Broken Leader 8042 Oscilloscope  - Page 1
Forum section: electronics / repair

author: Julian Garcia
author_rank: Newbie
timestamp: January 30, 2021, 07:07:46 am
content: Hello, I just got this rather nice Leader 8042 Analog Oscilloscope (A rebadged Kenwood 5130??)And when I got it, the controls seems rather wonky and doesn’t seems to register well, so I disassembled it to spray some contact cleaner. Which it does fix that, but now there’s new problem.The display now get all flickery, traces, readout is all flickery, I though I somehow bumped into the vertical control pot, but after spending a day looking in Kenwood service manual, I cannot find where that pot is.I’ve tried reseating every connector, soldering any suspicious looking joints, but no dice.So, out of ideas. I decided to asks here, maybe I’d get some more insight into the solutions.What’s your guys opinion? Thanks in advance!

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 01, 2021, 08:28:58 am
content: Flickery as in brightness variations?Is focus also unstable?If so look for dust build-up or open (high value) resistors in the CRT supply.

author: Julian Garcia
author_rank: Newbie
timestamp: February 01, 2021, 11:48:54 am
content: Hi, thanks for your input.Flickery in this case means that the image keeps rolling, I suspect it had to do with the horizontal ramp waveform not being drawn correctly.Also, while I was trying to fix it yesterday, one of the component fell off.It appears to be a neon lamp and after looking it up in the service manual, its a part of the horizontal board, which seems consistent with my suspicious.I don't quite know what function does these neon lamps served, but my best guesses is its used to create a ramp waveform via capacitor charging up to a certain voltage and these neon lamp arcs which discharge the caps and causing them to recharge again, generating a ramp waveform.So I just ordered some replacement lamps, I'll probably replace both lamps in the circuit since the first one that fell off, I see some signs of burning, which might be due to arcing.I'm stand to be corrected, anyway. Thanks again! I'll be sure to update one I got the replacement parts.(I also enclosed a short gif of the symptoms and the broken part with a portion of board schematic)

author: FFMX666
author_rank: Newbie
timestamp: February 22, 2021, 09:42:09 am
content: Do you have datasheet or Use guide for leader 8042 oscilloscope,and Could you give it to me ,My email address is 2324223682@qq.com.thank you

author: FFMX666
author_rank: Newbie
timestamp: February 22, 2021, 09:46:11 am
content: It is better to have the circuit diagram of the leader 8042 . I want to fix it. Thank you

author: Gyro
author_rank: Super Contributor
timestamp: February 22, 2021, 11:43:38 am
content: The Neon lamps are normally in the CRT cathode / grid circuit as protection against over-voltage or insulation breakdown within the CRT during power up / power down. You might see them flicker briefly when you turn off the scope, but they should not normally be lit. If one is actually physically broken then there is no choice but to replace, but otherwise there would be no reason to.It is a very long time since Neon lamps were used in oscilloscope timebase circuits. They were very slow and non-linear.