EEVblog #804 - HP1740A Oscilloscope Repair - Part 2 - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: October 05, 2015, 09:33:37 pm
content: Follow Dave as he tracks down an elusive intermittent thermal fault in a HP1740A analog oscilloscope.Can you figure out the culprit before Dave does?https://teespring.com/warrantyvoid2

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 09:56:45 pm
content: Grab a towel Dave and throw over the scope and let it cook for a while.A trick the Gray Beards and Old Crones know...

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 10:26:26 pm
content: One other comment...The diode bridge should be mounted with a few mm of space between it and the board.

author: Romain
author_rank: Regular Contributor
timestamp: October 05, 2015, 11:12:41 pm
content: Quote from: EEVblog on October 05, 2015, 09:33:37 pmCongratulations Dave! Not an easy one at first sight!I also suspected the primary in the first video, but your extensive testing of voltages and currents proved me wrong. Well done again

author: Luddi
author_rank: Newbie
timestamp: October 05, 2015, 11:13:36 pm
content: broken bond-wireLEDs sometimes get a buildin blinkmode when this happens

author: HP-ILnerd
author_rank: Frequent Contributor
timestamp: October 05, 2015, 11:32:36 pm
content: Great video Dave! Nice to see you win one!Any chance of you debugging something with an old analog scope? That could be instructive!

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 11:38:42 pm
content: Dave Saves another one from the landfill. Good work.

author: EEVblog
author_rank: Unknown
timestamp: October 05, 2015, 11:43:54 pm
content: Quote from: wilfred on October 05, 2015, 11:24:38 pmI did use hot air in the previous video.By the time I narrowed it down there was no point, I knew it was the diode bridge or solder joints under.

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 11:54:13 pm
content: Quote from: EEVblog on October 05, 2015, 11:43:54 pmGood job Dave.

author: tautech
author_rank: Super Contributor
timestamp: October 05, 2015, 11:59:43 pm
content: Nice fix Dave, like you I have never seen a bridge go intermittent either.

author: EEVblog
author_rank: Unknown
timestamp: October 06, 2015, 12:04:12 am
content: Quote from: tautech on October 05, 2015, 11:59:43 pmIt could have been ugly if I didn't catch that, or it didn't play ball.

author: c4757p
author_rank: Super Contributor
timestamp: October 06, 2015, 12:09:33 am
content: That whole general area in my 1744A looks like it has been running very hot for a long time. Even yours shows a bit of discoloration in the PCB, though notnearlyas bad (my connector is brown and brittle and the PCB very dark). HP really did abuse their rectifiers in this design.

author: tautech
author_rank: Super Contributor
timestamp: October 06, 2015, 12:12:13 am
content: Quote from: EEVblog on October 06, 2015, 12:04:12 amHell yeah, you were real lucky there.

author: Radio Tech
author_rank: Frequent Contributor
timestamp: October 06, 2015, 12:27:27 am
content: Great repair Dave.Seen a lot of strange things with diode bridges but never like that one.Most I see are failed short!

author: tautech
author_rank: Super Contributor
timestamp: October 06, 2015, 12:36:51 am
content: Quote from: EEVblog on October 06, 2015, 12:04:12 amDave, could you post 2 screen shots of the un-smoothed DC waveform of of that bridge in good and failure modes.Many would be interested to see them I'm sure.

author: bushcrafter
author_rank: Newbie
timestamp: October 06, 2015, 12:43:47 am
content: Great video Dave! I really learned a lot! I'm a noob when it comes to electronics. Keep up the great work! 😁

author: bushcrafter
author_rank: Newbie
timestamp: October 06, 2015, 12:49:06 am
content: Another great video Dave! Keep up the good work! 😁

author: apis
author_rank: Super Contributor
timestamp: October 06, 2015, 01:24:04 am
content: Good work Dave, very interesting!Quote from: wilfred on October 06, 2015, 12:39:48 amTheres a whole video about that: "EEVblog #279 - How NOT To Blow Up Your Oscilloscope!"

