Author Topic: Very nice HP1745A with little problems.  (Read 18488 times)

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Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #25 on: October 10, 2014, 05:34:30 pm »
Waveform TP3, there is practically nothing.   :(   DLY'D GATE
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #26 on: October 10, 2014, 05:48:24 pm »
Waveform TP3.
My bad.
I measured the wrong place.
The case is similar to TP4 and TP1.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #27 on: October 10, 2014, 06:23:14 pm »
I do not know, have not touched it yet, but I feel it is nothing wrong with 19kV.  :)

I will continue to investigate.
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #28 on: October 11, 2014, 05:33:28 am »
I may see a problem in the gate output waveform which is TP1, waveform 4, and the first one you posted.  That rising edge should be squarer and it will be affected by trimmers C11 and R12 shown on service sheet 3 which is PDF page 93.  That by itself could explain the dim part at the beginning of the sweep.

Page 5-4 of the service manual tells how to adjust C11 and R12.

If that waveform cannot be made "flatter" on the top and especially after the leading edge, then there may be something wrong around that circuit.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #29 on: October 11, 2014, 06:32:13 am »
Hey David, I'm just awaken now, drink my coffee, so forgive me I'm lost in my posts.
So which waveform is bad according to your opinion?
I have noticed, the frequency of the signals almost double than expected, except waveform 2, which is chopped mode.
Thank you for your valuable help!
I have now some works must to do.
Buying for weekend, dishwashing, housekeeping etc... but I hope afternoon I will work on this wonderful equipment.

Kind regards,

Gyuri
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #30 on: October 11, 2014, 06:39:49 am »
On the service manual sheet I have marked with lightblue the corresponding waveform.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #31 on: October 11, 2014, 01:56:27 pm »
So what I don't understand waveforms 1,3, and 4.
These are all of them (according to Service Sheet 3) 500 Hz signals, with 2 ms period.
In contrast, I am getting 714 Hz signals (1.4 ms) in all three position.
And waveform 1 is slightly asymmetrical.
Waveform 4 also not suitable spaced.
It is divided in 5/8 - 3/8, and I get about 27/35 - 8/35 ratio.

What a surprise its ratio is the same what I got at waveform 1.
Only it has 8/35 - 27/35 ratio.

I don't know where and how can be set it to 500 Hz.
I do not know what the consequences of this, but I do not like it.
« Last Edit: October 11, 2014, 02:20:23 pm by TGyuri »
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #32 on: October 11, 2014, 03:19:18 pm »
I may see a problem in the gate output waveform which is TP1, waveform 4, and the first one you posted.  That rising edge should be squarer and it will be affected by trimmers C11 and R12 shown on service sheet 3 which is PDF page 93.  That by itself could explain the dim part at the beginning of the sweep.

Page 5-4 of the service manual tells how to adjust C11 and R12.

If that waveform cannot be made "flatter" on the top and especially after the leading edge, then there may be something wrong around that circuit.

Maybe that measure wasn't so good, so I give it another try.
I think it is not so bad.
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #33 on: October 11, 2014, 03:25:54 pm »
The timing in these waveforms (except for transition timing which the adjustments I discussed above have to do with) depends on the sweep settings.  That little short bump at the top of #4 is the brightness being increased during the intensification of the delayed sweep.

If you want to get an idea of what is going on and what you are looking at, measure #4 while changing the sweep, trigger holdoff, and intensity controls.

What #4 is really showing is the intensity control of the CRT with positive values yielding a brighter trace.  The most negative portion of the waveform is the period when the trace is blanked.  It even says below the graph, "amplitude varies with intensity".

What we do *not* want to see is a slow rising edge or in this specific case, a gradually rising section *after* the fast rising edge.  The top of the pulses in #4 should be flat except for the short bump from the intensified sweep.
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #34 on: October 11, 2014, 03:28:26 pm »
I may see a problem in the gate output waveform which is TP1, waveform 4, and the first one you posted.  That rising edge should be squarer and it will be affected by trimmers C11 and R12 shown on service sheet 3 which is PDF page 93.  That by itself could explain the dim part at the beginning of the sweep.

Page 5-4 of the service manual tells how to adjust C11 and R12.

If that waveform cannot be made "flatter" on the top and especially after the leading edge, then there may be something wrong around that circuit.

Maybe that measure wasn't so good, so I give it another try.
I think it is not so bad.

That photo looks fine.  What changed it?

