EEVblog #803 – HP1740A Analog Oscilloscope

Dave saved this classic from the dumpster, a HP 1740A 100MHz dual channel analog oscillocope from the mid 1970’s.
It wasn’t supposed to be working so this is a look at the problem, a mini teardown, and some basic PSU troubleshooting.
Update: Even after 4 hours it still doesn’t fail with the covers off. So likely to be some sort of thermal issue as suspected.
Dave saved this classic from the dumpster, a HP 1740A 100MHz dual channel analog oscillocope from the mid 1970’s.
It wasn’t supposed to be working so this is a look at the problem, a mini teardown, and some basic PSU troubleshooting.
Update: Even after 4 hours it still doesn’t fail with the covers off. So likely to be some sort of thermal issue as suspected.
Forum HERE

Acknowledgments
The 1740A design group was led by Stan Lang until the start of pilot production when he transferred to another project. The design team included Jim Garner, mechanical design including the vertical attenuator switch, Eldon Cornish, who designed the horizontal section, and Van Harrison who designed the CRT circuits, power supplies, and gate amplifier.

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26 comments

  1. Hi Dave,

    HP’s competition to the Tek 465. In some ways superior.

    I have an absolutely mint one of those that one of my best friends left me when he passed over 20 years ago. He bought it new from HP. I even have the original shipping box.

    I would try the blanket and hair dryer technique to snake out any possible thermal issues. Too bad you are not in the USA, I would drop by and give it a go. I have worked on quite a few HP scopes in this series. Mostly aged out of tolerance resistors, bad caps (tantalums and Sprague C30’s) and once in a while a transistor.

    Good luck with it. It is a good analog scope.

    Sam
    W3OHM

  2. Oh, wow. A 1979 date code from a TI low-power (L, not LS) TTL chip. I was a new hire, green as grass. Every Tuesday morning, a product marketing engineer in the Digital Circuits Division, about 12 feet away from my desk, would read out the piece parts loading to the manager in El Salvador, in Spanish and VERY LOUDLY (because the telephone connection was poor). That would determine the quantity of each TTL type that would go through packaging and test that week.

  3. As I just suggested on gooogle+ could the fault be due to the mains input voltage selection switch or circuitry.
    If we are intermittently switching out or in some of the mains side transformer coils, because the voltage selector switch is bad, that might result in the output side voltages
    dropping.
    Might be worth spraying all of those iffy contacts with contact cleaner before you start going down too many other rabbit holes.

  4. Hi! Did I miss the results of the battery discharge to zero volts video?

  5. Hello Dave,

    You can use your Flir thermal camera to find hot spots on boards and look for potential points of failure. Maybe that helps.
    Other suggestion: if the inside of the scope look clean, maybe somebody else tried to it earlier and made it worst. Maybe is good idea to find fresh solder joints and compare changes with schematic.
    Great video, thanks!

  6. Thinking similar to Andrew Hull , watching power consumption may be a clue to unhealthy transformer behavior .
    Was it quiet enough to hear any signs of distress ? ( Check fuses not over sized ! )

  7. Hi Dave,

    I would suggest to check the solder joints of board-to-board connectors first.
    I also have a 1740 and had a 1741 too. They both produced miscellaneous errors due to cracked solder.

    Cold solder joints are the common issues of these HP scopes.

    (See this: http://www.eevblog.com/forum/testgear/hp-1741a-oscilloscope-restoration-project/30/)

    Best regards!

  8. Sniff it, Dave. I would have thought you have a nasal adapter with a pipe that you can direct to a PCB to sniff things out. Come on, we all know your olfactory skills. 🙂

  9. I am very familiar with the Tek 465. The feature set seems about the same as this HP scope and the block diagram appears to be a duplicate. I really like the looks of the mechanical design of this HP scope. For example, pulling boards on the 465 requires use a of soldering iron to detach wires. In my opinion, the timebase switch on the 465 is a stunning piece of industrial design. Might be interesting to do some side by side tear downs of competitive vintage (or modern) gear and compare. Many people might be surprised by how much they will have in common.

  10. Hi, nice collector !
    I would consider checking the main transformer for a small short-circuit when its temp gets above certain threshold. Maybe using the Flir camera, with the body wrapped inside cellophane, so you get thermal reading with almost the same convection as if the case was closed.
    Also would use a scope instead of the meters in order to see the noise on the various power lines.
    But of course you’d need to reproduce with the cover open first, as it looks like related to temperature (and that’s where the cello might help),
    (Maybe don’t neglect gravity too, you test it open upside-down. The chemical gels in the caps, or a short-circuit in the transformer might be sensitive to that…)

    Otherwise you’d have to follow the repair manual: disconnect all from the PSU, connect dummy loads, and see what happens ! Disconnect/reconnect one by one… a PITA
    Best regards,
    Steve

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