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Old 23rd Mar 2020, 9:12 pm   #1
Kopferkingel
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Join Date: Apr 2010
Location: Prague, Czech Republic
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Default LEADER LBO-301 stops sweeping after 5 seconds

Iīm experiencing a rather peculiar problem with my LBO-301 oscilloscope. A small, 1 channel scope. When I leave it sitting for some time and turn it on, it sweeps just fine and after 10 seconds o so it loses sweep and only dot is present. The trace just flickers a little and goes straight to dot. Iīve measured all transistors - they appear just fine. Iīve replaced all bad capacitors. Iīve checked the power supply voltages - they are just fine. I am able to move the dot up and down, left and right. Magnification, focus, intensity, everything works. The sweep never starts again, even in "auto" mode and I havenīt been able to trigger it externally. When I apply signal, the dot changes into vertical line. I am trully desperate, because Iīm simply running out of ideas. Iīd like to ask if anyone could point me to the right direction or at least share some ideas. (I have the service manual and digital scope but I deal with VHS/Beta VCRs). Thanks for any advice.
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Old 23rd Mar 2020, 10:57 pm   #2
WME_bill
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Default Re: LEADER LBO-301 stops sweeping after 5 seconds

Leader LBO301 troubles.
It seems that the Y system is operating correctly, as you get a vertical line. And also the X amplifier is probably working, as you can move the spot side to side.
The CRT supplies seem working. So also is Trace Bright-up.
But Trigger and Timebase are not.
The symptoms of working for a few seconds and then fails suggests a transistor or diodes (or perhaps less likely another component) failing as it warms up or comes under stress.
I am not a supporter of the remove and check everything philosophy, as it can often create more problems. And a quick transistor test is not going to show up the kind of fault you have here.
I suggest: Wait for fault condition. Set all trigger to Normal, no signal and TB speed to Ext. This gives fixed conditions to the Time base with a central CRT spot.
Voltage checks can now mean something, so check all voltages. The circuit diagram is very good and gives lots of voltages. To be sure, start with the power rails. The +27v rail seems only to the TB, so monitor that.
Then try following waveforms, using your other scope from trigger through. Trigger (Q601 toQ604), if set to Auto should show a pulse waveform at TP601 and after the schmitt bistable stage Q605/606, TP602. Varying Stability will affect this. Or feed in a Y input signal and normal trigger.
Then trace this pulse to the TB input (Q608) and output (J601) and Q610 emitter. The end of trace is at TP603.
When checking voltages, remember the emitter-base(+) is about 0.6V, also with diodes anode(+) to cathode. The collector will be much nearer the power rail. So others voltages you can work out relatively easily.
You are not looking for a few millivolts, but a large difference from what you expect or is marked.
wme_bill m0wpn.

Last edited by WME_bill; 23rd Mar 2020 at 11:03 pm.
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Old 24th Mar 2020, 12:04 pm   #3
Kopferkingel
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Default Re: LEADER LBO-301 stops sweeping after 5 seconds

Thank you very much for the hint. Iīve been able to get the reading from TP601, this is OK and as it should. The next test point TP602 shows just nothing or nonsense. The PCB however is the one without any component descriptions and the parts displacement is slightly different from the schematic. This is massively frustrating. Also the disassembly is impossible without complete desoldering of all leads and switches. This is just nightmare to work on! I donīt think Iīll be able to fix this one, since I donīt understand where the TP601 signal should lead to and I canīt even look at the part side of the PCB properly to inspect the components. I think Iīll just have to move on and let it be.
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Old 24th Mar 2020, 1:46 pm   #4
Bazz4CQJ
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Default Re: LEADER LBO-301 stops sweeping after 5 seconds

I don't think this will be useful to you, but for what it's worth, I have a Leader scope (different model) - the only problems I've ever had with it were tantalum capacitors going short circuit.

B
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Old 24th Mar 2020, 3:15 pm   #5
The Philpott
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Default Re: LEADER LBO-301 stops sweeping after 5 seconds

Would fault tracing with selective application of freezer spray be of use? (whilst taking precautions against electric shock)

Dave
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Old 24th Mar 2020, 4:25 pm   #6
Kopferkingel
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Default Re: LEADER LBO-301 stops sweeping after 5 seconds

Iīve tried the spray already. This was no use. As far as the capacitors are concerned, there are only electrolytic, plastic film and ceramic capacitors. Tracing any signal here is next to impossible since there are no parts descriptions on the solder side of the PCB and the parts side canīt be inspected either. It is tightly held in place with bunches of wires.
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