Heathkit Oscilloscope Burned Out Display Tube - Page 1
Forum section: electronics / repair

author: enteranickname16
author_rank: Newbie
timestamp: July 04, 2022, 09:55:00 pm
content: I have hit a bit of a dead end on my project of restoring an old Heathkit OM-2 oscilloscope. Somehow someway, the CRT's filament got burned up. When I was testing and calibrating the unit (according to the similar OM-1's manual being I couldn't find this scope's), I had connected the 1V P-P output to the horizontal input which caused the scope to make a sparking noise and the spot on the tube to slowly lose intensity.I triple checked the voltages at the transformer and tubes sockets. I replaced all of the capacitors and checked the values of every resistor and pot. I even powered on the unit after the fact and did the same thing which led to no sparking noise or anything. I know I will need a new 5BP1 tube to get the unit to function, but I really don't want to ruin another expensive tube without fixing whatever went wrong in the first place.Thanks ahead of time for any helpful comments or suggestions.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 05, 2022, 11:00:01 am
content: Please check continuity of filament pins on the tube.

author: enteranickname16
author_rank: Newbie
timestamp: July 05, 2022, 11:41:21 am
content: I just looked at the scope this morning and check that and got a reading of 14 ohms. I figured with that maybe there was a slim chance the tube would work so I turned the unit on only to be disappointed. After cycling the power the continuity read ~265 ohms. Which I find all this strange being that when I was messing with the scope yesterday, it read around 80 kilohms.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 05, 2022, 01:50:31 pm
content: Perhaps clean out the old solder form the pins of the tube and resolder with new solder, if you can.

author: enteranickname16
author_rank: Newbie
timestamp: July 05, 2022, 07:35:42 pm
content: Do you mean on the socket? I went over those connections when initally restoring the scope. If you mean in the base of the tube, I haven't touch that besides ensuring that it wasn't loose and attempting to clean the pins. I will reassess those connections to make sure they are clean, though.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 05, 2022, 08:37:21 pm
content: It occasionally happens with tubes that have pins soldered to lead out wires (anything with a plastic base, more or less). Never junk a tube like that without re soldering the pins first to conform the problem is internal.Short of some real screwy problems IDK how what you did could have cased a failure. AFIAK what usually scraps those old service oscilloscopes is either people turning them into clocks or the paper and foil high voltage caps shorting and taking the HV winding in the power transformer with it.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 06, 2022, 10:57:58 pm
content: I do mean the tube base. Sometimes the solder in the pins corrodes and the connection is intermittent, different ohm readiigs etc.I do not know exactly why this happens but it does occur on the filament pins more often.It is a big problem with filaments that are also the cathode (not isolated cathode)

author: enteranickname16
author_rank: Newbie
timestamp: July 07, 2022, 01:07:42 am
content: You were both right, it was the connections in the base of the tube. I removed it and all of the wires were black with oxide. Put it back on and everything works great. Thanks for the advice, as I had no idea those bases could be removed so easily, or that they were such a probable failure point.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 07, 2022, 06:23:14 pm
content: It should be securely adhered to the glass, common failure on CRTs, most of the TV restoration guys fix it with a non corrosive (no vinegar odor) RTV silicone or 5 minute epoxy.