Author Topic: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser  (Read 859 times)

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Offline EEVblog

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EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« on: August 09, 2019, 12:32:35 am »
Old school, they don't make'em like this any more.
Teardown of the HP 3785A Jitter Generator & Receiver with beautiful HP bubble LED displays.

 

Offline Bud

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #1 on: August 09, 2019, 02:03:28 am »
Wow, this thing is filled with gems! Never seen right angle ceramic trimmer caps  :-+
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Offline cbutlera

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #2 on: August 09, 2019, 10:41:09 am »
The socket visible at the edge of the frame is the top of an extender card, to raise any of the other cards out of the chassis for easy debugging.  Another beautiful bit of HP attention to detail.
« Last Edit: August 09, 2019, 10:55:24 am by cbutlera »
 

Offline Fungus

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #3 on: August 09, 2019, 10:57:46 am »
Gotta love the resistor-color-coded board tabs. Any EE will know the exact order of the boards without any of that "numbering" rubbish.




(Surprised Dave didn't spot that...  :-// )

« Last Edit: August 09, 2019, 11:21:43 am by Fungus »
 
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Offline dr.diesel

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #4 on: August 09, 2019, 11:23:10 am »
I would felt obligated to replace the battery and clean the board before tossing back in the dumpster!

Offline Heartbreaker

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #5 on: August 09, 2019, 03:51:31 pm »
Gotta love the resistor-color-coded board tabs. Any EE will know the exact order of the boards without any of that "numbering" rubbish.

Not only are the boards numbered, the cabels are too. Two color bands in each end - one indicading board number and the other cable (or connector?) number.
 

Offline Heartbreaker

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #6 on: August 09, 2019, 04:01:50 pm »
The socket visible at the edge of the frame is the top of an extender card, to raise any of the other cards out of the chassis for easy debugging.  Another beautiful bit of HP attention to detail.

My first thought was that any of the boards could be inserted into this extender card (location), but at 14:15 the backplane is seen at the extender card end, and the extender card is not inserted into a connector. So its intended use must be to relocate it to any given location to debug the board at that location.
 

Offline cbutlera

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #7 on: August 09, 2019, 09:02:55 pm »
The socket visible at the edge of the frame is the top of an extender card, to raise any of the other cards out of the chassis for easy debugging.  Another beautiful bit of HP attention to detail.

My first thought was that any of the boards could be inserted into this extender card (location), but at 14:15 the backplane is seen at the extender card end, and the extender card is not inserted into a connector. So its intended use must be to relocate it to any given location to debug the board at that location.

That's right.  Common HP practice for equipment of this era.  I have more old HP boat-anchors than it is wise to admit in polite society, and a collection of extender cards to help maintain them, some of which came mounted inside the device, such as in this case.

I've seen these Jitter Analyser's for sale on that addictive web site from time to time.  But try as I might, I can't think of even the smallest justification to buy one, damn it!  It's probably best that I don't find one in a dumpster.  I don't have the willpower of dr.diesel.  I would test it, fix it, calibrate it, then put it on the shelf as another headache for whoever has to deal with all this stuff when I'm gone.  (I can hear the TEA thread calling, but so far I have managed to resist.)
 

Offline floobydust

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #8 on: August 09, 2019, 09:49:48 pm »
It's a work of HP art and is literally full of gold. Amazing to see such a thoughtful design.
You have to wonder if engineers who aren't pushed and rushed end up making a better product, out the door faster?

Why didn't Dave give us the bubble LED porn? I have some on an old LED watch, TI-30 calculator and a few in my parts drawers.
Sure would like a look at the display board...

The HP reference oscillator's jitter, it would be interesting to see 25 years later if we can actually measure it.
 

Offline bsfeechannel

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #9 on: August 10, 2019, 10:01:31 pm »
This kind of test gear was an absolute must for companies producing transmission telecom equipment back in the day.

Not only HP made them, but also Rohde&Schwartz and Anritsu.

Because in the Plesiochronous Digital Hierarchy (PDH), the tributaries of a multiplexer are almost (Plesio-) but not synchronized, stuffing bits have to be added to the fastest tributaries so that their bit rates are lowered to accommodate the slower tributaries.

Since this operation is statistical in nature, it adds a "random" jitter to the pulse train, which translates in errors and, ultimately, in noise at the reception.

These companies had to comply by law to the CCITT standards, otherwise they could not sell their stuff. So they needed a reliable jitter analyzer to guarantee an absolute compliance before certification.

This construction pattern of a motherboard and several plug-in daughter boards was the standard of the era not only for the test equipment, but also for the very telecom equipment for which they were designed.
 
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Offline EEVblog

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #10 on: August 11, 2019, 07:28:57 am »
Why didn't Dave give us the bubble LED porn? I have some on an old LED watch, TI-30 calculator and a few in my parts drawers.
Sure would like a look at the display board...

I mentioned that, it would have been a lot of work to get the whole thing apart, not worth it.
 

Offline EEVblog

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #11 on: August 11, 2019, 07:32:36 am »
Gotta love the resistor-color-coded board tabs. Any EE will know the exact order of the boards without any of that "numbering" rubbish.



(Surprised Dave didn't spot that...  :-// )

Because not everyone sees everything at all times, especially when you are trying to talk and produce a video at the same time.
Given the lack of any mention of this in the Youtube comments from 10's of thousands of other people, I don't feel so bad missing it.
So well done in spotting it!
 

Offline Barny

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #12 on: August 11, 2019, 07:33:06 am »
Dave should take a look at the poor US&A made battery on one of the card.
This little acid container started to get a little bit incontinent.
Green fluff is under the battery and on IC's around it.

It would be good to take care of this bevore it gets really bad.
-> starting 8:08 in the video
« Last Edit: August 11, 2019, 07:35:34 am by Barny »
 

Offline BrianHG

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Re: EEVblog #1237 - Old School Teardown: HP3785A Jitter Analyser
« Reply #13 on: August 11, 2019, 11:06:02 am »
Dave, when you were scoping the clock output seeking for a jitter while playing with the settings, have you considered that only the 'Data Output' contains the jitter?

This way, if someone wants to see the jitter on a scope, they can lock one channel on the clock and see on the scope's second channel true width of the jitter to the left and right of the true reference clock output.
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