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Old 08-11-2018, 06:49 PM
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maxhifi maxhifi is offline
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Hickock OSC-4 oscilloscope troubleshooting.

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
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Old 08-12-2018, 02:32 PM
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The electrolytics should be replaced next. Try control cleaning spay, some use WD-40, on the pots. You might want to not do this on the brightness and focus pots because they carry high voltage. Then check the resistor values.
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Old 08-12-2018, 09:41 PM
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Cool, I did clean the pots, will do the caps next. The issue is more like when a TV has a very critical horizontal hold - the waveform can appear double, or totally out of sync, and then pop into sync with a weird setting of the controls, for example when the horizontal gain is way up

I did remember I do have another scope, an old EICO from the 1950s. I dug it out of the garage, and unbelievable it works properly (I partially recapped it about 25 years ago) .

I think it may be useful to use the EICO to figure out what's going on inside the OSK-4.
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Old 08-13-2018, 12:34 PM
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So after doing some reading on the subject, it looks like the issue is "oversynchronization", as per the image on page 36 of this book:

http://www.tubebooks.org/Books/smith_oscilloscope.pdf

I'm going to have to dig back into it and figure out why that's happening. At least I have the EICO 425 on my side now, to assist with troubleshooting.

I read another document by tektronix, about oscilloscope troubleshooting, and it says to isolate the problem with the cover on - good advice I think, I never recap before troubleshooting, but this one I tried to do a quick fix, and ended up just knocking it out of calibration.
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Old 08-15-2018, 10:44 PM
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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.
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