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