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Old 3rd Oct 2018, 12:07 am   #1
Skywave
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Question HP 1715A bandwidth question

HP 1715A dual-beam oscilloscope.

Doesn't get used very often; main 'scope is a Tek. 2465. However, powered up the '1715A yesterday and discovered that the beam finder push-button switch was 'iffy': a squirt of switch cleaner fixed that.
Whilst it was (and still is) on the bench, though I'd run a few basic spec. / performance tests. And that's when it became head-scratching time. The claimed bandwidth (vertical display channel) is 200 MHz. Now to me, that says that at 200 MHz, the response is -3 dB of its LF response - say at 5 MHz. But I measured -3dB at 80 MHz. (Same on each channel). That can't be right, can it?

Source: HP 8640B plus short length of RG-55 (53 Ω) terminated in a 50 Ω calibrated load at input to 'scope. Level from S/G 100 mV rms.

Any relevant comments are cordially invited: thank you.

Al.
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Old 3rd Oct 2018, 7:20 am   #2
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Default Re: HP 1715A bandwidth question

Something's wrong somewhere, but you'll need something else to arbitrate to find it.

David
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Old 3rd Oct 2018, 8:15 am   #3
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Default Re: HP 1715A bandwidth question

Possibly compensation. Depends on what vertical amplifier setting you are on. If you’re using an RF signal generator I’d assume that’s a relatively low signal level. On the 5mV/div setting the vertical bandwidth is only 150MHz for example. I had a similar problem on my old 1740A and it was a duff capacitor just after the delay line on one of the compensation networks. Took me four days to find it.

I’d personally check the signal generator on a known good scope or power meter. Then check the cable. Then check the terminator. Then follow the check out procedure on the scope from the service manual and work out where it starts to go wrong.

Also worth making sure the internal termination isn’t jammed on as well (more dirty switches!)

Or me being as lazy as I am these days, swear at it, Chuck it on an auction site for spares repair and use the working one.
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Old 3rd Oct 2018, 9:08 am   #4
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Default Re: HP 1715A bandwidth question

My 1725A (275Mhz) is 3dB down at 167MHz both channels. Source Racal 9081 100mV.

Peter

Last edited by peter_scott; 3rd Oct 2018 at 9:24 am.
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Old 3rd Oct 2018, 9:27 am   #5
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Default Re: HP 1715A bandwidth question

Check your method by doing the same test on the 2465.
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Old 3rd Oct 2018, 9:50 am   #6
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Default Re: HP 1715A bandwidth question

Quote:
Originally Posted by peter_scott View Post
My 1725A (275Mhz) is 3dB down at 167MHz both channels. Source Racal 9081 100mV.
I think my cable was inadequate. If I use a 1 metre RG-59 with a 75 to 50 pad I get 233MHz as my 3dB point.

Peter

Last edited by peter_scott; 3rd Oct 2018 at 9:56 am.
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Old 3rd Oct 2018, 11:58 am   #7
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Arrow Re: HP 1715A bandwidth question

Quote:
Originally Posted by barrymagrec View Post
Check your method by doing the same test on the 2465.
Yes: perhaps I should have mentioned in my OP that I did indeed do that. The 2465 returned -3 dB at 235 MHz.
All of the test equipment used was not only verified on that 2465 but also on my Advantest spectrum analyser (without the external 50 Ω termination fitted at the analyser's end of the cable, of course).

With regard to the 1715A, the one thing that puzzles me is that the (poor) response is same on both vertical channels - which suggests that there is, by design, a common link between the two - which, at a 'first think', seems strange. Time to dig out the manual & study the cct. with that in mind. (But there is still the nagging thought that I might have overlooked something very basic & potentially obvious! )

Al.

Last edited by Skywave; 3rd Oct 2018 at 12:09 pm.
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Old 3rd Oct 2018, 12:41 pm   #8
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Default Re: HP 1715A bandwidth question

I bet its the output amp or the hybrid of doom if it's both channels.
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Old 3rd Oct 2018, 12:59 pm   #9
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Default Re: HP 1715A bandwidth question

There was an issue with the delay line used in the 1740A and others - though not the 1715A unless that came to light later - which caused the symptoms described. The HP briefing for this can be found here: http://bee.mif.pg.gda.pl/ciasteczkow...1983-01-02.pdf - it tells you how to check the delay line. Hopefully it's something else - there's plenty else in common with the two channels - but it's easy enough to eliminate.
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Old 3rd Oct 2018, 2:04 pm   #10
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Arrow Re: HP 1715A bandwidth question

Quote:
Originally Posted by MrBungle View Post
I had a similar problem on my old 1740A and it was a duff capacitor just after the delay line on one of the compensation networks. Took me four days to find it.
Hmm: thanks. That comment, and the one by Mark H. (post 9), seems like a good place to begin an investigation.

