Hi Everyone;
I have an LBO-516 scope, getting a bit 'long in the tooth' but too nice to throw out. I works perfectly for a few moments after turn on, then there is no trace.
Has anyone gone through this with their unit?
Thanks for any help, Peter in Canada
I have an LBO-516 scope, getting a bit 'long in the tooth' but too nice to throw out. I works perfectly for a few moments after turn on, then there is no trace.
Has anyone gone through this with their unit?
Thanks for any help, Peter in Canada
How does trace go away? Does it fade or slide off screen to top or bottom?
Could make a big difference.
Just looked and manual IS available on bama.edebris.com/manuals/leader/
Doc
Could make a big difference.
Just looked and manual IS available on bama.edebris.com/manuals/leader/
Doc
Hi and thanks for your response. There is no trace at all today. When it disappeared, it was instant, and not sliding in any direction. I have a little knowledge (enough to get me in trouble).
Disappearing rather than shifting off screen, I would assume deflection circuits OK.
Instant disappearance, not fading, can I assume filament in CRT is OK?
I set up scope as per manual with the test probe 1 on the calibration voltage terminal on the front panel. Even when there is no trace, the 'triggered' light is on, and off with probe removed from ch 1 BNC input.
I have left the scope for an hour, now there is a trace, all normal and gentle tapping on pcbs, slight flexing of scope chassis (with cover removed) cannot product the malfunction.
I will turn the unit off, let it cool, and try again. I would certainly appreciate any thoughts, thanks, Peter
Disappearing rather than shifting off screen, I would assume deflection circuits OK.
Instant disappearance, not fading, can I assume filament in CRT is OK?
I set up scope as per manual with the test probe 1 on the calibration voltage terminal on the front panel. Even when there is no trace, the 'triggered' light is on, and off with probe removed from ch 1 BNC input.
I have left the scope for an hour, now there is a trace, all normal and gentle tapping on pcbs, slight flexing of scope chassis (with cover removed) cannot product the malfunction.
I will turn the unit off, let it cool, and try again. I would certainly appreciate any thoughts, thanks, Peter
I do not know this particular 'scope, but the very first rule with all analog scopes is to measure every supply rail (DC and ripple).
Very doubtful that ripple is the problem here, but a missing or low rail can give such symptoms as the tube beam is turning off.
Next is to check the HV supplies, but only if you are SURE what you are doing!
Very doubtful that ripple is the problem here, but a missing or low rail can give such symptoms as the tube beam is turning off.
Next is to check the HV supplies, but only if you are SURE what you are doing!
When trace is there, how is the brightness? (Intensity) How well can you see it at high sweep rates? You say trace returned after the scope was left on a while. Turn it off for ten minutes, does trace reappear within a minute?
What I am concerned over is the state of your cathode material in your CRT. Less so from your statement the disappearance was instant. That sounds more like perhaps a bad filter cap that arced internally and took down a supply voltage. Scope might temporarily recover from such. Loss of cathode material would result in dimmer and dimmer trace over time, but would seem to get better after heater in CRT is left on for longer periods.
Cathodes are coated with thoriated tungsten. When heated by filament this releases electrons which get accelerated toward the front of CRT by the high voltage on front. This electron beam strikes the phosphor on the face, causing it to emmit photons. Depleted cathode material equals fewer free electrons equals dimmer trace.
So... Your trace going away suddenly is a good thing! It means that your CRT is probably okay.
Should problem come back I'd look at your power supply caps.
Doc
What I am concerned over is the state of your cathode material in your CRT. Less so from your statement the disappearance was instant. That sounds more like perhaps a bad filter cap that arced internally and took down a supply voltage. Scope might temporarily recover from such. Loss of cathode material would result in dimmer and dimmer trace over time, but would seem to get better after heater in CRT is left on for longer periods.
Cathodes are coated with thoriated tungsten. When heated by filament this releases electrons which get accelerated toward the front of CRT by the high voltage on front. This electron beam strikes the phosphor on the face, causing it to emmit photons. Depleted cathode material equals fewer free electrons equals dimmer trace.
So... Your trace going away suddenly is a good thing! It means that your CRT is probably okay.
Should problem come back I'd look at your power supply caps.
Doc
Thanks Cliff;
I thought because I saw the 'triggered' light functioning as it should when a signal was applied, that most of the unit (and LV power supplies) were probably OK.
Regarding the HV supply for the CRT, the operator's manual mentions a meter with 2kV capacity. I would suspect that the CRT B+ was higher. I have some 'fly by night tv repair experience from the 70's. Is there a way to test the CRT HV with out out a special meter? I guess shorting the HV lead to the chassis would be a bad idea?
I thought because I saw the 'triggered' light functioning as it should when a signal was applied, that most of the unit (and LV power supplies) were probably OK.
Regarding the HV supply for the CRT, the operator's manual mentions a meter with 2kV capacity. I would suspect that the CRT B+ was higher. I have some 'fly by night tv repair experience from the 70's. Is there a way to test the CRT HV with out out a special meter? I guess shorting the HV lead to the chassis would be a bad idea?