author: dentaku
author_rank: Frequent Contributor
timestamp: October 06, 2015, 01:27:11 am
content: FinallyThat was a Shariar level long repair video that turned out well. Of course his repair videos can take many days to put together and he has access to some amazing broken stuff considering his location and job.

author: retrolefty
author_rank: Super Contributor
timestamp: October 06, 2015, 01:32:00 am
content: Good video. Troubleshooting is never as simple as one might expect, it's as much art as logic and experience is king.Chasing rabbits down the hole?My side of the pond (at least in my circle) called it chasing rabbits into the tall weeds.

author: EEVblog
author_rank: Unknown
timestamp: October 06, 2015, 01:44:38 am
content: Quote from: wilfred on October 06, 2015, 12:39:48 amThe board was pulled out. I wasn't going to replace it all just so I could do that.QuoteI've done a whole 24min video on it. This video was getting long enough already, I can't keep including detail in every video every time I mention it.

author: poot36
author_rank: Frequent Contributor
timestamp: October 06, 2015, 02:51:59 am
content: I really liked this repair video. Would it be possible for you to do a repair of the Tektronix scopes you got in a previous mailbag?

author: edpalmer42
author_rank: Super Contributor
timestamp: October 06, 2015, 04:13:10 am
content: Well done, Dave! I still can't wrap my head around how a bad diode bridge can drop the DC voltage on the capacitor without causing high ripple. Too weird!Ed

author: crispy_tofu
author_rank: Super Contributor
timestamp: October 06, 2015, 04:54:23 am
content: Wow, who could've thought it was that?LOL'd at 'Has it permanently failed... That's what we want!'

author: Xistornut
author_rank: Newbie
timestamp: October 06, 2015, 05:58:34 am
content: Dangit! I posted to the part 1 Youtube comments instead of here, could have saved you some time, Dave.See my part 1 Youtube comment. I suggested the diode bridges, and those connectors. I had almost the same problem with my HP scope, 35 years ago, and it was a flaky bridge. I see from all the comments that no one else had ever seen a flaky bridge, and, come to think of it, in 45 years+, I've never seen another one. HP must have picked up all the flaky ones, i guess. Those things are everywhere.Wonder how many of those scopes were affected. Funny none of your readers had had one come by.My bridge had a tiny crack in it, but it was also a battery/AC option scope, and the same part handled the Ni-cad charging, so it worked hard. At 250ma in your scope, tho, it just has to be flaky parts.Sorry I didn't realize that this spot is better to post. I'm registered, and I'll be good now! (P.S. Posted under D. Stuart on part 1, I choose xistornut my old ebay name here, as it's quite appropriate.)My minibio: Started hobbying at a very early age, bought my first soldering iron with money from my 8th birthday, and have been slinging solder ever since. Started working at 13 for an ex-US navy man, learning to repair FM two-way radios. (He's the one that pounded into me, "Check the power supply first!" A very useful lesson, huh? I was working there when the HP scope fizzled. Then a stereo shop, trade school, CB shop, Radio Shack as a TRS-80 (and CB) tech. Associated Press as a tech, fully trained on 100 year old teletypes. Very useful, that! Civil servant tech at a big Calif airport, repairing all sorts of cool stuff. Traffic signals, boiler and chiller controls, fiber optics, micro-controllers, CCTV, access control huge PA systems, biometrics, DVRs and so on. Retired, still hacking. Think my bloodstream's about 10% lead based.Enjoy your videos, good dissections, entertaining. Definitely the most DMMS I've ever seen used in one session! Saves a lot of poking around, and way handy when trying to spot those sneaky ones! I like.Keep up the good stuff. Looking forward to the next ones. Stu

author: MauriceS
author_rank: Contributor
timestamp: October 06, 2015, 06:01:37 am
content: The voltage regulators look awfully close to LM723's in TO-100 (metal can) package...http://www.ti.com/lit/ds/symlink/lm723.pdf

author: Xenix
author_rank: Newbie
timestamp: October 06, 2015, 06:58:29 am
content: Amazing video! Very informative and entertaining for a hobbyist like me

author: ornea
author_rank: Unknown
timestamp: October 06, 2015, 07:41:47 am
content: Great video Dave. I think I would have been scratching my head for some time on that one.After repairs always amuses me when you show the customer the faulty component and they say "Is that ALL it was?". Repairs take time to setup, tear down, go down those herring holes ,fix and put back together.

