LG OS-5100RB problem - Page 1
Forum section: electronics / testgear

author: CX8JD
author_rank: Newbie
timestamp: April 23, 2014, 04:00:59 pm
content: Hello!I have a problem with an oscilloscope LG OS-5100RB. LA image disappears and I was able to determine that there is a problem with the CRT DRIVE BOARD, specifically in the high-voltage module. I tried to get a replacement but this model is no longer manufactured oscilloscope few years ago.Any of you have any idea how to replace the damaged component or where I can buy a device that has an HV-MODULE that can replace?Attached are some photos and the circuit with the defective part.thanksregardsFernando

author: zinda
author_rank: Newbie
timestamp: March 29, 2015, 01:03:02 pm
content: HI dearFirst of all I feel HT black box is not the thing to be worn out so quickly. please let try again if you have time, then review small parts behind that bit by bit. I am sure you can have the solution.Further if you have the catalog the complete circuit of this board and relevant boards, please share with me, also the picture of board from both sides, but in good resolution. I think I can help you as I can.I have some similar model of EZ scope. may be the voltages and parts components value I can check here and tell you t verify. This can be a good combination if you agree.I would be looking forward to hear you, if you may like my suggestion.regards.

author: LA9QV
author_rank: Newbie
timestamp: June 20, 2018, 04:40:05 pm
content: I also have a problem with the response of the "volts/DIV" encoders on my OS-5100RB scope.Have tried cleaning them with contact spray to no avail. There are other controls also which do not respond correctly, but jump up and down randomly instead of increasing and decreasing with movement. Therefore I would like to get a manual for this instrument, which is working fine apart from this

author: ulyanoff
author_rank: Contributor
timestamp: January 02, 2020, 09:07:04 pm
content: Hi ! I also ask for help with the service manual for the OS-5100RA model, anyone can help with any documentation .

author: hugarefe
author_rank: Newbie
timestamp: February 25, 2020, 09:31:17 pm
content: Hello. Y will try to relate my experience with an OS-5100RA with the same crt drive board.The first symptom was F741 burning aparently without reason at intervals, at the beginof weeks, after days, and finally hours. Simultaneously and sporadically with this fault,the crt filament no turn on, in spite of having a good oscillation level on collectorof Q746 (2SD613F). Dont try to measuring the filamet voltage with a tester, becauseif the transistor is working, you will have 1.47KV with respect to ground. After verifyingthe crt´s socket, inclusive changing it, I noticed that diconecting the crt´s socket,and measuring the conduction of the filament winding from the H-V Module, it measuredopen. So that taked out the H-V Module, and after cuting the black plastic cover inthe line of the two points that feed the filament, I found that the two wires are solderedto a printed circuit at the top of the module and was VERY bad soldered.To removing the pink sealant used as insulation, I used a hot air blower ataproximately 180° centigrades, (356 fahrenheit) and a screwdriver. To isolating this again I used hot glue and a layer of PTFE of 0.5mm thicknes, (0.019685 inch) covering the cutted parts.As the voltage at this point is 1.47KV, you can use maylar instead of PTFE.After this, the filament always turned on with Q746 working ,but keeped burningF741 sometimes. After some days F741 blowed and Q746 shorted. After checkingthe H-V Module´s conection to the colector of Q746 have found the same fail as thefilament. After applying the same procedure as with the filament, the conectionswas fixed. BUT after trying several transistors to replacing the 2SD613F without success, found the datasheet where says that the "F" means hfe from 160 to 320. After comparing with other of my oscilloscopes, an Tek 7623, noticed that the transistors used in the HV tranformers in oscilloscopes must work in the lineal region,and therefore the base bias and coupling are low. This means that on the colector you must have a sinusoidal waveform. In my case, Y found an old good quality TIP31 (Motorola) that worked OK. About the schematics Y have found that is very similar to LG Goldstar OS-3020, in particular the file "LG Goldstar OS-3020 Schematics PT2.pdf" page 5. I hope this information can help you.Hello. I found two photos that I took when I did the first repair of the H-V Module.

author: tongbajiel
author_rank: Newbie
timestamp: July 27, 2020, 08:45:19 pm
content: Hello CX8JDCan you share or send me OS-5100RB schematic please..i have same item and its no display problem..maybe same as your problem to...tanks..

author: CX8JD
author_rank: Newbie
timestamp: February 01, 2021, 02:40:09 am
content: Hi!Sorry but I haven't been on the blog for a long time.Thank you very much for the suggestions. I have not yet been able to repair my oscilloscope, it is possible that something similar to what hugarefe described is happening, in my case I only have the image dimmed and smaller than usual, at first intermittently but after some time remained permanently. Sometimes the F741 fuse burns for no apparent reason and after replacing it, the image appears on the screen again. I have not been able to measure any faulty components so I still believe the problem is with the high voltage module. I have tried to warm up the module a bit to see if it changes its behavior but it has not. At some point with a high voltage tip that they loaned me, I was able to measure that the 10Kv voltage was much less than that value.I upload the circuits that the factory sent me a long time ago (unfortunately they are scanned in very low quality), and photos of the printed circuit board.I have seen that there are many model os-5100 oscilloscopes for sale for parts, but no os-5100rb, does anyone know if the model 5100 carries the same high voltage module as the 5100rb?If anyone comes up with an idea of ​​something to measure or test, I appreciate it. For now I think I am going to try to disassemble the HV module and then I will comment on the results.RegardsFernando

author: hugarefe
author_rank: Newbie
timestamp: March 31, 2021, 11:02:27 pm
content: In first place, thank you very much for sharing this schematic. I've looking for it since 2007, when y bought it. About your problem, check first the tube socket, I had to changue it. If ok, check on page 7, Q747, Q748, and relative components. These are part of the sistem of the automatic HV control. By example, if C746 becames with loses, the circuit will believe that the HV is high. If this is ok, you will have to check all the HV capacitors, already that due to that this circuit works in lineal mode, if there is any one with loses, this will increase the comsuption of the entire circuit.When I had the trouble with the HV MODULE the problem was total, on or off, never dimming.I hope this can help you, and thanks again.

author: sprit
author_rank: Regular Contributor
timestamp: January 27, 2023, 05:41:26 pm
content: I have same Model as you. The problem was this tiny HV transformer. Did anybody known how to remove this pink chemical? Its so weird

author: hugarefe
author_rank: Newbie
timestamp: January 30, 2023, 02:59:39 pm
content: Hello spirit. The only way I have found was with an hot air blower at approximately 180° centigrades, (356 fahrenheit) and removing it carefully with an screwdriver.

author: Gearbreaker
author_rank: Contributor
timestamp: October 22, 2023, 03:58:58 pm
content: I just picked up an OS-3020D. It is brand new and I have has it running for about 15 minutes. The text on the screen is about half the brightness of the trace. Is there a way to raise the text brightness? I am not sure if the issue is the scope or me. I am now used to LCD displays which are easy to read. With the LG I need to dim my room lights to be able to see the display. ):