Hi Thaumaturge;
When there is a trace, the brightness ( and focus, actually all functions, I think) are completely normal, no fluctuation or fading. I will do some on/off tests, time periods as you suggest. At first I thought something warming up caused the failure, now it appears warming up resolves the problem. I'm really hoping it is a bad connection somewhere, but it is a complex unit (to me) and many components are not accessible without some serious disassembly. Thanks again, Doc, I will let you know regarding the time periods verses trace appearance
When there is a trace, the brightness ( and focus, actually all functions, I think) are completely normal, no fluctuation or fading. I will do some on/off tests, time periods as you suggest. At first I thought something warming up caused the failure, now it appears warming up resolves the problem. I'm really hoping it is a bad connection somewhere, but it is a complex unit (to me) and many components are not accessible without some serious disassembly. Thanks again, Doc, I will let you know regarding the time periods verses trace appearance
TV needs 10Kv plus, because of the relatively enormous screen size hence deflection angles.
AFAIR, scopes use 2 to 3 Kv.
An HV meter can be bodged using a resistor chain. I would use 9 10MR in series inside a polythene (fish tank air?) tube. With the standard meter 10MR Rin, this will give 10:1 at 100MR impedance. Adjust R's to taste.
Remove tube from fish-tank first .... Obviously this is not for the novice. 10% tolerance is quite good enough to check it is there or not. It can be left on the meter 20v range to give 2Kv FSD, and wait for the fault.
AFAIR, scopes use 2 to 3 Kv.
An HV meter can be bodged using a resistor chain. I would use 9 10MR in series inside a polythene (fish tank air?) tube. With the standard meter 10MR Rin, this will give 10:1 at 100MR impedance. Adjust R's to taste.
Remove tube from fish-tank first .... Obviously this is not for the novice. 10% tolerance is quite good enough to check it is there or not. It can be left on the meter 20v range to give 2Kv FSD, and wait for the fault.
Hi Cliff; I feel a little foolish, now the scope will not malfunction, good I guess. Regarding your HV test setup, did you mean 910 Meg ohm? Wouldn't this value be hard to find in my electronics graveyard? Could I use a 10 Meg and a 1 Meg, in a series voltage divider to ground. With my Fluke meter across the 1 Meg.
Is 11 Meg to much of a current draw for the HV supply. Just seems that a voltage divider is safer and more accurate, if I don't know my meter's internal impedence
And don't worry, fish are long gone, (previous HV experiments, gone horribly wrong)
Thanks for all your help so far, Peter
Is 11 Meg to much of a current draw for the HV supply. Just seems that a voltage divider is safer and more accurate, if I don't know my meter's internal impedence
And don't worry, fish are long gone, (previous HV experiments, gone horribly wrong)
Thanks for all your help so far, Peter
sorry, I meant 9 pieces of 10megohms in series = 90MR. Easily found cheap components.
All small DVMs I have seen (not very many!) Have attenuators giving 10eg input impedance except the very lowest range (200mV?) which can be =>~
Of course you can use any values and do the maths. I like a long chain as the voltage stress is shared, but no use for 'scope probes as the bandwidth is about 10Hz!
All small DVMs I have seen (not very many!) Have attenuators giving 10eg input impedance except the very lowest range (200mV?) which can be =>~
Of course you can use any values and do the maths. I like a long chain as the voltage stress is shared, but no use for 'scope probes as the bandwidth is about 10Hz!
Thanks Cliff; Maybe I should rig this up and test it while it is working. I will look for a Voltage figure on the schematic, to test my rig.
Thanks again, from Canada
Thanks again, from Canada
BTW, service manual is here:
LEADER LBO-516 SM Service Manual free download, schematics, eeprom, repair info for electronics
EHT is 1950v.
General design and construction is Tektronix 475 era, so very serviceable.
I just cannot remember (20 years!) what is the common cause of beams turning off and on. 😕
LEADER LBO-516 SM Service Manual free download, schematics, eeprom, repair info for electronics
EHT is 1950v.
General design and construction is Tektronix 475 era, so very serviceable.
I just cannot remember (20 years!) what is the common cause of beams turning off and on. 😕
The Fluke high voltage probe uses a 1.09 gigohm resistor in a voltage divider with the 10meg meter input in parallel with another resistor for a 1000:1 ratio. Mouser has 1Gohm Vishay resistors for under $8. Rated up to 15KV. Add a 90Meg in series. I think the other resistor is a 100Mohm.You'd have to calculate it out. FHV2001G00FNEB Vishay / Techno | Mouser
Doc
Doc
Hi thaumaturge,
I would be very worried about flash-over if I were you. The creep distance is pretty long on HV probes. That is what those discs are for.
Hi Peter,
What you should do is buy an HV probe, don't attempt to make one. Resistors come with a voltage rating as well, and that changes things a bit.
