any difference?

You really need to find a service manual.  I think the HP1746A service manual covers the most or all of the HP1745A.  Artek Manuals has that one:

http://artekmanuals.com/manuals/hp-manuals/
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #12 on: October 10, 2014, 08:38:46 am »
Hey David,
Thank you for your answer and questions as well!
Do you think it's too dim?
https://hbpeza-ch3301.files.1drv.com/y2pIuG0UF3x5abjKciibiCYYNU9UBEXnooDXE6wl9xT8MJ8TkXXdeaT1zcyk-ylsBn2W4bF5m42g69ikxbKpZab90_n9_tWWxABiQL_TZsAX80/Img00460001_1.JPG?psid=1
Yes, if we see at the beginning of the signals, certainly is.
Photograph is cheating some extent.
What can you see, it is two 10 MHz signal from my TOELLNER TOE7711 generator.
(Is there anybody who has service manual for this?)
A channel is main output triangle, B channel its ECL output.
I have the manual from Keysight:
http://literature.cdn.keysight.com/litweb/pdf/01745-90901.pdf

There are more pictures:
https://onedrive.live.com/?cid=E894CB7D50B65FBF&id=E894CB7D50B65FBF%21857
« Last Edit: October 10, 2014, 10:11:32 am by TGyuri »
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #13 on: October 10, 2014, 09:39:41 am »
At 100 MHz things are worsened.
See attachments.
Trigger is stable, but the beginning of signal dimmed further.
First pic at 50 ns, without 10x mag., second with mag., somewhere in the middle, third one is at the beginning of signal.
So, Dear David, if you can show me the way, according to the service book, it would be good.

Many Thanks,

Gyuri
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #14 on: October 10, 2014, 01:36:57 pm »
...Can you disconnect the CRT anode or the voltage multiplier and see if that makes any difference?

You mean which connectors?
Can you show me in the linked manual?
Otherwise, I respect the high voltage, and that 19 kV does not sound good. :)
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #15 on: October 10, 2014, 01:55:04 pm »
Hey David,
Thank you for your answer and questions as well!

Keep in mind that I do not have any experience with HP oscilloscopes.  I am a Tektronix dude but I do have a Tektronix 7603 which is roughly comparable as far as having a large CRT.  The HP1745A and the others in that series look like fine oscilloscopes and based on their high PDA (post deflection acceleration voltage), I suspect they should produce a somewhat brighter and clearer display than a Tektronix 7603.

Quote
Do you think it's too dim?

It looks dim and unfocused to me but someone more familiar with the HP1745A or HP1746A and how their large CRTs should look will know better.

Quote
Yes, if we see at the beginning of the signals, certainly is.
Photograph is cheating some extent.
At 100 MHz things are worsened.
See attachments.
Trigger is stable, but the beginning of signal dimmed further.
First pic at 50 ns, without 10x mag., second with mag., somewhere in the middle, third one is at the beginning of signal.

I noticed that as well.  It could indicate an unblanking problem or a problem with the z-axis amplifier which could also cause a dim display.

On analog oscilloscopes, it is usual for the trace to become fully unblanked before 1 or 2 divisions into the sweep.  Yours does not seem to show this until 3 or 4 divisions but it is difficult to tell without knowing the time/div setting you used.

Quote
I have the manual from Keysight:
http://literature.cdn.keysight.com/litweb/pdf/01745-90901.pdf

So, Dear David, if you can show me the way, according to the service book, it would be good.

Well, that is handy.  I spent some time looking for the service manual yesterday and came up with nothing.  I am looking at PDF page 91 and it seems I guessed right about the post deflection acceleration voltage of 19 kilovolts.

I would disconnect the 19 kilovolt lead going to the CRT, ground it to the chassis, and then check the display again to see if it gets dimmer or otherwise changes.  PDF page 90 shows that it has a high voltage plug at the end of the lead going to the CRT which plugs into the high voltage multiplier (A6 on PDF page 91 in the lower right corner).  The alternative is to disconnect the high voltage multiplier from the inverter but I do not know how easy that would be.

Alternatively if you have access to a high voltage probe, the output of the high voltage multiplier could be measured directly after disconnecting the lead.

The usual precautions should be taken when doing any of the above since it *is* a high voltage circuit.  Do not mess with the high voltage connections when the oscilloscope is turned on.  It might be a good idea to have someone monitoring you.

I am basing the above idea on a Tektronix oscilloscope we diagnosed a couple months ago as having a bad high voltage multiplier which displayed a similarly dim and slightly unfocused display.

If doing the above is a bad idea, I am sure and hope some other EEVBlog member will say so.

If you want to start off with something safer, then I would check the waveform at test point 1 shown on PDF page 93 which is the z-axis amplifier output.  PDF page 92 shows what the waveform at that point should look like.  You will need another working oscilloscope to do this however.

