EEVblog #540 - HP35670A DSA Repair - Part 3

More progress (or is it?) on the HP35670A DSA repair.

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[uNQuDZ]

I would trace back from the source output and see why it's putting out +15v, there's definitely an opamp shorted to the positive rail.

[whuffo]

Don't give up! Just solve the problems one by one; don't focus on the system, focus on the sub circuits. Fix the source output first, then move on from there. Step by step and you'll get there - eventually. I've seen worse than this before and prevailed, sometimes it takes longer than we'd like but the feeling of accomplishment at the end makes it all worthwhile. Identify problems, solve them, move on to the next problem. Repeat as necessary. That's how it's done.

[worldgeektube]

Whether you fix it or not I've learned a lot from this series about troubleshooting.

[BeerTower]

The opamps are connected in parallel to the power rail, not series. Opamps are generally on a separate supply to digital stuff, so hopefully this means that the ASIC hasn't been blown!

[brianhilligoss]

I can say, i am learning alot from this video. Being a beginner I am learning alot about trouble shooting. Please consider doing more trouble shooting videos.

[ArumesYT]

7:05 Remember this DSA was designed for the northern hemisphere. Ofcourse your voltages are upside down.

[DOZZZ]

I can understand Dave quite well. I have a tek TDS540 Scope i've been working on for some time now. After fixing all the faulty caps on fthe boards, there is still an error persistent. And now I don't know wether I should keep on trying or just get rid of it as it is... Dave, I hope you can find the motivation to keep on this nice piece of electronic...

[FNIX_Productions]

i hope you continue the series, this has interested me since the first one

[ForViewingOnly]

Hard luck Dave. If you give up on the fault-finding (and I wouldn't blame you), maybe you'll find some replacement boards on eBay sometime, or pickup another faulty DSA that you can strip for parts. I enjoyed the series anyway mate, keep us updated if you have a Eureka moment.

[KrisBlueNZ]

I'd measure resistance from the VRef test points to the output pins on nearby op-amps. The reference voltage rails are probably driven by op-amps, either directly or through a low resistance. Then use visual inspection or continuity testing to trace the circuitry around the op-amp and try to trace back to the voltage source. Luckily it's all through-hole with everything in plain sight. A schematic would be great but a bit of reverse-engineering would be worthwhile for a piece of gear like that!

[dahappiehippie]

Love this series :) even if it's proving a bugger to fix, I've learned much more about troubleshooting than any other videos out there!

[joblessalex]

Each time getting closer, let's hope ya get it this time!

[alancordwell]

That ADC waveform is telling you something- think that, unfiltered, it would be a stepped sinewave between + and - limits. If you still have a -ve reference that is out, you may well see something like that... I think you're very close now! DAC -> anti-aliasing filter -> 'source' output driver: the end is in sight! Hopefully there will be a part 4?!

[jurva-markusvehasmaa]

Have to say this, its been very interesting see you try to hunt down he problem and fix those found step by step... Quite interesting to see a fault that's not easy to repair for a change.

Sure would like to see rep of manufacturer to step up and diggup you one of those schematics. As someone who does small odd repair jobs now and then i do preciate the availability of schematics.

[EEVblog]

Yes, troubleshooting is pretty much hopeless without easy board access. I need an extender board or cable, or else this on is going nowhere.

[KostantisX]

The connection of devices on a power rail is like the connection of home appliances in your house. In parallel. If instead of 110 you have 200V it will blow them all. What you said could happen if the failure mode of one of the devices was shortcircuit but usually it's not.

[theLuigiFanProductions]

I hope you can fix the Signal Analyzer, even though from the looks of it, it dosent look good. I hate it when countless things are possibly dead with little to no way of figuring out what.

"Will Agilent send you the schematics? Did you ask them? You ARE a pretty big name they're bound to cater to."

I too was thinking if you asked them that they would probably do so, its no harm in just asking.

Hope you can fix the DSA and good luck (you probably will need it).

[whiskeyify]

I used to work in a electronic repair shop, and often didn't have diagrams for stuff, so you have to just hunt & peck around like Dave is doing here to find the trouble.

[crazzzik]

This is the best adventure I've seen in years! It is almost like archeology! Those few that can fix it are scattered throughout the world and only Brave Dave (C) takes the challenge of reviving an ancient relic! Best part is that it is not a fantasy fiction, IT IS REAL! There are no cheat codes or walkthroughs. We get real challenges and real emotion. Also, about 10 to 20 thousand people on a search of a forgotten schematic in hope to save HP35670A DSA unit. GEEK PORN

[MauroTamm]

It's great that it's not fixed!!!! This means more videos for it!! :D

Most likely probing is better then replacing all of the chips.
Like comparing output on the opamps (well not that clear data, its in circuit) but perhaps some have no output at all or very unusual values.
Perhaps also trace the swapped -/+ voltages as much as possible - perhaps the block diagram is correct and it has swapped due to some unusual failure somewhere and causing all the problems.