I have repaired 'scopes in the past for Leader. My shop is in a dismantled mess right now as I upgrade to bench 3.4 . Should you run into trouble, I can see if I can help. I do have a proper HV probe - for fixing 'scopes as it happens. I once worked for an equipment calibration - service company called Transcat where I did this kind of work as well.
-Chris
I would be very worried about flash-over if I were you. The creep distance is pretty long on HV probes. That is what those discs are for.
Hi Peter,
What you should do is buy an HV probe, don't attempt to make one. Resistors come with a voltage rating as well, and that changes things a bit.
I have repaired 'scopes in the past for Leader. My shop is in a dismantled mess right now as I upgrade to bench 3.4 . Should you run into trouble, I can see if I can help. I do have a proper HV probe - for fixing 'scopes as it happens. I once worked for an equipment calibration - service company called Transcat where I did this kind of work as well.
Correct. But you may have a failure in a deflection crkt, or in the blanking pulse area. Of course, the HV could be down as well. Your HV is probably between 1.5 and 1.9 KV, but a look through the manual would give you the correct figure. The higher end 'scopes can go higher, except that today they are almost all LCD displays. You do have a nice oscilloscope there, perfectly serviceable for most things you might run into. You can PM me if you need help at some point.
-Chris
1. Metrology Engineer Litton Allied Technology 5 years.
2. Senior Metrologist Test Lab Company 4 years.
3. Senior Repair Stabro Laboratory 4 years.
4. Chief transmitter engineer KXIV UHF 1.62MW ERP 4 years.
5. Chief transmitter engineer UofU 2 DTV, 2 VHF, 1 FM 6 years.
6. Senior Repair tech Sperry UNIVAC cal lab. 1 year.
7. Transmitter Engineer KRSP 2 years.
8. Senior microwave prototype tech Montec div E-Systems 1 year.
I've repaired and calibrated MANY HV probes including Fluke, Tek and HP (capacitive divider). I also constructed and safely used my own HV probes.
The link I posted is for a Vishay metal film HV resistor rated to 15KV. On a properly constructed HV probe I would feel safe using it up to 12KV. It would certainly be safe using it to measure 2KV HV input to the capacitor multiplier on a scope.
You do as you feel comfortable. But current going price on a commercial HV probe is about $120 for a Fluke. The 15KV resistor is under $8. Don't trust Vishay's rating? Mount it on a three foot dowel and feed it into a $5 DVM. Safe enough?
Doc
Alternately, you can buy the resistor kit for the Fluke probe right from Fluke part number 939335.
Thanks to everyone for all your help, and thanks to diyaudio. I have had some professional experience with audio and electronics, but little formal training. This forum is reviving my love for my favourite hobby, and my abilities.
Thanks again, Peter in Hillsdale, Ontario
Thanks again, Peter in Hillsdale, Ontario
Hi thaumaturge,
Please assume the intended person for advice knows nothing (or not enough) and advise accordingly. You did not attach any warnings to your advise and while you are free to take whatever risks you want, you have a duty to keep other folks safe.
Metrology labs differ greatly in quality and safety procedures. You would not be approved to use a self made HV probe. Any lab manager who would allow such a thing would be canned pretty much right away. Given what we both know, I think it is telling that I purchased an HV probe from Fluke, although I am knowledgeable enough to build one. I have also built very HV regulators that use basically a built in HV probe. I tend to use at least two HV resistors in series with gaps in the PCB to lengthen the creep distance.
You have to respect very high voltages. Anyone working in an area that might come into contact with those high voltages would need the official warnings from you if you are advising them. If someone were to receive a shock, it may not really injure them, but the resulting gash from involuntary muscle contractions could very easily exceed the probe costs by the medical costs. In Ontario we have almost free health care, but I would like to think you would properly warn everyone, not just those with pay as you go health care.
Please, keep safety in mind at all times. You can zap yourself, but protect others from poor judgement. You and I both know the risks involved first hand.
-Chris
Please assume the intended person for advice knows nothing (or not enough) and advise accordingly. You did not attach any warnings to your advise and while you are free to take whatever risks you want, you have a duty to keep other folks safe.
Metrology labs differ greatly in quality and safety procedures. You would not be approved to use a self made HV probe. Any lab manager who would allow such a thing would be canned pretty much right away. Given what we both know, I think it is telling that I purchased an HV probe from Fluke, although I am knowledgeable enough to build one. I have also built very HV regulators that use basically a built in HV probe. I tend to use at least two HV resistors in series with gaps in the PCB to lengthen the creep distance.
You have to respect very high voltages. Anyone working in an area that might come into contact with those high voltages would need the official warnings from you if you are advising them. If someone were to receive a shock, it may not really injure them, but the resulting gash from involuntary muscle contractions could very easily exceed the probe costs by the medical costs. In Ontario we have almost free health care, but I would like to think you would properly warn everyone, not just those with pay as you go health care.
Please, keep safety in mind at all times. You can zap yourself, but protect others from poor judgement. You and I both know the risks involved first hand.
-Chris
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