Leader scope lost its sweep - Page 1
Forum section: electronics / repair

author: sweatybetty
author_rank: Contributor
timestamp: September 13, 2020, 08:19:51 pm
content: Hello all!New guy here looking for some help. I'm working on a Leader LS8106A scope that has lost the horizontal sweep.I have replaced all the capacitors on the power supply board, all voltages check out OK.I'm hoping someone can point out which part of the circuit controls the sweep. The schematic isn't marked as to which section is which.I'm going to attempt to attach the FSM to this post, don't know if I'm allowed.thanks all sb

author: sweatybetty
author_rank: Contributor
timestamp: September 13, 2020, 08:22:58 pm
content: Well either the file is too large or I'm too new here to use the attachment feature.

author: bdunham7
author_rank: Super Contributor
timestamp: September 13, 2020, 08:28:27 pm
content: "Lost its sweep" is a bit general.Is the screen blank or do you see a dot that can be moved around with the position buttons?You could have a problem with triggering, the horizontal sweep generator or the horizontal amplifier, or perhaps something else. Can you get the scope to work in XY mode?

author: sweatybetty
author_rank: Contributor
timestamp: September 14, 2020, 12:10:56 am
content: With the horizontal position knob centered, the trace (just a dot) is off the right side of the screen. Turn the knob full left and the trace is just right of center. In X-Y position, just the opposite.My understanding is that there is a sawtooth pattern generated that drives the trace across the screen.Also, please forgive me if I am using the wrong terminology.thanks again sb

author: bdunham7
author_rank: Super Contributor
timestamp: September 14, 2020, 01:06:39 am
content: I tried to talk another forum member through a similar repair and he is either waiting for some probes or has given up, I'm not sure which. It may not be easy! What other test equipment do you have and do you have a link to a schematic?So, in general, the horizontal sweep generator will generate that sawtooth pattern, but it will only start when told to by the trigger circuit. The sweep signal is fed into the horizontal amplifier which connects to the horizontal deflection plates in the CRT. The problem could be anywhere in those systems, or in their power supplies.In XY mode, there is no sweep and the dot is moved horizontally by the X input and vertically by the Y input. Any input on the X channel should turn the dot into a horizontal line. Try that first--you may have to adjust the input level to match the signal, but you should get a line, or for very low frequencies, a moving dot. Do you have a good signal generator?

author: floobydust
author_rank: Super Contributor
timestamp: September 14, 2020, 01:45:29 am
content: Is the CRT readout text also off screen?https://www.eevblog.com/forum/testgear/leader-ls8106-oscilloscope/You could post the service manual toThe Boat Anchor Manual Archive BAMA

author: sweatybetty
author_rank: Contributor
timestamp: September 16, 2020, 01:03:50 am
content: Hello.bdunham7As far as equipment I have another scope, several VM's and a signal generator. Ill try your advice about X-Y mode tomorrow afternoon.floobydustThe readout text is on screen as it should be.Im going to try attaching the horizontal board schematic just to see if I can post a small pic. If not, Ill try BAMAthank you both

author: sweatybetty
author_rank: Contributor
timestamp: September 16, 2020, 01:29:52 am
content: More than 1 way to skin a cat. I also sent the entire file to BAMA.

author: sweatybetty
author_rank: Contributor
timestamp: September 17, 2020, 12:12:14 am
content: Short video. 5hz sine wave at 1.5V. I was moving the H-pos control back and forth.

author: ambrosia heart
author_rank: Regular Contributor
timestamp: September 22, 2020, 03:28:08 am
content: Quote from: sweatybetty on September 13, 2020, 08:19:51 pmThe sweep circuit of your oscilloscope has 6 parts:(1) schmitt circuit [not shown](2)sweep gate circuit(3)free run circuit(4)ramp generator circuit(5)hold off circuit(6)sweep stop circuit and(7)sweep selection circuitHow to measure:see your upload 48-49 & 8106hbmake sure ALL concerned circuits supplied with proper voltage.(0)Oscilloscope in auto mode with intensity at minimium, i.e. Spot not seen.(1)measure waveform at X+ & X- output first. If no waveform, measure preceding stage marked with a cross in green.(2)If no waveform at ALL (1), measure waveform marked in red 1,2 & 3 located in Q.(3)Is waveform at Q pin 2 marked in red (4) in High, otherwise, no sweeping in auto mode.(4)If (3) is high, check ALL components with with arrowsWhen an oscilloscope under test changes time base, so does sweeping waveform on screen.

author: ambrosia heart
author_rank: Regular Contributor
timestamp: September 24, 2020, 05:28:38 am
content: waveforms

author: ambrosia heart
author_rank: Regular Contributor
timestamp: September 24, 2020, 05:31:18 am
content: waveforms

author: sweatybetty
author_rank: Contributor
timestamp: September 25, 2020, 07:21:27 pm
content: ambrosia heartQ110Collector 1st pic 5mv/divBase/emitter 2nd pic 5mv/div

author: sweatybetty
author_rank: Contributor
timestamp: September 25, 2020, 07:24:11 pm
content: Q112 Q307 and Q308same pattern all 3 terminalsSame thing on pin 2 IC108all at 5mv/div

author: sweatybetty
author_rank: Contributor
timestamp: September 25, 2020, 07:39:14 pm
content: Pin 13 IC's 109 & 1105mv/div

author: sweatybetty
author_rank: Contributor
timestamp: September 25, 2020, 08:54:46 pm
content: Found the waveform. Its coming off the -8V supply. Going to check the power supply board.Waveform is gone. -8V is now normal. Everything points to IC108, might order a new one.

author: ambrosia heart
author_rank: Regular Contributor
timestamp: September 26, 2020, 02:22:05 pm
content: Quote from: sweatybetty on September 25, 2020, 07:21:27 pmActually, no sine wave will observe in this circuit. Replacing faulty IC108 may get it fixed. If not fixed, repair rampgen. circuit. The X plate amplified signal drives electron beam sweeping across screen. See waveform with x10probe. IC108 pin 2 signal also shown in auto and fix mode, it gives a straigth line in norm & single mode.

author: sweatybetty
author_rank: Contributor
timestamp: September 27, 2020, 04:51:30 pm
content: Ordered a couple new IC's and sockets. Ill give an update when I get it installed.Appreciate the help