Oscilloscope repair - Page 1
Forum section: electronics / repair

author: Massa
author_rank: Newbie
timestamp: July 23, 2025, 12:13:39 pm
content: Hi, I recently got my hands on an old analog oscilloscope, the UNAOHM G505 A DT, and I wanted to repair it because it is my first oscilloscope. It seems to be almost unknown online but i managed to get the schematic diagram from the seller (even though the oscilloscope is from 1975).It is working fine, but only when the core of the transformer is isolated from ground, when i connect it to it via the screws that are on the back of the oscilloscope, it arcs to che chassis; the strange thing that happens is that whenever it is connected directly and in a good way to ground it still arcs from those places that are lightly connected.As far as I can tell the arching seems to be high voltage DC because sometimes the arc takes time to have place.After some on time the arching frequency increases.The arching messes up with the image on the screen, it looks like the CRT shuts off or bugs for a second, unfortunately for the moment I can't provide you with a video because the sweep time knob is broken and a replacement is on its way and it doesn't work without but i have a video of the arching (after the frequency increase)This is the link to the videohttps://drive.google.com/file/d/1Aw2GW_tqGgq-ucYD3-kaKef1URC5M9I-/view?usp=sharingThank you for your help and time and sorry for the bad english.Ivan

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 23, 2025, 12:33:39 pm
content: Hi,welcome to the forum.Looks like the transformer has a short to the core/can.Better trash the scope and get an save instrument.Good luck

author: coromonadalix
author_rank: Super Contributor
timestamp: July 23, 2025, 01:45:20 pm
content: if the scope works fine or seems to work finei have no schematics ... but yes there could be some problems ?you can use an 1:1 isolation transformer BUT you work ONLY with the Line and Neutral, no Ground / Earth ever be userin the past i've seen some tv and instrument having say : an non earth grounded case, in some cases it was neutral connected ...

author: Runco990
author_rank: Frequent Contributor
timestamp: July 23, 2025, 06:05:25 pm
content: I'd be very careful with that one. You could kill something that you are probing with that scope. High voltage will find it's way through your probe.That scope was probably gotten rid of over this problem. If you can't solve the arcing, I would NOT use it.There are many scopes out there for cheap, don't try to use a potentially unsafe one. Also see if there is a capacitor going from live to case ground, like on many old radios and gear. A leaking cap could be causing that as well. The schematic should tell you.

author: Gyro
author_rank: Super Contributor
timestamp: July 23, 2025, 06:16:30 pm
content: I certainly wouldn't trash the scope at this point.I would start with a physical examination of the transformer terminations, that's where there is the greatest chance of a short to the core (as long as the bobbin is in decent state). I'm sorry I haven't downloaded your video, I don't like google, maybe you could post an image of the transformer. Hopefully it is something as simple as cut insulation on one of the transformer leads, which can be fixed by adding an additional sleeve.From your description it sounds as if the fault is on one of the secondary windings - as long as the scope chassis is properly mains grounded, a primary short to the core would immediately blow the 500mA mains fuse.The time to trash the scope is when the transformer overheats or blows the mains fuse due to a shorted turn in the windings. Until then, there is always hope.Don't be tempted to remove the mains ground. In this case, I don't think it would change the symptoms anyway.P.S. A likely location may be on transformer wires 11 and 12, the 6.3V heater supply to the CRT. These are held at high negative potential by the EHT supply multiplier, so most at risk of intermittent insulation breakdown.

author: Massa
author_rank: Newbie
timestamp: July 23, 2025, 08:32:23 pm
content: Hello and thanks for the replies, That's some photos of the transformer.I would like to repair this oscilloscope because it was a gift from my professor (he knew it had issues but not this bad).

author: Gyro
author_rank: Super Contributor
timestamp: July 23, 2025, 10:14:00 pm
content: That transformer looks pretty good for its age. It's difficult, even with the images, to check every possible wire insulation angle for damaged insulation, so a careful inspection of each wire is needed (without stressing the entry points to the bobbin. The bobbin itself looks to be good quality (unless you can see any cracks).I would be tempted to add insulation with plastic sheet wherever possible between the bobbin and laminations, closely fitting pieces down the sides between the outside of the windings and the laminations (it looks as if there is space to fold these a little way around the ends of the bobbin). Also pieces sandwiched between the vertical edges of the bobbin and the laminations, top and bottom, extending the bottom ones out over the mounting flange where the wires might touch.Filling all the gaps in the ends of the bobbin with a couple of heavy coats of Polyurethane varnish will be a good idea too. Warming the transformer will help it flow into the gaps more easily and speed up drying. Do each end separately to avoid too much mess and allow to dry thoroughly before powering up. Warm the transformer well before you start to drive out any moisture too.I (like you) think the most likely breakdown point is on the wires to CRT heater as they are at negative EHT potential (present across C7 - don't try to measure it with a DMM!) I'm not sure which wires these are but I would add an extra close fitting PVC sleeve to each one and embed it fully in the wet varnish to provide a good insulation seal. It is probably worth doing the same with the mains primary wires too to prevent flashover to the mains.This is a bit experimental, but if you reinforce the insulation at all possible points between the windings and the laminations, you will hopefully fix the specific weakness. I don't think it can make it any worse.

