Author Topic: EEVblog #536 - HP35670A DSA Repair  (Read 16168 times)

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Offline EEVblog

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EEVblog #536 - HP35670A DSA Repair
« on: October 18, 2013, 09:45:12 pm »
Dave repairs a HP 35670A Dynamic Signal Analyser.
Well, kinda...

! Private video
 

Offline JoannaK

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #1 on: October 18, 2013, 10:40:12 pm »
Have to say, that 40;Mhz oscillator looks indeed quite a strange..

Well, at least the machine has plenty of self-diagnostics in it now that it manages to boot up. Please keep us informed how this progress.


 

Offline free_electron

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #2 on: October 18, 2013, 11:15:08 pm »
IMMEDIATELY clean off the square rubber feet on that top board that have turned to gunk ! Remove all of em !
The other ones need to come off as well they eventually all turn to gunk. that gunk is conductive. 3M has admitted they had a problem with those.  a plasticiser in the rubber reacts over time and the thing basically 'melts'.

i have sene tha t problem in many a machine that has these square rubber bump feets.

that thing on the crystal is not a resistor but a color coded inductor
« Last Edit: October 18, 2013, 11:17:50 pm by free_electron »
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Offline nitro2k01

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #3 on: October 18, 2013, 11:36:20 pm »
Obvious thing to try next: All other linear regulators you might find. Whatever fault took down the 7912 probably took down some other stuff as well. If another rail is gone, obviously all the analog tests are going to fail.

Edit: The video embed link says private video. You can probably fix this by doing a dummy edit to the post.
Whoa! How the hell did Dave know that Bob is my uncle? Amazing!
 

Offline LaurenceW

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #4 on: October 19, 2013, 12:04:27 am »
Yeah, the number of listed "fails" is so widespread, that I'd rather expect a single, common cause. My money (not all of it, obviously) is on a dead power supply rail.

At least, if Dave can't fix it, it will make an exciting Dumpster find for somebody else :-DD
If you don't measure, you don't get.
 

Offline exp1

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #5 on: October 19, 2013, 12:09:33 am »
I would heavily suspect anything connected to the +/- 18v rails, looks like when the PSU broke the 18v rails soared and took out the oscilator's reg. it could have taken out some analog/mixed parts too.

Btw, did you check the main fuse in the PSU? that could be the broken thing in the AC side.
 

Offline EEVblog

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #6 on: October 19, 2013, 12:27:35 am »
Obvious thing to try next: All other linear regulators you might find. Whatever fault took down the 7912 probably took down some other stuff as well. If another rail is gone, obviously all the analog tests are going to fail.

Yep, quite likely, that was going to be my next plan of attack. May not be easy to measure some though, will have to see.
 

Offline centon1

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #7 on: October 19, 2013, 03:04:42 am »
Noob question.

Does the DSA self-test care if the frequency has not been 'tweaked' again?
 

Offline EEVblog

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #8 on: October 19, 2013, 07:25:22 am »
Does the DSA self-test care if the frequency has not been 'tweaked' again?

Almost certainly not. The DSA can't know the reference freq is off unless it has another ref freq to compare it to, which it doesn't.
 

Offline Tothwolf

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #9 on: October 19, 2013, 07:28:02 am »
You might also try checking for shorted tantalum capacitors, especially the SMD versions. Tantalum caps tend to fail short when exposed to power spikes or excessive ripple (which punches a hole in the oxide insulating layer). I've seen many tantalum capacitors fail in this manner when a PSU failed (and even when there was just excess ripple due to dried out electrolytic filter capacitors). With enough current, tantalums can even catch fire and/or explode when they short.
 

Offline algorath

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #10 on: October 19, 2013, 10:10:06 am »
how much did you pay for it? and how can a psu repair possible cost 1600$ ? what the hell did they do with it?
 

Offline Experimentonomen

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #11 on: October 19, 2013, 11:29:21 am »
I noticed a core on one of the transformers looks cracked, the printing on one looks interrupted and shifted up/down as if a crack appeared. Dunno if this could count for anything though as the psu was working at first.
 

Offline R_G_B_

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #12 on: October 19, 2013, 01:25:13 pm »
I had something simular happen with a oscilloscope turned out I had a short on a cap. Pulling the volatage down to zero and everything else connected to the same rail. So I would not be to worried by the amount of errors that have been loged if all these compents are running from the same source. Just a thought. Try powering up each board seperatly and test the voltages on each board. 

