LG OS-502RB oscilloscope -- Channel 1 vertical position problems - Page 1
Forum section: electronics / repair

author: Coffeemugger
author_rank: Newbie
timestamp: December 15, 2018, 12:52:43 am
content: Hello, inexperienced person hereI recently received an LG OS-502RB (probably close to the OS-5020 but I don't know!) from eBay. It seems to be working in every way except that the channel 1 vertical adjustment settings has a weird set of symptoms.Upon turning the knob all the way to the right, there channel 1 trace appears from the bottom of the screen. Turning the knob all the way to the left causes the text readout to move upward slightly. Finally, by applying pressure around the vertical adjustment knob, I can make the knob appear to function normally, but it's very sensitive to touch.I've attached some videos in case of any confusion, hopefully they convey the symptoms accurately.Symptoms:https://youtu.be/SBvdoSAT028Vertical adjust behaving normally sporadically:https://youtu.be/4SGaFo8DG8gSent from my Moto G (5) Plus using Tapatalk

author: BFX
author_rank: Frequent Contributor
timestamp: December 15, 2018, 08:43:27 am
content: Obviously there is some bad solder join or pot is bad (pot leg is loose/broken ). Open it and check.

author: Coffeemugger
author_rank: Newbie
timestamp: December 15, 2018, 03:39:54 pm
content: Glad you think so, I was hoping it was a bad pot but thought it could be the vertical preamp -- or worse, the tube! *shutters*I'll open it up today in that caseHey, how do I thank you for the answer?EDIT: NVM, thanked.

author: Coffeemugger
author_rank: Newbie
timestamp: December 17, 2018, 05:51:17 pm
content: As far as I can tell, the potentiometer didn't have a good connection to the board. Initial testing of the pot on the board gave me a full (or close to full) resistance range from 0 to 9.1 kohms vs the channel 2 pot which gave a range of 0 to 2.3 kohms. I thought that the difference could be due to resistors parallel to the ch2 pot. So I desoldered the ch1 pot, tested it off the board (giving me 0-9.1k again), then resoldered it, and that seemed to have fixed it as the resistance now ranges from 0-2.3kohms between the end and base.I'm in the process of putting the scope back together. There doesn't seem to be a repair manual readily available online for either the os-502rb or the os-5020, and the closest I could find was for the os-9020. In addition, I failed to take adequate photos of the connectors on the way in. So I had to piece together the connections again based on the board labels (very helpful) and my grainy photos.Edit: to be more clear, everything is reconnectedI didn't get any magic smoke when powering up the scope, but the readout on the display is pushed to the left side so that the right side of the display has no output. Text closer to the right appears distorted. The horizontal position knob articulates the readout further to the left. Traces are nowhere to be found.Video:https://youtu.be/HlJirt_9LZwNote: the shifting and movement of the readout up and down was there when I got the scope. Giving the side of the case a good tap has usually fixed that.Hypotheses:*I switched two connectors on the horizontal amp.Or*A connection between the horizontal board and crt is disconnected.Or*In the process of taking apart and putting the scope back together, I inadvertently turned an internal trimmer on the horizontal board.Next steps:I'll be searching for a repair manual for the os-502rb or at least os-5020. In the meantime, does anyone have one of these? A link would be very much appreciated!A bad connection or switched connections would mean that I should completely take the scope apart again, document every connector, every destination, and whether I made that connection.A turned potentiometer would mean that I need to turn a potentiometer in the opposite direction and then calibrate the thing.If any of this sounds wrong just let me know... Better than learning the hard waySent from my Moto G (5) Plus using Tapatalk

author: Coffeemugger
author_rank: Newbie
timestamp: December 19, 2018, 05:39:08 pm
content: Not sure what to do at this point... My limited troubleshooting ability may be insufficient to the problem.$40 parts maybe...