Analog oscilloscope ONLY working for a split second when restarted - Page 1
Forum section: electronics / repair

author: osmax_br
author_rank: Regular Contributor
timestamp: November 04, 2024, 05:43:26 pm
content: I have a goldstar (LG) 9020g oscilloscope, its problem that it doesn't give any output , so I turn it off and then immediately on and it flashes for a brief second and then no output againI am suspecting the hv circuit is malfunction , the problem is that I have a medical condition call HighVolt-Phobia and I don't know for sure how to test it without getting killedMy question1- sources on the internet where someone explains how to deal with crt high voltage circuit (I want him still alive just to make sure 😀)2- do you have any suggestions for the malfunction cause (maybe it is not the hv circuit) ?3- channels where people fix similar things4- general advice5- THX A LOT <3*There is 2 photos for my little device and the fast flash

author: tggzzz
author_rank: Super Contributor
timestamp: November 04, 2024, 06:35:42 pm
content: Might be HV, might be the LV PSU, might just need a good internal clean.Is the fault a new fault?A 10s gurgle reveals the service manualhttps://gepengser.weebly.com/uploads/9/6/3/5/9635683/os_9020_g_lg-goldstar_1.pdf

author: osmax_br
author_rank: Regular Contributor
timestamp: November 04, 2024, 06:44:35 pm
content: Yup it is a new faultBefore that it was working but with dim display and then it stopped working , I don't think it is a crt problem because the crt is flashing on startupAnd side note , six months ago it returned working suddenly and I still dont know how it worked , however it worked a few timesI have the service manual , just need to see someone on yt working on similar HV circuit just to know that I am not risking myself+ All the functions of the oscilloscope are working perfectly fine (function gen , and the xy functions ) and as far as I know they use the same low voltage psu as the crt (maybe I am wrong but that's what I believe )Thanks for your reply

author: tggzzz
author_rank: Super Contributor
timestamp: November 04, 2024, 07:30:56 pm
content: Quote from: osmax_br on November 04, 2024, 06:44:35 pmThe service manualoughtto indicate the "interesting" areas.Certainly the connections at the other end of the tube could bite, and the other one could be a "sticking plaster" on the side of the next with a thick cable coming from it. The latter takes alongtime to discharge.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 04, 2024, 07:40:33 pm
content: Quote from: osmax_br on November 04, 2024, 05:43:26 pmI'll just say that that is agoodcondition to have. You'll live longer.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 04, 2024, 07:45:10 pm
content: HiFirst you need part 3 of the online service manual to get the schematics.If you are not trained or experienced with HV circuits, DO NOT work on them to avoid personal injury and possibly death. There is some 1.9KV on the CRT Cathode/1st GridI have had a quick look at the HV/CRT circuits in the service manual, the design is at best very poor. My best guess is that the fault is in the CRT high voltage circuitry.A quick look on a certain auction site shows numerous faulty such LG scopes for sale, I wonder why.I have been repairing, and back in the day designing CRT scopes, for the last sixty five years so I do know what I am doing with them. Plus I am still very much alive still.G Edmonds