author: Stonent
author_rank: Super Contributor
timestamp: October 06, 2015, 07:47:32 am
content: Quote from: retrolefty on October 06, 2015, 01:32:00 amAt one point he said chasing red herrings down a rabbit hole.

author: sakujo7
author_rank: Contributor
timestamp: October 06, 2015, 09:04:47 am
content: Seems odd that it looked like a +15V problem...but it was a +5V bridge problem...but there wasn't a huge ripple on the output of the bridge. That 0.25V drop also seemed too low.So, current theory is that the +5V was fed mostly or entirely from the output of U2 (which can operate on its own without an external transistor), through Q5 which was acting like a diode rather than a switch. This added +250mA onto the 15V rail, causing it to hit current limit.Had the +15V been a bit beefier, this failure could have gone undetected.

author: oldway
author_rank: Super Contributor
timestamp: October 06, 2015, 09:29:55 am
content: Quote from: sakujo7 on October 06, 2015, 09:04:47 amYes, that is the right explanation of what happened.

author: German_EE
author_rank: Super Contributor
timestamp: October 06, 2015, 10:25:04 am
content: Up the garden path chasing a red herring down a rabbit hole so that you can get your winner winner chicken dinner.And they say that German is confusingAh well, a good long repair video with lots of useful tips, just try not to work on live scopes with both hands please.

author: dr.diesel
author_rank: Super Contributor
timestamp: October 06, 2015, 10:27:10 am
content: Of the many hundreds of repairs i've made, never seen a diode bridge fail like that either.

author: wojt
author_rank: Newbie
timestamp: October 06, 2015, 10:43:52 am
content: I started to suspect the bridge when the ripple trace on scope showed to be 50 Hz. After a good bridge it should be 100 Hz. Apparently one of the diodes was gone already.

author: EEVblog
author_rank: Unknown
timestamp: October 06, 2015, 11:19:01 am
content: Quote from: ornea on October 06, 2015, 07:41:47 amYep, this one could have easily taken a lot more time than it did.

author: EEVblog
author_rank: Unknown
timestamp: October 06, 2015, 11:19:30 am
content: Quote from: Xenix on October 06, 2015, 06:58:29 amThanks, glad you liked it. And welcome to the forum!

author: Rasz
author_rank: Super Contributor
timestamp: October 06, 2015, 01:15:45 pm
content: Nice one Dave.Personally I would probably start by substituting power rails with bench psu, or isolating psu and loading rails manually, but your fault was so intermittent and random it would probably not help at all in locating it.Quote from: ornea on October 06, 2015, 07:41:47 amIts a bad idea to show them the culprit from consumer relations point of view. Consumers dont understand what goes into repair, and if you show them failed part will often think they are getting ripped off, even more if you mention its $0.1. It might be good for your ego, not much else.some rants about billing customers:

author: jolshefsky
author_rank: Regular Contributor
timestamp: October 06, 2015, 01:36:53 pm
content: This is probably one of Dave's most informative videos. It demonstrates that you need to be systematic about troubleshooting.Anyone who's fixed anything fast knows the feeling of adoration when we take a guess, replace a part, and everything works—but that's kind of toxic. It makes us lazy.Dave proposed a hypothesis then tested it based on the criteria. The resistance to abandon the theory was palpable in the video—the primary-side problems seemed so promising. But when it failed to manifest when testing the unregulated voltages. Then the hypothesis that there was a downstream problem with the horiz./vert. boards was also promising, but it too was dispelled when it wasn't an over-current problem.The thing I take away from this is, "don't guess" or at least "don't take your guesses on faith." Divide the problem somewhere—the power supply is a good start: "is the power supply output okay?" If no, then you can start troubleshooting the power supply. Divide at another point: "is the transformer output good?", then another point, "is the unregulated power good?" ... and at any point you've narrowed it down to "point A is good" and "point B is bad" so you can figure there's a problem between point A and point B.Another thing—this also from just getting older and less cocky—if there is troubleshooting information, follow it. I say "from just getting older" because when I was younger, I'd think "why do are we doing things the hard way? we can skip this and that..." but now I know enough to recognize when someone is more experienced than I, and to just do what I'm told until I understand why it's done that way. Troubleshooting information just might be written by someone with more experience than you—particularly with what you're currently working on. So use it until you hit a dead-end.