...Can you disconnect the CRT anode or the voltage multiplier and see if that makes any difference?

You mean which connectors?
Can you show me in the linked manual?
Otherwise, I respect the high voltage, and that 19 kV does not sound good. :)

None of your photos show the area so I am not sure how to point it out to you.  The description of the area in the service manual on PDF page 90 is cutoff on the left hand side.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #16 on: October 10, 2014, 02:15:39 pm »
Thank you for your detailed answer, David!
I have found meanwhile what you suggested.
I will be careful, because I don't want to kill this beauty, and of course not myself.
And I'm not in hurry, so I will listen anyone else opinions.
And I will try to learn this scope, read manual and try understand it, as I can, you know, my crappy english, and my knowledge in electronics, which is similar than hen's knowledge to alphabet.
So now I do not pull apart any connectors for the time being.
I can't do so high voltages in check anyway, because I do not have instrument for this.
If I get a detailed script how can I perform this test, what you suggested for me, I will do it.

Kind regards,

Gyuri
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #17 on: October 10, 2014, 03:09:11 pm »
Thank you for your detailed answer, David!
I have found meanwhile what you suggested.
I will be careful, because I don't want to kill this beauty, and of course not myself.
And I'm not in hurry, so I will listen anyone else opinions.

As far as I know, disconnecting the high voltage at that connector which just pulls straight apart is safe.

Quote
I can't do so high voltages in check anyway, because I do not have instrument for this.

I do not have one either.  I have a 6 kilovolt high voltage probe which I use to check the CRT cathode voltage which is in the -2000 to -3000 volt range but for checking the PDA voltage which is much higher, typically a 40 kilovolt probe is used.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #18 on: October 10, 2014, 03:22:45 pm »
I would disconnect the 19 kilovolt lead going to the CRT, ground it to the chassis, and then check the display again to see if it gets dimmer or otherwise changes.

You say, pull apart that connector on the thick red cable, and grounded it?
It doesn't kill the multiplier?
Just ask, because I never ever doing similar.
You know, I only once repaired a CRT television, when my wife flooded the flower on it.
There was some burns on the PCB, I have repaired it with some tiny copper wires.
It still works. :)
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #19 on: October 10, 2014, 04:00:59 pm »
I would disconnect the 19 kilovolt lead going to the CRT, ground it to the chassis, and then check the display again to see if it gets dimmer or otherwise changes.

You say, pull apart that connector on the thick red cable, and grounded it?
It doesn't kill the multiplier?
Just ask, because I never ever doing similar.

The connector pulls straight apart with the plug going to the CRT and the socket going to the multiplier.  The terminal inside the socket going to the multiplier is enclosed which shields it from exposure so that is pretty safe.  The terminal on the plug going to the CRT is exposed and may or should be safely grounded to the chassis.  Do not touch it however because the CRT may have residual charge still present.

The CRT should still work without the anode connection to the high voltage multiplier but it will be dim and not as well focused.  By comparing it to your previous results, you should be able to tell if the high voltage was originally missing which I suspect may be the cause for the dim display.

The reason I suggested this test is that it is relatively easy to do and requires no special equipment.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #20 on: October 10, 2014, 04:12:39 pm »
Meanwhile, I was thinking about (although this is difficult for me :) ) it is a nonsense, to be grounding the wire on the side of multiplier, so I misunderstood you.
CRT side of that thick wire is, what must to be grounded.
I think, I will be waiting with it, altough I'm very curious, what is the problem.
I'm performing checks now on side 92 in the manual first.
 

Online Alex Eisenhut

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Re: Very nice HP1745A with little problems.
« Reply #21 on: October 10, 2014, 04:32:33 pm »
I have one of the 1741A storage scopes as a daily driver at my service bench.  The 1745/46A’s came out in 1983 and were right at the end of the 10 year run of the HP 1700 series scopes.   With this series in the late ‘70’s HP finally had scopes that could match Tektronix head to head in performance and reliability, and they eventually passed Tek in scope sales once digital scopes started becoming popular.   Despite the Tek 2225 being a few years newer than it, the 1745A when working right should be able to trigger on any signal a Tek 2225 can trigger on, so I suspect there is an issue with your 1745A somewhere. 
 
-Mark-

Lucky. I had a 1741 for a while but the tube was weak. I ended up donating it to a local hacker space.
*Except AC/DC adapters on eBay. Avoid them all!
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #22 on: October 10, 2014, 04:43:09 pm »
I'm searching for test points.
I have found number 4, it is almost like exactly as expected, if we doesn't count that its frequency almost double that we waiting for.
Or I'm doing something wrong.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #23 on: October 10, 2014, 04:57:38 pm »
Here the situation is similar:
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #24 on: October 10, 2014, 05:20:21 pm »
Waveform TP2, almost good.
« Last Edit: October 10, 2014, 05:22:31 pm by TGyuri »
 


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