Hung Chang 5510 100MHz Oscilloscope repair

[Flavius]

Hello, I got my first oscilloscope from my uncle, but channel 1 is very finicky and unreliable.
Most of the time it simply doesn't work, nothing showed up but sometimes it does show up as an straight GND lines and that's it, when it does show up its not coming off the display although ch1 isn't selected unless other channel is selected.
Very rarely (2 times) it does work normally, and it registers the probe, but when no channel is selected its not coming of the vcr display.

I've also attached some schematics that I found online, incase that does help.

Thanks!

[Flavius]

I should also mentions that the vertical positioning doesn't work.

[Grandchuck]

Does the vertical positioning work on channel 2? 
Do you have the experience to work safely?
Can you measure the supply voltages and verify they are correct?

[Flavius]

The vertical positioning does work on ch2.
I sure do have the experience to work safely with high voltages.
I measured the supply voltages and they are correct.

I highly suspect that the vault is in the attenuator, perhaps some relay. I believe tilting and "smacking" the oscilloscope does help fix the problem.

[C-47]

No specific guidance, except that environmental conditions in Indonesia can tarnish / corrode exposed selector switch contacts as I see potentially existing on your unit. A good quality switch / contact cleaner (not WD-40) and exercising it can sometimes work wonders. Good luck with the restoration.

[Flavius]

Thanks! Sure will try to check each and every selector switch!

[Zenith]

This sounds very much like dirty or tarnished switch contacts. It's a common problem with old scopes.

You may need to partially dismantle the scope to reach the switches properly. I've found it's better to apply switch cleaner to rotary switches with a cotton bud and that way you remove the dirt rather than just displace it. Very often the slide switches are enclosed and can't easily be opened but there's usually a hole or two through which you can push the extension tube of the contact cleaner can and squirt in contact cleaner, and then operate the switch 20 or 30 times.

I'm not keen on spraying contact cleaner at switches with lots of it ending up on twitchy RF circuitry.

Other things that can cause problems are bad soldered joints, which aren't always easy to spot. The flexing of the joint when the switch is operated can cause it to fatigue in time.

It's far easier to fix a scope if you have proper schematics. I don't know whether they were released for that model.

[Flavius]

Thanks a lot for the recommendations! I sure have done lots of research regarding to the schematics but doesn't seems to find them anywhere. I've been also checking each switch and selector using a multimeter, it's sure is lots but I'm working on them slowly but surely. Cheers!

[Zenith]

You might try emailing Hung Chang, or Protek (with whom they seemed to have a close association) for the service manual. Certainly some Protek products seemed to be rebadged Hung Chang.

https://www.gsi-protek.com/untitled-c1kt5

I get the impression they were never highly focused on support and that analog scopes are now a fading memory, but it has to be worth a try.