author: station240
author_rank: Unknown
timestamp: October 06, 2015, 01:42:53 pm
content: You were kinda right about a current limit being involved, just it was the 50% limit as a result of half the rectifier being out of the circuit. The meter test setup shows the current drops in half at the same time the voltage does.My guess is the diode bridge is physically constructed with the 4 diode dies bonded between the pins, so it's possible for one diode to loose connection. Maybe the pins got bent when it was originally installed.Btw I do have a faulty diode that behaves like this, its one of the big bolt in ones and the input terminal got twisted so it's now loose.

author: vindoline
author_rank: Unknown
timestamp: October 06, 2015, 02:01:07 pm
content: Great video. I love the troubleshooting videos!

author: DanielS
author_rank: Frequent Contributor
timestamp: October 06, 2015, 04:19:30 pm
content: I have seen diodes fail open or short. Intermittent though is something new.

author: boffin
author_rank: Unknown
timestamp: October 06, 2015, 05:21:07 pm
content: Like many others, I'd love to see the output of that bridge, pre and post failure on a scope.Any chance you can hook it up to 9VAC, give it a load, and slap the Rigol on it and take a couple of screen shots?I assume one of the four diodes in the bridge goes open?

author: retrolefty
author_rank: Super Contributor
timestamp: October 06, 2015, 06:26:40 pm
content: Quote from: boffin on October 06, 2015, 05:21:07 pmI'm not so sure. Recall the ripple voltage on shown on the scope was not bad when the voltage output dropped from the bridge. Again it seemed to just go high resistance which I too had never recall coming across. But the test you suggest would still be worth seeing.

author: jancumps
author_rank: Unknown
timestamp: October 06, 2015, 06:49:17 pm
content: Quote from: jolshefsky on October 06, 2015, 01:36:53 pmOr even "that you need to be analytical...". A fully systematic person would have followed the service manual first straight down into that rabbit hole

author: tautech
author_rank: Super Contributor
timestamp: October 06, 2015, 07:20:30 pm
content: Quote from: retrolefty on October 06, 2015, 06:26:40 pmI too wanted to all to see the frequency of that ripple. I missed it as Dave had V/div set very low.wojtspotted it:Quote from: wojt on October 06, 2015, 10:43:52 am

author: HighVoltage
author_rank: Super Contributor
timestamp: October 06, 2015, 07:47:40 pm
content: Quote from: wojt on October 06, 2015, 10:43:52 amVery good point, wojtGreat fixing video!Thanks Dave

author: Ampere
author_rank: Regular Contributor
timestamp: October 06, 2015, 08:16:44 pm
content: What a strange problem. I was under the impression that diodes could only fail open. A catastrophic failure or none at all.

author: pickle9000
author_rank: Super Contributor
timestamp: October 06, 2015, 08:19:14 pm
content: @DaveSeeing as how the bridge made for such an entertaining video why not frame it and hang it on the wall with the episode number?These troubleshooting skills are what makes a good engineer, hobbyist, repair person. You cover a great deal in the video, an excellent job. Congrats.