I have run across oscilloscopes before that had problems with the intensity control signal where it gradually rose after unblanking yielding a dim CRT trace during the beginning of the sweep which yours seems to be doing.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #35 on: October 11, 2014, 03:56:09 pm »
I have changed from TEK 2225 to TEK 2440.
But what most important, changed a cheap 100MHz probe to an original TEK P6137 probe. I ashamed myself, but even that cheap probe gave me a better signal shape if I calibrated it. Excuse me.
So, do you think it is an appropriate signal?
I can see an overshoot after rising edge.
So this is, what I should remove with setting of C11/R12, am I right?
Is it worth it a try?
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #36 on: October 11, 2014, 03:59:45 pm »
Have you an idea about timing mismatch what I experienced?
Thank you for your patience!
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #37 on: October 11, 2014, 04:11:46 pm »
Detailed view of rising edge waveform 4:
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #38 on: October 11, 2014, 06:00:22 pm »

If you want to get an idea of what is going on and what you are looking at, measure #4 while changing the sweep, trigger holdoff, and intensity controls.


Yes, of course!
I have played with these settings, very interesting.
Even I have made an exactly similar waveform what I missed permanently, only setting TIME/DIV Vernier and Trigger Holdoff. But it doesn't have practical results.
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #39 on: October 11, 2014, 06:03:36 pm »
I have changed from TEK 2225 to TEK 2440.

Hmm, good choice.  The Tektronix 2440 is an excellent vintage from 1990.

The 2225 is probably just fast enough at 50 MHz to make the rise time measurement discussed below.

Quote
But what most important, changed a cheap 100MHz probe to an original TEK P6137 probe. I ashamed myself, but even that cheap probe gave me a better signal shape if I calibrated it. Excuse me.

Doh!

This would completely explain the observed slope on the flat part of the pulse.

Quote
So, do you think it is an appropriate signal?
I can see an overshoot after rising edge.
So this is, what I should remove with setting of C11/R12, am I right?
Is it worth it a try?

It looks fine to me now or at least the top part does but see my comment below.

The overshoot can be corrected with C11 and R12; that is what they are there for.  I would certainly tweak them for the best shape.

The rise time of the pulse looks slow to me given that the oscilloscope's vertical delay line is 100 nanoseconds.  I think the rise time should be less than 1/4 of that or faster than 25 nanoseconds.  Maybe that is related to the apparent low CRT intensity or is a completely different problem.  Adjusting C11 and R12 may help resolve it or not.  Maybe someone who has one of these oscilloscopes can confirm whether a 140 nanosecond rise time is typical for this series.

This slow rise time easily explains the original problem of not being able to see the pulse from the Jim Williams avalanche pulse generator and it was next on my list of things I was going to recommend checking.  Figuring out why the rise time is slow if adjusting C11 and R12 does not fix it will not be easy.

These are the kinds of problems I love to solve.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #40 on: October 11, 2014, 06:18:36 pm »

These are the kinds of problems I love to solve.


Yeah, just like me!

But the things are aggravating on my part to what I wrote about hen and ABC. :)
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #41 on: October 11, 2014, 06:59:28 pm »
Interesting.
C11 here is not trimmer, it is fixed.
Maybe tomorrow I will check it.
Setting trimmer pot. R12.
I have achieved worse results.
We will see!

Kind regards,

Gyuri
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #42 on: October 14, 2014, 07:34:24 am »
I was wrong, again.
It was because my dimmed eye.
(I have bought this scope mainly because its nice big screen.  :) )
It is a trimmer cap, just not that shape what I have known.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #43 on: October 14, 2014, 01:39:51 pm »
I have carried out settings described in paragraph 5-17.
I have got slightly better waveform, (see attachment) but it hasn't affected rise time.
It hasn't affected dimmed trace at 50ns neither.
So I performed that test, what David suggested me.
I disconnected 19 kV connector.
The scope was turned off at least one hour before.
I was very careful. But not enough.
As I pulled apart connectors, I have dropped that end of cable, which has a naked pin.
As David wrote it earlier, it has a residual charge for sure!
It has produced a very nice blue lightning, with some thundering.
So it certainly had some kV in it.
And now, what happened?
I was worried about, what went south because of lightning.
Because I didn't seen it, where the lightning was striked.
But I was lucky.

Be very-very careful, if you doing this!!!

Well, so this test proved, my feeling about a good PLD was right.
And what shows the screen, without PLD?
I have got a much smaller and dimmer picture.
So PLD doing its job.
« Last Edit: October 14, 2014, 01:52:49 pm by TGyuri »
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #44 on: October 14, 2014, 05:26:22 pm »
...

As I pulled apart connectors, I have dropped that end of cable, which has a naked pin.
As David wrote it earlier, it has a residual charge for sure!
It has produced a very nice blue lightning, with some thundering.
So it certainly had some kV in it.
And now, what happened?
I was worried about, what went south because of lightning.
Because I didn't seen it, where the lightning was striked.
But I was lucky.