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Offline M0BSW

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #13 on: October 19, 2013, 01:29:50 pm »
 I'd love  to find something like this, where if you have the time you can investigate one part then on to the next, satisfaction must be fantastic when completed. maybe a hobbyist wouldn't care about not been viable repair, why would it matter in a hobbyist world, you are a lucky man to find this stuff Dave.
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Offline M0BSW

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #14 on: October 19, 2013, 01:32:39 pm »
how much did you pay for it? and how can a psu repair possible cost 1600$ ? what the hell did they do with it?
They probably gave the owner some Grade " A "bull shit I should think
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Offline sleibson

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #15 on: October 19, 2013, 03:17:07 pm »
Another great video Dave. You are a natural!  :-+

I suspect there's one root cause for all the self-test failures. With the ADC clearly not working, the analog section cannot take any measurements. Thus any tests requiring the ADC to work must fail. Since the self-test is complaining about the ADC FPGA, that's the place to start. I don't think that you pulled that board in the video, but if it's another Actel ACT 1 FPGA in a socket, the first step is resocketing that FPGA after using a bit of contact cleaner with the hope that a bit of corrosion is the culprit.

The good news is that the entire digital section seems operational with perhaps the exception of the missing I2C peripheral.

The 39.xxxx MHz crystal oscillator in this instrument is a wonder. It takes up a huge amount of board space! I would have guessed that with all the attention and space given it, this oscillator was supposed to be some sort of ultra-stable clock source for all of the spectral measurements this DSA is supposed to make, but then I would have expected an oven-controlled crystal if stability was that important. I would generally cheat in this part of a design and just use a canned crystal oscillator in a metal can with a 14-pin DIP footprint. It would have been a lot smaller and my designs generally were crimped on board space. Also, it wouldn't need -12V. You could certainly afford one of those in a $20K instrument.

Strange that something killed the primary section of the "new" or "repaired" power supply. A spike that got through and also killed the negative voltage regulator? I can't tell from the video, but it looks like HP bought that supply from a power supply vendor instead of developing one on its own.
« Last Edit: October 19, 2013, 03:18:46 pm by sleibson »
 

Offline Rory

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #16 on: October 19, 2013, 03:23:47 pm »
Doesn't it seem like HP/Agilent gear is designed that when they break, they fail catastrophically?
 

Offline sleibson

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #17 on: October 19, 2013, 04:17:18 pm »
This instrument is 18 years old (based on the date codes) and there are many indications that it's gotten rough treatment. Crud in the BNCs, corrosion on the display, general tarnish and crud inside. It's seen a lot of action.
 

Offline bktemp

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #18 on: October 19, 2013, 04:19:56 pm »
The datecode on the FPGAs is 94/95. The datecodes on the transformers also say 94/95. My guess would be that they have repaired the power supply. Otherwise I would expect a never datecode.

How does the DC input works? I see no second regulator path. From the placement of components I would guess that both inputs use the main switching transformer (between the L shaped heatsink and the secondary side). Therefore the other transformer must be a 12-28Vdc -> 300Vdc (or something like that) converter driven by a H-bridge (the 4 mosfets mounted on the chassis next to that transformer). The big red toroidal inductor could be the filter inductor. It could also be a PFC inductor but that was uncommon in 1994.
If it is done that way, then it is strange why the AC input is not working. It must something simple like a broken fuse, bad switch contact or connector.
 

Offline Saulius

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #19 on: October 19, 2013, 05:34:19 pm »
For how much did you get this device?
 

Offline vaualbus

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #20 on: October 19, 2013, 07:18:54 pm »
As I've arleady written on youtube, I've read the schematics diagram on the service manual and apparently,on each board there is voltage regulator to change the +/-18 to other voltages, so if the psu fail and give to much voltage that regulator will die but the baord work.
Dave do a follow up please.
 

Offline Zbig

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #21 on: October 19, 2013, 08:19:58 pm »
These yellow ICs are wicked cool  :-+ I want a yellow microcontroller so badly now. Or a DAC. Or anything. No, yellow tantalum caps won't cut it.
 

Offline david77

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #22 on: October 19, 2013, 08:43:07 pm »
The yellow things are probably resistor networks not IC's, sorry to disappoint.
 

Offline LoyalServant

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #23 on: October 19, 2013, 09:27:19 pm »
Looking forward to the follow-up.

I wonder if the I2C bus is being dragged up or down.... common culprit in a lot of consumer gear at least.
 

Offline EEVblog

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Re: EEVblog #536 - HP35670A DSA Repair
« Reply #24 on: October 19, 2013, 09:50:29 pm »
The 39.xxxx MHz crystal oscillator in this instrument is a wonder. It takes up a huge amount of board space! I would have guessed that with all the attention and space given it, this oscillator was supposed to be some sort of ultra-stable clock source for all of the spectral measurements this DSA is supposed to make, but then I would have expected an oven-controlled crystal if stability was that important.

My guess would be it's a Butler oscillator. How low drift and low phase noise it is I don't know. Probably not hugely special in the phase noise department given the low frequency measurements.
It could simply be a carry-over from an old design, before canned oscillators cut the mustard?
 


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