author: BFX
author_rank: Frequent Contributor
timestamp: October 06, 2015, 08:28:25 pm
content: Finally most interesting video.My suggestion is that you could make one repair video blog every 1-2 months

author: nfmax
author_rank: Super Contributor
timestamp: October 06, 2015, 08:58:32 pm
content: I see from one of my old lab notebooks that on 9th July 1996, I replaced a faulty bridge rectifier in the low voltage PSU of my HP 1740A. For one minute I thought it was the exact same part, but no, it was A16CR4, on the +15V supply. My unit has serial number 1751G03795; what is the serial number of the unit you fixed? The bridge rectifier solder joints all look OK on mine (however I still need to fix the delayed timebase!)

author: open loop
author_rank: Regular Contributor
timestamp: October 06, 2015, 09:47:09 pm
content: Never seen a diode bridge fail in that way, been fixing stuff for years as a hobbyist and a pro...

author: jh15
author_rank: Frequent Contributor
timestamp: October 06, 2015, 11:31:47 pm
content: 1. Put each internal diode on a curve tracer. Heat it or load it to self heat. Show us it opening in real time.2. In the 70's I came across a pro recording problem with the bridge doing exactly the same thing. This was a portable Revox 15ips on a DBX pro system, so noise floor was around -90db.We kept hearing an elusive 120hz hum in the recordings, ruled out line balance, shields, etc, not 60hz anyway. No light dimmers, etc. Puzzling...Then I said turn off the noise reduction... the tape machine itself was quietly showing some intermittent 120hz down in the tape hiss. The DBX was pushing the noise down most of the time.It was a bridge rectifier exactly like your form factor.To double check, a cheap hair dryer showed it up repeatedly.A hair dryer and freeze spray shortened many a troubleshooting time in my days of very-low-scale ic's and through hole components.I also would have tried some percussive troubleshooting early on.

author: AF6LJ
author_rank: Unknown
timestamp: October 07, 2015, 03:08:58 am
content: Quote from: Rasz on October 06, 2015, 01:15:45 pmThat totally depends on how you handle it. I have found that customers can appreciate the work that goes into troubleshooting when they see the insides of a similar piece of gear.You get the "that looks complicated, I couldn't possibility do that" from a third the next third just gets glassy eyed.Of that last third only a small number complain about paying the repair bill.

author: han
author_rank: Frequent Contributor
timestamp: October 07, 2015, 03:45:57 am
content: ThanksAF6LJ for the video link.

author: bitwelder
author_rank: Super Contributor
timestamp: October 07, 2015, 05:04:59 am
content: Thanks Dave! Very 'satisfying' troubleshooting video.About the weird fault of the bridge rectifier, I'd like to see a follow-up video with the characterization of that component (in its failure mode). Are all diodes failing inside it degrading at the same time (or perhaps at same temperature)?

author: yym
author_rank: Contributor
timestamp: October 07, 2015, 07:21:24 am
content: So what was the problem with the other rails?In the manual it said that the 15v is the reference for all the others so how a failed 5v rail caused all the others to fail?

author: tautech
author_rank: Super Contributor
timestamp: October 07, 2015, 07:47:37 am
content: Quote from: yym on October 07, 2015, 07:21:24 amRead this post and the next one carefully:https://www.eevblog.com/forum/blog/eevblog-804-hp1740a-oscilloscope-repair-part-2/msg771391/#msg771391

author: VK5RC
author_rank: Unknown
timestamp: October 07, 2015, 08:00:30 am
content: I really liked the smile Dave had after the problem found, solving a problem like that gives you a real sense of satisfaction.Well done Dave , educational and entertaining.

author: retrolefty
author_rank: Super Contributor
timestamp: October 07, 2015, 11:09:20 am
content: Quote from: EEVblog on October 06, 2015, 11:19:01 amBut Dave, you went one quite a bit in that 2nd video how much faster youcouldhave located the problem if you weren't filming the effort. Both those statements can be true, so not too useful observation. It always takes what it takes.Just an observation from an old grey beard troubleshooter, you seem to jump between steps/decisions too quickly and spend too little time analyzing each step/decision, resulting in too many trips down the hole/tall weeds.Sometimes that can't be helped due to no documentation to work from, but in this example you had full manual/schematic to work with and help with the best troubleshooting steps and checks.This is all meant in the spirit constructive feedback. In this case we still don't have a good enough post analysis of why this intermittent failed rectifier is giving the symptoms seen on all voltage rails, further information is needed on the component failure mode and how that effects the rest of the power supply voltages. Not many of us could even attempt to try and troubleshoot something while filming it in real-time. I still really enjoyed this video and thank you for it's creation, I personally can't talk and troubleshoot/think at the same time, my mind does not allow that skill.

