HP1707A oscilloscope restoration

[picofarad]

Hello,
some of you may remember that 7 years ago I purchased a HP 1707A oscilloscope, which worked for a little bit and then stopped working. I also wrote a topic looking for help for its repair (https://www.eevblog.com/forum/repair/help-with-hp-1707a-oscilloscope-repair/). Now, it's been almost 5 years since I put my hands on the scope, a lot of things have changed (like getting a EE degree ahah) and I've acquired other gear.
Finding myself stuck in the current lockdown situation, I've decided to start from scratch with A COMPLETE RESTORATION of the oscilloscope. So this thread will be both a journal for the whole trip and also a place where to ask for help (my EE knowledge expanded through the years, but I can't know everything!).

What is wrong with the scope:
- Power supply: in the original post from 7 years ago, the main suspect was the HV section; now I think it's probably a problem in the LV converter board.
- some pot knobs are missing/disintegrated/in bad shape. For sure, one vertical amplitude knob, the trigger knob and the time base selector knob need to be replaced (they are missing).
- internal cabling checking needed: the almost 50 yo heatshrink is slowly melting away.

Available test equipment for the project:
- several multimeters, lab power supply, function generator, one channel 10 MHz scope, one of those 20$ all in one component testers, an ESR meter, an octopus component tester (most of this stuff is DIY).

I also found the service manual for this very scope. Here's the link: http://hparchive.com/Manuals/HP-1707A-Manual-75Mhz-Scope.pdf

In the following days/weeks I will post updates.

[christopher iles]

Hello,
I have an HP1741A lying in my workshop. It doesn't work, powers up and there is a spot on the screen which indicates that eht is there but no timebase.
Maybe some parts may fit your project.
Kind regards,
Chris, Somerset, England.

[picofarad]

Thanks for your reply.

So, while i'm waiting for some components, i started modelling some of the missing knobs in order to eventually 3D print them. The first one is the voltage attenuator knob. I used Freecad to model it, and i have to say, it's a great pain to work with. But i eventually figured out... Here's a screenshot of the result:



I don't plan to use a brass insert as the original did, mainly because i have no idea where to get them.

Other than that, until now I've completely dismantled the scope and started checking the cabling and replacing some heatshrink tubing.

[JKKDev]

I don't want to put you down but I believe there's no way you'll be able to 3D print that knob and have it look half decent (huge overhang, small sharp points,...). Maybe consider something like this. Of course it won't look original but at least they will have a good feel to them. If they are not the correct diameter look for "brass insert knobs" on eBay. There's a metric s***ton of them Good luck with the repair tho

[dzseki]

Don't underestimate the brass inserts. You need small screws to fix the knob: will the plastic knob hold the screw? No...

[picofarad]

Indeed using brass insert would be way better. I really can't find them on ebay, the only viable way seems to be buying cheap knobs and extracting the insert using a soldering iron...

I don't want to put you down but I believe there's no way you'll be able to 3D print that knob and have it look half decent (huge overhang, small sharp points,...).

Well, for now i'm just trying. I don't have a 3D printer at home (i have one at work), so i think i'll have to do some prototypes first. That type of knob you suggested wont work, unfortunately, as the pot it's mounted on is a double one, so there's also another small knob on top of it.

[tunk]

I have used set screws in 3D printed parts. E.g. make a 2.5mm hole,
after printing drill it out to 2.5mm and then use a M3 thread tap.
If it will work for daily use over years is another question.
How accurate is that 3D printer? Unless it is very accurate, you
may want to experiment with the diameter of the center hole so that
it fits snugly. The wing may possibly be prone to breakage.

[picofarad]

More modelling: the timebase selection knob. The knob was totally missing, but from some photos i figured out it is the same as the trigger channel selection one. So here's the model:




Still don't know how it will print... I will put the STL files here anyway, maybe they'll be useful to someone else.