Leader LBO-514 warmup time - Page 1
Forum section: electronics / testgear

author: Kostas
author_rank: Regular Contributor
timestamp: December 22, 2015, 08:58:24 pm
content: Hello!I recently got my first oscilloscope. It's not a great one, it's a 10MHz Leader LBO-514, but I got it for free, so I can't complain. After all, any scope is better than no scope. It has only seen very light use for the last 30 years or so, it actually looks new. Everything seems to work fine, but there's only one thing that bothers me. The traces take about 20 to 30'' to appear on screen, depending on the intensity setting and probably the ambient temperature (the "lab" can get quite cold at times). So, is it normal, or is it a sign of underlying problems?

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2015, 10:00:30 pm
content: CRO's do vary in warmup times, 20-30 seconds seems long but for that model I'm unsure.As with any old scope you could check the LV PSU power rails for ripple as the E-caps are getting on in age whether its been sitting around or not.

author: Kostas
author_rank: Regular Contributor
timestamp: December 22, 2015, 10:06:27 pm
content: So, you mean the low voltage psu might be malfunctioning to some extent and affect the performance of the filaments of the tube?

author: tautech
author_rank: Super Contributor
timestamp: December 22, 2015, 10:13:05 pm
content: Quote from: Kostas on December 22, 2015, 10:06:27 pmNot precisely.The CRT tube heaters are normally AC.I was thinking that maybe excessive ripple on the LV DC rails is maybe affecting the warmup.Just check the LV DC rails are to spec, voltage and ripple and visually check for bulging/leaking caps then use/enjoy it.

author: Kostas
author_rank: Regular Contributor
timestamp: December 22, 2015, 10:16:29 pm
content: Thanks Tautech!

author: Seekonk
author_rank: Super Contributor
timestamp: December 22, 2015, 10:30:12 pm
content: This is a long shot, but try unplugging the CRT socket and reinserting it. Corrosion on the filament pins may be dropping the voltage. Reality is that even if it wasn't used much, the tube can get gassy. There was a thread here about rejuvenating a CRT tube. Not permanent but it can get you a couple years.

author: Kostas
author_rank: Regular Contributor
timestamp: December 22, 2015, 11:34:46 pm
content: Ah yes, crt rejuvenation. Years ago, we had a crt tv that took ages to show anything. Sound was ok, everything else was ok, but no picture for something like 15'. The repair guy warned that it might work for some months, but eventually it will do the same. Well, it did work for about 3 months, albeit with a little distorted colours.

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2015, 12:00:51 am
content: Quote from: Kostas on December 22, 2015, 11:34:46 pmShould you need it, a link from some previous thread a while ago:http://www.ke5fx.com/crt.html

author: Kostas
author_rank: Regular Contributor
timestamp: December 23, 2015, 10:12:57 am
content: Ok, I took a quick look inside. Its very clean for it's age, as in squeaky clean. All electrolytic capacitors look fine and they're made by Nippon Chemi-Con. I found 4 test points: +15V, -15V, +250V, +1750V. The low voltage ones are +-14.7V, off by only 2%. The 250V was 235 or thereabouts, still reasonably close. Is there a chance that the electrolytic capacitors have "aged" too much while the scope was sitting there unused for years?Anyway, FWIW, I made a ring oscillator on a breadboard with a 74LS04. I used 3 gates and got this waveform:x10 probe, 0,1V/div, 0,1us/div, so roughly 35MHz frequency and 1,1V amplitude. Normal gate output was 4,2V. I guess this means -11,6dB at this frequency.

author: tautech
author_rank: Super Contributor
timestamp: December 23, 2015, 10:40:14 am
content: Quote from: Kostas on December 23, 2015, 10:12:57 amPossibly.They mayget better as they re-form with use.That the 15V rails are low is more important.Check each for ripple using your DMM set to AC mV, they each should be less than 10 mV, under 5 better.Presuming your DMM is accurate and ripple is low the voltages should be adjusted exactly to spec for better scope accuracy.Stay away from the HV, it's usually best left alone unless you have HV gear.

author: Kostas
author_rank: Regular Contributor
timestamp: December 23, 2015, 04:09:39 pm
content: Quote from: tautech on December 23, 2015, 10:40:14 amThey might be getting better. I checked again and it seems it got a tad quicker.Quote from: tautech on December 23, 2015, 10:40:14 ammV AC scale? Hehe, nope! I probably have the lamest DMM here, a cheapo Mastech MS8221A.There's only a 2V AC scale. FWIW, the reading was 0,001V. I also found the schematics of the scope. The power supply doesn't have any integrated voltage regulators. Two 5,6V zener diodes are used to get the +-15V and there aren't any trimmers to adjust voltage. I don't have very high accuracy expectations from this anyway, it already seems good enough. I also tried to measure the 6,3V AC for the tube's heaters, but its tough to get there. In any case, the scope as is is perfectly usable, I was just worried about the screen turn on time.Anyway, thanks a lot to both of you.