author: SeanB
author_rank: Super Contributor
timestamp: October 07, 2015, 04:01:07 pm
content: I have had plenty of rectifier diodes die, either short ( most common, though the fix then is often not going to happen as something will have smoked), open circuit or like Dave's thermally.Probably internally the one die was not properly bonded to the wire end, and then with time and thermal cycling, it finally finished cracking off and then there was separation from the bond with heat generated in the die. You see this with LED's which are run at a high current, where they will flicker as the die undergoes thermal cycling with a cracked bond wire end. Typically to do this take a cheap 5mm white LED ( cheapest eBay specials are best) and take a string of around 20 with the leads slightly bent open on inserting into the breadboard so the die is stressed. run at 40mA and leave for a day or so to get one or two that flicker, though the whole series string will do it.

author: vasonline
author_rank: Contributor
timestamp: October 07, 2015, 04:24:50 pm
content: I once had a bad diode in my Mvison T4 Digital Terrestrial TV receiver.Sometimes the tuner stopped working. At random it seemed.The rest of the unit, OSD, playback from USB still worked.Checked PSUFound that they used two diodes in series to drop the voltage to the tuner's microprocessorOne of the diodes dropped 2-3V instead of 0.7VBut if you let the T4 cool down a bit it started to work again.Indeed, after shutting off the T4 a bit, the diode dropped the expected voltage of 0,7VReplaced diode, and good ever since.Greetings

author: rsjsouza
author_rank: Super Contributor
timestamp: October 07, 2015, 07:44:56 pm
content: Quote from: SeanB on October 07, 2015, 04:01:07 pmThanks for that SeanB.I was starting to think that I was the only one that saw this more than once (about 10~20 times throughout the years).

author: hollor
author_rank: Newbie
timestamp: October 07, 2015, 08:09:21 pm
content: Looking at the schematic once again, one possibility why all the rails dropped seems to be that some of the regulation circuitry (for instace U3 on the -15V rail) is fed by the faulty +5V.I've seen a similar topology on my Iwatsu OS-6612 DSO. All the rail regulation is referenced to one single rail voltage and I accedentally damaged the scope than I shorted the +5V rail while troubleshooting. oops..Thanks for the great repair video, very educational!

author: oldway
author_rank: Super Contributor
timestamp: October 08, 2015, 06:06:41 am
content: Quote from: hollor on October 07, 2015, 08:09:21 pmThat's not right: if +5V is faulty as it was (only 1V drop), it will not change working of U3.Moreover, this would not explain why +15V voltage also drop.The right explanation has already be done by sakujo7...Please read his post !

author: vlad777
author_rank: Frequent Contributor
timestamp: October 08, 2015, 11:52:24 am
content: Come on MAN!!Send that heavy thing to Europe.I'll tell you what country.You already have too many scopes

author: guenter
author_rank: Newbie
timestamp: October 09, 2015, 12:17:57 pm
content: Hello Dave,connect the rectifier to an componenttester of your scope. You will see a jumping line on the faulty diode.

author: rdl
author_rank: Super Contributor
timestamp: October 09, 2015, 03:07:47 pm
content: What's inside that selectable voltage AC power supply?

author: Rasz
author_rank: Super Contributor
timestamp: October 09, 2015, 04:12:47 pm
content: Quote from: guenter on October 09, 2015, 12:17:57 pmand later decap it in nitric/sulfuric acid, whole episode right there

author: chris_leyson
author_rank: Super Contributor
timestamp: October 09, 2015, 07:50:51 pm
content: Nice one DaveSend it back to GE and get them to do a failure analasys