author: Massa
author_rank: Newbie
timestamp: July 23, 2025, 10:44:36 pm
content: Hi! Thank you for your help! I found the issue, it was the secondary going to the crt heater as we were thinking.I also found something about the transformer, the secondary going to the voltage testpoint ~ is roughly capable of delivering enough current to the heater so i connected that to it (just to test it, then i removed it), and it worked!I will find another transformer that is able to easily deliver enough current to the heater

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2025, 09:26:21 am
content: You're welcome. It's good to get confirmation that it is that specific secondary. I hadn't noticed the other 6.3V secondary next to it. I wonder if they are wound with the same gauge wire and with the same level of insulation - obviously it wouldn't be possible to use it for the ~ test point at the same time due to the DC elevation. [Edit: If they both have the same level of insulation, it might be possible to swap them though, what are the wire colours of the '~' winding? (this will help to understand their relative position in the winding layers)]Finding a replacement 6.3V transformer may be interesting. Whilst the current requirements are very low (probably 300mA), the high primary-secondary insulation requirement may be more difficult. At one time, it used to be possible to get 'repair' transformers for CRT TVs with heater-cathode leakage problems but these are probably long gone. Check the primary-secondary test voltage spec for any candidate transformers, I don't see an EHT voltage marked on the schematic but C7 is rated at 2kV so I would expect the EHT to be maybe 1kV - 1.5kV max.I'm still curious about where the existing secondary is breaking down to the transformer metalwork (presumably, I can't see how breaking down to another secondary instead would cause the transformer-chassis arcing symptom that you are seeing) so there is still a chance of fixing the breakdown location. What colour are the CRT heater wires on the transformer?

author: Massa
author_rank: Newbie
timestamp: July 24, 2025, 12:24:52 pm
content: Hello!The crt heater wires are the light blue ones and the other 6.3V supply is purple (that goes to ~ test point).I noticed longer heat up times with the purple wire connected, also it seems that the arching appens even when only one of the crt cables are connected (i discovered that because I soldered it wrong and the cable disconnected lol).Anyway, what should I look for if i want a replacement transformer? Or do you think that it is easier to just try to repair the winding.Thank you for your help!

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2025, 01:53:25 pm
content: Ah ok, the light blue CRT winding is the outermost winding then. That makes sense from the point of view of extra insulation from the other secondaries. The purple winding is further in, it probably doesn't have the same level of insulation from the other secondaries and, from what you say, has a lower current capability too.Finding another filament transformer would mean finding one with sufficient primary-secondary insulation. Split bobbin types (primary and secondary side by side) normally have the best isolation, but are still only intended to mains supply with transients, test to around 4kV from memory. You might get more clearance if you buy a dual 6V secondary (6.3V is almost impossible to find), most are dual secondary, and unwinding the outer one. Alternatively removing the secondary completely, counting the number of turns, and replacing with better insulated wire (eg. Triple insulated, as used in SMPS transformers) and adding another 5% to the number of turns to bring the voltage up to 6.3V.Personally, I would go for a repair. It will save finding another transformer and mounting it, arranging the mains primary connection etc. As it is the outermost winding, and the leads are far from the core, I think it is breaking down between the sides of the bobbin (probably at the ends where thegreyblack insulation doesn't quite meet the end plastic, and the laminations. Check this area (4 places) for trapped debris, dirt etc. I would then insert some plastic sheet (similar to what you would find in heat sealed blister pack packaging (on tools, batteries etc.) and insert down the sides of the bobbin, folding around the gap at the ends of the plastic bobbin and extending the full thickness of the laminations plus 5-10mm on each side. Overhead transparency plastic or plastic folder material would be ideal. I can't see where else the winding could be arcing to the metal of the transformer instead of to other windings.I have roughly drawn on one of your images...P.S. I didn't notice the string of 10 PL180Z Zener diodes shunt regulating the EHT. That puts the heater winding at -1800V (-1900V max) relative to the metalwork.

author: Massa
author_rank: Newbie
timestamp: July 26, 2025, 03:47:16 pm
content: Hello, sorry for not updating you these last few days.I fixed the transformer doing what you've advised me to do! Thanks a lot for the help!I reconnected everything and the oscilloscope works, exept for the sweep time knob which is on its way, but that's an easy fix.Still I can't thank you enough, I think that the next step will be checking if the components on the horizontal and vertical amp (especially the capacitors) are within tolerance.It's a beautiful piece of equipment for a beginner like me (I just graduated in an electronics highscool and going to UNI) and I am super happy to use it.Still Thanks you a lot for the help))

author: Gyro
author_rank: Super Contributor
timestamp: July 27, 2025, 04:48:51 pm
content: That's excellent news, well done! You're welcome.It's always good to be able to restore a piece of test equipment to a useful - and appreciated, state. Looking at the components on the schematic, there is nothing in there (apart from maybe a burned out transformer or the CRT) that would stop you from keeping it running indefinitely, at minimal cost. It includes plenty of fuses too, always helpful.Enjoy!P.S. As long as the H and V pass basic tests, I would try to avoid the temptation to mess with it too much.