Hickock OSC-4 oscilloscope troubleshooting.

[maxhifi]

I've been doing a lot of service work lately, and my oscilloscope has got worse and worse. Basically, keeping it in synch has become extremely critical. I have to mess with the Vernier and sync amp a lot to get it to be stable, and then when the amplitude of the signal changes, it again goes out of synch. I did a quick replacement of all the paper capacitors today, but the electrolytics are still original. Also, I have tried swapping tubes around, and replacing them, to no end.

Of course, I would like to check the waveforms the synch amp is making, but unfortunately this is the only scope I have.

Any help would be appreciated, it's my first time working on the scope.

Ref: https://www.oscilloscopemuseum.com/o...4s40-1437.html

 

[zebulon1]

I love my test equipment and I have reasons for using the models I have, regardless of others opinions.
So I have to ask. What is your reason to keep this particular one in cal?

 

[maxhifi]

I love my test equipment and I have reasons for using the models I have, regardless of others opinions.
So I have to ask. What is your reason to keep this particular one in cal?

I've had it since the early 1990s, and it's always been my eyes and ears when troubleshooting. I have tried newer scopes, but I really prefer the big round screen, and the control layout of this one. With audio stuff you don't need a high end scope to see signal distortion, or power supply ripple, etc.

I also like how it has a proper attenuator on the input, and some nicer features than my extremely basic EICO 425. The 425 definitely wins in appearance though. I have been studying how scopes work, and I believe the issue is called over synchronization. I dug out the old EICO, and amazingly it actually works after not being used for almost 20 years. I did change the filters back in the 1990s. I think I can use the EICO to check the sync waveform in the OSK-4, and get things sorted out.

 

[elnaldo]

I've seen the picts you linked, and the device is full of those old carbon resistors, which use to drift from specs, sometimes a lot. I'd start measuring the resistance in circuit (so you don't butch the unit ), every resistor, checking if any of them measure MORE than the rated value. If it measures more, that's not OK. Check specially the plate resistors at the tubes. They use to fail.

Also, replacing the capacitors is a must in such an old device. Just one of the caps with leaks or out of specs could affect the way the unit works. This are precision devices, so everything should be withing specs.

You report that the unit works, but not stable. That could mean some component out of specs but not totally ruined.

 

[maxhifi]

So it turned out to be a bad main filter capacitor. It was only noticeable in the sync amp, because the other amplifiers are push-pull, and cancel hum, while the sync take off point is single ended and does not cancel hum.

All it took was probing the sync input to the oscillator with my EICO 425, and then observing the massive power supply ripple on top of the sync signal.. The ripple was causing the scope to not be able to synchronize properly with any signal. I temporarily connected a capacitor in parallel with the bad filter capacitor, and suddenly everything works properly.

This should have been more obvious to me, but since the scope makes a straight green line, I just assumed the filters must be OK - well, live and learn! I will proceed with a full replacement of all the electrolytic capacitors, and then follow the calibration instructions.

I'm going to restore the EICO 425 while I am at it, it did a great job helping me diagnose the OSK-4.

 

[zebulon1]

It's always so obvious after you figure it out. But you know that I'm sure.
Good job and now your very cool o scope is back up and running.

 

[maxhifi]

It's always so obvious after you figure it out. But you know that I'm sure.
Good job and now your very cool o scope is back up and running.

Thank you, I am happy to have it up and running. And now I know how simple oscilloscopes work. The EICO 425 is actually pretty nice too, better than I gave it credit for. It's interesting how primitive it is, it doesn't blank the retrace line, and there is no limiter on the sync signal.

Now that the scope is sorted out, I can get back to the hi-fi equipment which is getting piled up. I doubt I will ever want new test equipment, the old tube test gear has so much personality, and is great to use.

 

[elnaldo]

Good job ! I'd check the plate resistors, just in case, or replace them all. They use to go out of specs.

 

[maxhifi]

Good job ! I'd check the plate resistors, just in case, or replace them all. They use to go out of specs.

Will do! Thankfully the documentation in my first post makes it easy to spec replacement parts. Makes me think I should find the time to go through my VTVM, and other vintage test gear.