EEVblog #804 - HP1740A Oscilloscope Repair - Part 2

[EEVblog]

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

[AF6LJ]

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...

[AF6LJ]

One other comment...
The diode bridge should be mounted with a few mm of space between it and the board.

[Romain]

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?

Congratulations 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 

[Luddi]

broken bond-wire
LEDs sometimes get a buildin blinkmode when this happens

[HP-ILnerd]

Great video Dave!  Nice to see you win one!
Any chance of you debugging something with an old analog scope?  That could be instructive!

[AF6LJ]

Dave Saves another one from the landfill. Good work. 

[EEVblog]

I am wondering why you didn't get a bit of hot air onto it if you thought it was possibly a thermal issue. You weren't shy about using the freezer spray a while back. If hot air came in a spray can would you have tried it?

I 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.

[AF6LJ]

I am wondering why you didn't get a bit of hot air onto it if you thought it was possibly a thermal issue. You weren't shy about using the freezer spray a while back. If hot air came in a spray can would you have tried it?

I 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.

Good job Dave.

[tautech]

Nice fix Dave, like you I have never seen a bridge go intermittent either. 

[EEVblog]

Nice fix Dave, like you I have never seen a bridge go intermittent either. 

It could have been ugly if I didn't catch that, or it didn't play ball.

[c4757p]

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 not nearly as bad (my connector is brown and brittle and the PCB very dark). HP really did abuse their rectifiers in this design.

[tautech]

Nice fix Dave, like you I have never seen a bridge go intermittent either. 

It could have been ugly if I didn't catch that, or it didn't play ball.

Hell yeah, you were real lucky there. 

[Radio Tech]

Great repair Dave. 
Seen a lot of strange things with diode bridges but never like that one.
Most I see are failed short!

[tautech]

Nice fix Dave, like you I have never seen a bridge go intermittent either. 

It could have been ugly if I didn't catch that, or it didn't play ball.

Dave, 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.

[bushcrafter]

Great video Dave! I really learned a lot! I'm a noob when it comes to electronics. Keep up the great work! 😁

[bushcrafter]

Another great video Dave! Keep up the good work! 😁

[apis]

Good work Dave, very interesting!

As a viewer more likely than most to blow up a scope by incorrectly grounding the probes I also think you should explain the problem just a little more. I don't think someone who didn't know the problem would have realised they need to find out more just from the brief comment alone that you made. I understand you don't want to bore your experienced viewers.

Theres a whole video about that: "EEVblog #279 - How NOT To Blow Up Your Oscilloscope!" 

[dentaku]

Finally 
That 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.

[retrolefty]

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. 

 

[EEVblog]

But, I was also thinking of trying to induce the problem on command  in the subsequent test setup.

The board was pulled out. I wasn't going to replace it all just so I could do that.

As a viewer more likely than most to blow up a scope by incorrectly grounding the probes I also think you should explain the problem just a little more.

I'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.

[poot36]

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?

[edpalmer42]

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

[crispy_tofu]

Wow, who could've thought it was that? 
LOL'd at 'Has it permanently failed... That's what we want!' 

[Xistornut]

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

[MauriceS]

The voltage regulators look awfully close to LM723's in TO-100 (metal can) package...

http://www.ti.com/lit/ds/symlink/lm723.pdf

[Xenix]

Amazing video! Very informative and entertaining for a hobbyist like me

[ornea]

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.

[Stonent]

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. 

At one point he said chasing red herrings down a rabbit hole.

[sakujo7]

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.

[oldway]

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.

Yes, that is the right explanation of what happened.

[German_EE]

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 confusing 

Ah well, a good long repair video with lots of useful tips, just try not to work on live scopes with both hands please.

[dr.diesel]

Of the many hundreds of repairs i've made, never seen a diode bridge fail like that either.

[wojt]

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.

[EEVblog]

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.

Yep, this one could have easily taken a lot more time than it did.

[EEVblog]

Amazing video! Very informative and entertaining for a hobbyist like me

Thanks, glad you liked it. And welcome to the forum!

[Rasz]

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.

After repairs always amuses me when you show the customer the faulty component and they say "Is that ALL it was?".

Its 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:

[jolshefsky]

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.

[station240]

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.

[vindoline]

Great video. I love the troubleshooting videos! 

[DanielS]

I have seen diodes fail open or short. Intermittent though is something new.

[boffin]

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?

[retrolefty]

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?

 I'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.

[jancumps]

This is probably one of Dave's most informative videos. It demonstrates that you need to be systematic about troubleshooting.

...

Or even "that you need to be analytical...".  A fully systematic person would have followed the service manual first straight down into that rabbit hole

[tautech]

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?

 I'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.

I too wanted to all to see the frequency of that ripple. I missed it as Dave had V/div set very low.

wojt spotted it:

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.

[HighVoltage]

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.

Very good point, wojt

Great fixing video!
Thanks Dave

[Ampere]

What a strange problem. I was under the impression that diodes could only fail open. A catastrophic failure or none at all.

[pickle9000]

@Dave

Seeing 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.

[BFX]

Finally most interesting video.
My suggestion is that you could make one repair video blog every 1-2 months
 
 

[nfmax]

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!)

[open loop]

Never seen a diode bridge fail in that way, been fixing stuff for years as a hobbyist and a pro...

[jh15]

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.

[AF6LJ]

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.

After repairs always amuses me when you show the customer the faulty component and they say "Is that ALL it was?".

Its 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:

That 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.

[han]

Thanks AF6LJ for the video link.

[/size]

[bitwelder]

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)?

[yym]

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?

[tautech]

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?

Read this post and the next one carefully:
https://www.eevblog.com/forum/blog/eevblog-804-hp1740a-oscilloscope-repair-part-2/msg771391/#msg771391

[VK5RC]

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.

[retrolefty]

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.

Yep, this one could have easily taken a lot more time than it did.

 But Dave, you went one quite a bit in that 2nd video how much faster you could have 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. 

 

 

[SeanB]

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.

[vasonline]

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 PSU

Found that they used two diodes in series to drop the voltage to the tuner's microprocessor
One of the diodes dropped 2-3V instead of 0.7V
But 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 0,7V

Replaced diode, and good ever since.

Greetings

[rsjsouza]

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.

Thanks 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).

[hollor]

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!

[oldway]

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!

That'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 !

[vlad777]

Come on MAN!!
Send that heavy thing to Europe.
I'll tell you what country.
You already have too many scopes 

[guenter]

Hello Dave,

connect the rectifier to an componenttester of your scope. You will see a jumping line on the faulty diode.

[rdl]

What's inside that selectable voltage AC power supply?

[Rasz]

Hello Dave,

connect the rectifier to an componenttester of your scope. You will see a jumping line on the faulty diode.

and later decap it in nitric/sulfuric acid, whole episode right there

[chris_leyson]

Nice one Dave

Send it back to GE and get them to do a failure analasys