Oscilloscope vertical mode selection repair (only add function works) - Page 1
Forum section: electronics / repair

author: osmax_br
author_rank: Regular Contributor
timestamp: January 28, 2025, 04:14:22 pm
content: Welcome back to another episode of a newbie asking professionals for help with repairing his oscilloscope.I have a Goldstar OS9020G oscilloscope. The oscilloscope, when put into "Add" mode, works perfectly fine (all vertical and horizontal settings work, and it works totally normal). However, when I change the mode from "Add" to (Ch1, Ch2, or Dual), the oscilloscope suddenly stops working (the screen outputs nothing). When returned to "Add," the CRT displays and it returns to working normally.I was suspecting a failure in the vertical control board (components around u401) .I have the schematic and will attach it below.I am somewhat of a beginner, so if someone can point out a possible failure in the circuit (a broken component) or a place to test the voltage, that would be appreciated.I just need a tip of the rope to get started. Thanks for your patience and your help.Here is a link to the service manual:https://www.electronica-pt.com/esquema/instruments/lg-os-5020g-oscilloscope-21183/

author: D2236
author_rank: Regular Contributor
timestamp: January 28, 2025, 05:45:17 pm
content: You may start with making sure U401 74LS74 works correctly.https://www.makeyourownchip.com/7474html"The vertical switching circuit determines the input signal or combination ofinput signals to be connected to the vertical main amp.Input signal combinations that can be displayed are selected by D FLIP-FLOPthat is controlled by the vertical mode switches and the X-Y position ofthe TIME / DIV switch.In the DUAL modes, both channels are alternately displayed on ashared time basis."

author: osmax_br
author_rank: Regular Contributor
timestamp: January 28, 2025, 06:47:10 pm
content: Alright, I seeThe ic is GD74LS74AP and it is a D type flip flopI will first test whether it is receiving voltage, and will probably see a few tutorials online about 74hc74 and how to test them properly

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2025, 07:11:30 pm
content: ^ I noticed that your reference to 74HC74 in the above post. Looking at the passive component networks on the Q and \Q outputs, you will want to swap it for another 74LS74 if it proves to be faulty. Logic functions are obviously the same, but differences in output characteristics could be enough to create misleading problems.

author: osmax_br
author_rank: Regular Contributor
timestamp: January 28, 2025, 09:36:58 pm
content: Thanks for the reply!You are correct, however I am thinking of checking the voltages on the Q and Q— and see whether it is faulityBut on the other hand I am wondering, since the ic has 2 type D flip flop, why on the schematic one of them is being used why not both?Also I am suspecting failure in the ch1 and ch2 path , since dual mode isn't working and the switch isn't connected to the icSorry if I am wrong , please correct meThanks in advance !

author: D2236
author_rank: Regular Contributor
timestamp: January 28, 2025, 10:04:43 pm
content: Quote from: osmax_br on January 28, 2025, 09:36:58 pmThe dual signal enables the clock generator (pg11) that feeds back to U401 on pin 3. This puts the 74LS74 in toggle mode by alternately enabling Ch1 and CH2 with Q and Qbar.

author: osmax_br
author_rank: Regular Contributor
timestamp: January 28, 2025, 10:09:42 pm
content: UnderstoodI am grateful for your time and replies , thank you