Hi - I'm not an EE, just a retired hobbyist, so this might be quite uninformed. I got an 80's-something Tektronix 2445 Scope with two of the four channels working, but some problems with the on-screen text overlays and channels 1 and 2. My "visual inspection" discovered an exploded capacitor on the main power board. So the fact that anything works at all seems a bonus. But before I order and replace some caps, perhaps I need to understand the circuit a bit better.

Part of the power schematic is attached, with the blown 0.068uF cap highlighed. It sits on the mains side of the rectifier diodes And it has another similar mate to the left on the schematic, before the chokes.

1. Can someone explain what the components to the left of the bridge rectifier are trying to do? (The E1001 and E1002 components are gas-filled surge protectors).
2. They have polarity, so that confuses me. And they're across a 230v mains supply. Maybe that is why this one blew up.
3. The oscilloscope has a selectable 115v/230v mains supply. I reckon the 250v rating on the capacitors (according to the replacement parts list in the repair manual) is probably pretty safe in the USA with 115v supplies, but at 230V there is little margin for error. Any comment on this is welcome.

If I replace the two caps with some 0.068uF caps rated at 400v or 600v, does this circuit demand something special: e.g. a polarized capacitor that can withstand more-than-usual reverse voltage? The two original components are quite large solid blocks rather than in metal cans.


Thanks for any advice.
Peter

 

Those components are there to filter the mains coming in to remove anything that the 'scope circuits wouldn't like and also to filter out any interference from the 'scope circuits escaping along the mains wire and upsetting other equipment.

The capacitor should be replaced with a 250VAC X-rated capacitor. The X-rating menas it is suitable for connection across the AC supply.
It is unlikely that replacing this capacitor will fix the other problems with the 'scope.

 

Those components are there to filter the mains coming in to remove anything that the 'scope circuits wouldn't like and also to filter out any interference from the 'scope circuits escaping along the mains wire and upsetting other equipment.

The capacitor should be replaced with a 250VAC X-rated capacitor. The X-rating means it is suitable for connection across the AC supply.
It is unlikely that replacing this capacitor will fix the other problems with the 'scope.

Thanks Albert. Got me reading a bit more about capacitor X and Y ratings, etc. so it was most useful.

 

And then there are XY caps.

Low value resistors in series with a cap can be thought of as a "snubber". Switch arc suppression or supression radiation into the mains.

The resistor in parallel with an inductor should tell you something, The current in an inductor can't change instantaneously; similarly the voltage across a capacitor can't change instantaneously. So, the LC circuit's primary purpose is probably inrush current limiting.

 

So, the LC circuit's primary purpose is probably inrush current limiting.

They are much too small to have much impedance at 50/60Hz - they are for much higher frequencies.
Inrush current is limited by RT1010 and RT1016, which may well be NTC. Possibly that big black disc to the right of the bridge in your photo.
https://en.wikipedia.org/wiki/Inrush_current_limiter

 

They are much too small to have much impedance at 50/60Hz - they are for much higher frequencies.
Inrush current is limited by RT1010 and RT1016, which may well be NTC. Possibly that big black disc to the right of the bridge in your photo.
https://en.wikipedia.org/wiki/Inrush_current_limiter

Thanks again Albert, I think this nails it. I went back to the service manual, the big black disk is RT1016 in the schematic, described as Resistor, Thermal, 5 Ohm, 7A/Deg C with manufacturer Rodan Industries code SG-6. The SG seems commonly used for Surge Guard. (I didn't get the meaning of 7A/Deg C, though). By contrast, RT1010 (the T is obviously for Thermal) has a more understandable 3.9%/ Deg C specification.