Hi All,
This is my first post here (sorry for the long one) and I am rather a newbie to electronics, so please be patient with me.
As one of my first projects, I picked a quite difficult one. I purchased very cheaply old oscilloscope Aaron BS-635 (also known as the later rebranded Hung Chang OS-635). I simply can't afford to spend $400+ on a new or used piece. The oscilloscope had a blown fuse and I was told that prior to that the CRT lit up for a short while and then the fuse blew.
I decided to give it a go. After all, "if people made it, people can fix it"
I replaced ALL the electrolytic caps throughout the oscilloscope (what the heck - they were only 30 or so ), as most of them were old and this is the first thing which normally gives up the ghost, I was told.
I managed to connect everything correctly (or at least as it was before I took it apart) with the help of the photos I was taking before each step of the way. I also labelled all the cables with the source and destination connectors numbers for a future reference.
While working on the Power Supply board, I also discovered an internal fuse, which was blown. Of course, I replaced it as well.
When I powered up the oscilloscope for the first time after assembling it, the CRT came alive!
But there was a lot of sparks going between the 4.5kV CRT cable mount (solder) on a high voltage PCB and the internal high voltage enclosure (grounded). There is a distance of about 8-10mm between the PCB and the enclosure.
Since there were already some parts missing when I bought it (e.g. the high voltage enclosure lid, as well as some plastic shields here and there - I found one loose inside not in its place and I can't find where it belongs), I thought of simply shielding/insulating between the PCB and enclosure.
This helped - at least to eliminate sparks going there. But they started to form between the legs of the high voltage ceramic (metal film?) caps! The legs were originally insulated in most parts by some sleeves, but there were tiny exposed lengths just at the PCB connections. Using insulating liquid ("liquid insulating tape"), I made sure to cover these tiny bits. This helped too.
But... then the sparks started to fly between the CRT 4.5kV cable itself (through its insulation!!!) and.. the tops of the caps!
That was the moment when the internal (high voltage) fuse decided it had enough.
I took out the PCB again, replaced the fuse and started to examine the PCB under the magnifying glass.
The first thing I noticed, the CRT cable insulation had several micro cracks (age), so I covered all of it in several layers of "liquid tape". Then I noticed that one of three Neon Indicator Bulbs had one leg missing (poor thing).
Here starts my problem. The original neon bulbs (N1, N2 and N3 on the schematic) are type NE38B. These are small "pigtail" tubes, about 5mm long (picture below). They are connected in series. Unfortunately, I can't find anywhere not only any replacement but even any data about them. Apparently, they don't exist. I also don't have any clue at what voltage they are rated, as the schematic doesn't mention any voltage on the rail they are on. The cap next to them is rated at 500v, so my guess is they should be similar. The highest rating I found the new ones available for is 250v. I don't know virtually anything about the neon bulbs (just read the basics today). The fact that they are connected in series is probably very important - does it mean that they could individually be a lower rating, so they "make it up in the gang"? I think that in series they work like resistors, so this would be my guess.
Could this be a reason for the sparking (too high voltage because of the faulty neon bulb)?
I haven't tested the actual voltage because I have no meter able to test 4.5kV...
The black top on the neon bulb (photo below) is a little bit of the "liquid tape" (by accident) but is irrelevant to the problem.
If anybody could help me with any comments/ideas, I would be extremally grateful!
This is my first post here (sorry for the long one) and I am rather a newbie to electronics, so please be patient with me.
As one of my first projects, I picked a quite difficult one. I purchased very cheaply old oscilloscope Aaron BS-635 (also known as the later rebranded Hung Chang OS-635). I simply can't afford to spend $400+ on a new or used piece. The oscilloscope had a blown fuse and I was told that prior to that the CRT lit up for a short while and then the fuse blew.
I decided to give it a go. After all, "if people made it, people can fix it"
I replaced ALL the electrolytic caps throughout the oscilloscope (what the heck - they were only 30 or so
), as most of them were old and this is the first thing which normally gives up the ghost, I was told.I managed to connect everything correctly (or at least as it was before I took it apart) with the help of the photos I was taking before each step of the way. I also labelled all the cables with the source and destination connectors numbers for a future reference.
While working on the Power Supply board, I also discovered an internal fuse, which was blown. Of course, I replaced it as well.
When I powered up the oscilloscope for the first time after assembling it, the CRT came alive!
But there was a lot of sparks going between the 4.5kV CRT cable mount (solder) on a high voltage PCB and the internal high voltage enclosure (grounded). There is a distance of about 8-10mm between the PCB and the enclosure.
Since there were already some parts missing when I bought it (e.g. the high voltage enclosure lid, as well as some plastic shields here and there - I found one loose inside not in its place and I can't find where it belongs), I thought of simply shielding/insulating between the PCB and enclosure.
This helped - at least to eliminate sparks going there. But they started to form between the legs of the high voltage ceramic (metal film?) caps! The legs were originally insulated in most parts by some sleeves, but there were tiny exposed lengths just at the PCB connections. Using insulating liquid ("liquid insulating tape"), I made sure to cover these tiny bits. This helped too.
But... then the sparks started to fly between the CRT 4.5kV cable itself (through its insulation!!!) and.. the tops of the caps!
That was the moment when the internal (high voltage) fuse decided it had enough.
I took out the PCB again, replaced the fuse and started to examine the PCB under the magnifying glass.
The first thing I noticed, the CRT cable insulation had several micro cracks (age), so I covered all of it in several layers of "liquid tape". Then I noticed that one of three Neon Indicator Bulbs had one leg missing (poor thing).
Here starts my problem. The original neon bulbs (N1, N2 and N3 on the schematic) are type NE38B. These are small "pigtail" tubes, about 5mm long (picture below). They are connected in series. Unfortunately, I can't find anywhere not only any replacement but even any data about them. Apparently, they don't exist. I also don't have any clue at what voltage they are rated, as the schematic doesn't mention any voltage on the rail they are on. The cap next to them is rated at 500v, so my guess is they should be similar. The highest rating I found the new ones available for is 250v. I don't know virtually anything about the neon bulbs (just read the basics today). The fact that they are connected in series is probably very important - does it mean that they could individually be a lower rating, so they "make it up in the gang"? I think that in series they work like resistors, so this would be my guess.
Could this be a reason for the sparking (too high voltage because of the faulty neon bulb)?
I haven't tested the actual voltage because I have no meter able to test 4.5kV...
The black top on the neon bulb (photo below) is a little bit of the "liquid tape" (by accident) but is irrelevant to the problem.
If anybody could help me with any comments/ideas, I would be extremally grateful!