Be very-very careful, if you doing this!!!

This is why I suggested testing other things and leaving this for last.  If the problem was solved in some other way, then messing with it would not be necessary.

Quote
Well, so this test proved, my feeling about a good PLD was right.
And what shows the screen, without PLD?
I have got a much smaller and dimmer picture.
So PLD doing its job.

This tells us some things.  Since the deflection accuracy was affected by the acceleration voltage, we know that the acceleration voltage is correct, that there is nothing wrong with the power supply, and HP used a CRT design were the PDA (post deflection acceleration) voltage affects deflection.  Since the screen was dimmer, we sort of know the acceleration voltage was present.

That leaves the slow rise time of the blanking pulse as the problem.  Like I said, I think it should be 25 nanoseconds or faster.  As shown it is slow enough that the avalanche pulse is over before the CRT beam finishing unblanking so there is nothing to see.  When you press the beam finder, it disables blanking making the pulse visible.

Now the question is why is it slow and why does it have all of that overshoot?  The two are likely related.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #45 on: October 14, 2014, 06:15:22 pm »
R12 and C11 are in an extreme position.
I don't think it is intentional, and the waveform isn't perfect. (Meant: Not designed in this way.)
It would be nice, if someone (Tekfan) would measure at incriminated TP4 one of his scope. :)
Today's day is almost equal with a victory.
At least I did not ruin anything.
I'm healing slowly.
And the weather was beautiful.
BTW, I'm wondering, that gate circuit able to make a signal, which is needed for a 25 ns leading edge?

Do not take it seriously, I have nothing to do with the electronics that I can decide the issue.
« Last Edit: October 17, 2014, 10:43:24 am by TGyuri »
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #46 on: October 14, 2014, 06:52:13 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.

So what I don't understand waveforms 1,3, and 4.
These are all of them (according to Service Sheet 3) 500 Hz signals, with 2 ms period.
In contrast, I am getting 714 Hz signals (1.4 ms) in all three position.
And waveform 1 is slightly asymmetrical.
Waveform 4 also not suitable spaced.
It is divided in 5/8 - 3/8, and I get about 27/35 - 8/35 ratio.

What a surprise its ratio is the same what I got at waveform 1.
Only it has 8/35 - 27/35 ratio.

I don't know where and how can be set it to 500 Hz.
I do not know what the consequences of this, but I do not like it.

I'm in trouble, why this signal is so asymmetrical?
Its frequency does not match with what is expected.
Lastly, it is the control signal for gate circuit, isn't it?
Any thought?
 

Offline David Hess

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Re: Very nice HP1745A with little problems.
« Reply #47 on: October 15, 2014, 04:05:36 am »
R12 and C11 are in an extreme position.

This indicates a problem but we already expected that.  If the circuit is slow, then these adjustments are not going to work.

Quote
BTW, I'm wondering, that gate circuit able to make a signal, which is needed for a 25 ns leading edge?

The similar Tektronix circuits are about that fast.  Generally they finish settling about 20 nanoseconds before the vertical signal appears and their transition times are similar.  Surprisingly it is really not that difficult to do.

Quote
I'm in trouble, why this signal is so asymmetrical?
Its frequency does not match with what is expected.
Lastly, it is the control signal for gate circuit, isn't it?
Any thought?

The main gate signal which is the one that matters right now is waveform 1.  Waveform 3 is the delayed gate signal which we do not need to worry about yet.

Transistors Q1 and Q2 in U1 amplify the main gate signal and their output drives the high voltage amplifier made up of Q1 through Q4 and the output is waveform 4 which is the one that needs to be fast.

I think the things to check now are the various resistor values around Q1 though Q4 to see if one is open.  There could be an open capacitor somewhere but that would be difficult to find.

Take a careful look at R9 above Q2.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #48 on: October 15, 2014, 06:02:57 am »
Thank you again, David!
I will follow your suggestions above.
I'm also thinking to do something similar, thanks for the confirmation!
You're an excellent teacher, and I know this because both of my parents was.
Although both of them were music teachers.
But excellence is not based on what should be taught, but how.
I think, you are one of the man, who taught me the most in this field, the other one is One and Only. (NP)
Kind regards,
Gyuri

I'm getting better slowly.
 

Offline TGyuri

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Re: Very nice HP1745A with little problems.
« Reply #49 on: October 15, 2014, 08:55:18 am »
I think I have found it!
Resistor R7 39.2K seems to be an open circuit.
So signal just going thru C3 which is parallel with it.
I'm going to solder.  :)

 


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