EEVblog #804 - HP1740A Oscilloscope Repair - Part 2

[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