A bit of background (FWIW).
I've owned this 'scope for about 20 ~ 25 years. It was given to me by my project manager in a company that I was then working for who, IIRC, said something like:

"This 'scope isn't really up to the job I need it for. Would you like it? I'll get a better one".
So I said "thank you" and disappeared with it before he changed his mind!

From that time and up to about a few years ago when I acquired a 2465, my main 'scope was a Tek. 475. So during that period, I never got around to fully testing the performance of this 1715A. And that makes me suspect that this fault has probably been there from before I acquired it.

Al.

Last edited by Skywave; 3rd Oct 2018 at 2:14 pm.
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Old 3rd Oct 2018, 4:53 pm   #11
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Default Re: HP 1715A bandwidth question

It wouldn't surprise me. The one I got hold of had a "cal failed" sticker on it. I suspect they had tried to cal it and it didn't perform so it was tossed.
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Old 4th Oct 2018, 2:48 pm   #12
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Default Re: HP 1715A bandwidth question

And now it's fixed - but it's all rather strange.

Attention was focussed on the delay line. There is an aluminium cage-frame around the line, which is effectively in the shape of a fat toroid. After a struggle, that frame was removed and the line examined. The outer braid has no sheath, so the braid is exposed to air. Upon applying pressure with the fingers, it seemed that the entire line was acting like one integrated block. I gave that block a substantial squeezing, which seemed to 'loosen up' the windings.

Near to that frame-cage is the vertical channel input board. The rear of that board - close to the delay line - has two screws securing it to the metal frame which also act as earthing points for that board. Upon inspection, those screws were not loose, but were not fully screwed down, (about a 1 mm. gap), so an earth connection at that part of the board was of questionable integrity. The two screws were replaced with shake-proof washers and the screws tightened down.

Next, bandwidth tests were run; results:
Channel A: -3 dB @ 200 MHz; Channel B: -3 dB @ 190 MHz. (Figures are ± 5Mhz). That's good enough for me!

So, two 'changes' were made. The 'fix': either or both? And, more importantly, is the improved performance permanent? (Well, within reason: nothing is ever 'permanent' ) Only time will tell, but it is somewhat irritating to not be able to definitively state "The cause was XYZ".

Closing comments.
I've gone into some detail above. The reason for that is that this fault - it's possible cause(s) and fix - were strange. Hopefully, all of the above will be of use to other owners of this model (and its variants) if they meet the same problem.

Al.
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Old 4th Oct 2018, 6:12 pm   #13
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Default Re: HP 1715A bandwidth question

Good find. At least it’s working.

I’ve fixed a lot of problems in Tektronix kit by doing up screws tight, particularly on the attenuators.
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Old 4th Oct 2018, 6:20 pm   #14
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Default Re: HP 1715A bandwidth question

Good fix.

Given the choice I`d always be using the 2465 though.
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Old 4th Oct 2018, 6:34 pm   #15
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Default Re: HP 1715A bandwidth question

Good find. Thanks for posting.

Peter
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Old 5th Oct 2018, 12:03 am   #16
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Arrow Re: HP 1715A bandwidth question

Quote:
Originally Posted by barrymagrec View Post
Good fix. Given the choice I`d always be using the 2465 though.
Which is my regular, default 'scope. For me, the two features that the HP does have over the Tek. are:
(a) the CRT focussing: much better on the HP;
(b) build style from a maintenance perspective: again, the HP wins - just. But then Tek. 'scopes have always been poor in that regard, IMHO.
OTOH, the Tek. is a much later product and has some very useful user-features that the HP doesn't have. But the 2465 is in a different league, so it's not really fair to make too much of a comparison. I've always regarded the 17xx range from HP as their answer to the 7xx series from Tek.
And finally, having a second 'scope is essential when you need a working 'scope to fix one that isn't!

Al.
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Old 5th Oct 2018, 12:15 am   #17
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Default Re: HP 1715A bandwidth question

I prefer the Teks myself. The HP’s timebase switching scared me when I saw how it was implemented. Reliability nightmare. Also don’t like the flood gun illumination - graticule is much nicer. The teks have very common and predictable failure modes which are quite easy to fix once you have some motivation, a 1/4” nut driver and a stiff drink

Definitely agree with more than one scope. I’ve got, err 5 now, 3 of which are working, one for repair and one parts mule so I’m happy.
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Old 6th Oct 2018, 7:30 pm   #18
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Talking Re: HP 1715A bandwidth question

Quote:
Originally Posted by MrBungle View Post
Definitely agree with more than one scope. I’ve got, err 5 now, 3 of which are working, one for repair and one parts mule, so I’m happy.
I see - so we can safely assume that you like plenty of scope in your work, eh?

Al.
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Old 6th Oct 2018, 10:54 pm   #19
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Default Re: HP 1715A bandwidth question

Groan
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