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Old 01-09-2019, 02:04 PM   #21
redwire
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Default Re: Leader lbo 508 oscilloscope help

Colour TV CRT needs around 25kV, too much for a single winding on a 50/60Hz mains transformer. Ferrite cores must need fewer turns (higher flux). Old tube-type colour TV's had no voltage multipler, they used one rectifier and a 25kV flyback winding. Those were huge.

This scope only needs 2kV and it's cheaper to add a 930VAC winding to the power transformer, instead of add another (flyback) transformer and oscillator.

Get rid of the gypsy capacitor meter, it is being fooled by high leakage current. You did measure leakage current or ohms at least on the caps?

The Marcons surely shot. There's five 0.5uF 1500V caps for the -2kVDC rail. C117 does a lot of work and usually goes open-circuit.
Could replace the Marcon's with MKP/PP 0.47uF 1600V or two 1uF in series to get the 1500V.
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Old 01-09-2019, 02:42 PM   #22
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Default Re: Leader lbo 508 oscilloscope help

Yeah I suppose we can't call them flybacks if they're not really flybacks Only TVs had flybacks...

My Tektronix scopes seem to do weird. They have CRTs as well, and the secondary anode is around 8KV or something like that at least. The secondary anode voltage is generated off of a low unregulated voltage (15V from the main SMPS) which goes through the "flyback" (okay, since there's a SMPS flyback topology we can call them flybacks) and a multiplier as well.

These voltages don't quite make sense to me I guess, I do recall older scopes only around 2KV, not sure why the Tek needs much higher voltage despite also using a monochrome tube of similar length as any other oscilloscope?
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Old 01-09-2019, 05:00 PM   #23
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Default Re: Leader lbo 508 oscilloscope help

Scope CRT voltage is all about speed. For a 20MHz scope, a few kV is all you need to light up the phosphor. Even for dual trace, which has half the time to scan the CRT. So dual-beam helped that.

But a 100MHz scope or one that also has to draw on-screen text, or 4-channel, uses up to 14kV. I think it's just how much time there is to excite the phosphor.
There is usually a first and second anode with a tapped multiplier to accelerate the electron beam, like in a TV.

The CRT anode-to-cathode voltage is the net potential, scopes usually run the cathode around -2kV and the anode is either grounded or up at +6kv to +10kV giving a total 8-12kV for the beam.
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Old 01-09-2019, 05:14 PM   #24
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Default Re: Leader lbo 508 oscilloscope help

That would explain it...well, not completely, but close enough.

I think both scopes use the same voltage but one is a DSO and really doesn't need the speed... They use the same tube, however.
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Old 01-09-2019, 06:52 PM   #25
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Default Re: Leader lbo 508 oscilloscope help

that explains why they said "fuck it" in the end and digitised the signal and used monitors instead
i wouldnt mind one of the old HP's like the 52 or 54 series, but idiots bid them to stupid levels in ebay!
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Old 01-09-2019, 11:02 PM   #26
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Default Re: Leader lbo 508 oscilloscope help

Analog scope CRT's use electrostatic deflection. H and V deflection plates and tight geometry to keep the screen linear. Limited acceleration (brightness), it's hard to bend a fast beam with an E-field more than 300V with transistors. The phosphor does the storage.

Old digital scopes use a monitor, a different CRT with electromagnetic deflection (yoke), like TV's so well suited for video.
The CPU buffers the waveform so slow scanning can be used, with constant refresh, RAM does the storage.

I think the last true analog+digital scope with electrostatic CRT is Tektronix 2465. It's a beast, super complicated. The on-screen cursors and text are done like as an extra analog channel with DACs.
I've used HP54600- it's digital (monitor) I figure. Played Asteroids on it, the HP easter egg. 8-bit ADC and a very good scope at capturing pulses.
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Old 01-10-2019, 12:50 AM   #27
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Red face Re: Leader lbo 508 oscilloscope help

Quote:
Originally Posted by redwire View Post
Get rid of the gypsy capacitor meter, it is being fooled by high leakage current. You did measure leakage current or ohms at least on the caps?
It's the cap function on my Aneng meter...the only thing I've got ATM, so I don't know if it's good or bad as per your saying
At the moment we currently don't know if the scope even fires up. If it does (which it most likely does according to the old man's sayings), then we could say the CRT section to be fine, so the cap would be also fine, or at least within workable tolerances...
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Old 01-10-2019, 07:38 AM   #28
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Thumbs up Re: Leader lbo 508 oscilloscope help

Giving this a read to learn some stuff. It's calling what we nowadays call a SMPS, a "high efficiency supply", so I know it's quite old but nevermind - it perfectly matches the relative age of this scope we're discussing. Keeps mentioning this "beam finder" feature...haven't seen anything of the sorts on this scope and I'm not entirely sure what it does. Simple logic would dictate it centers the waveform on the screen if it happens to be drawn outside the screen. The manual mentions this as a way of testing some stuff.
I shall come back once we actually do something to it. Right now it's just sitting on my desk...I would've gladly plugged it in by now, but I don't want the old geezer getting into my hair because the thing popped on my desk...let it pop on his if anything
Attached Files
File Type: pdf troubleshooting-scopes.pdf (3.57 MB, 4 views)

Last edited by Dannyx; 01-10-2019 at 07:55 AM..
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Old 01-10-2019, 11:04 AM   #29
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Default Re: Leader lbo 508 oscilloscope help

i used to put stuff like that outside for the first test - not worth filling the room with smoke or bits of wet paper and aluminium!!
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Old 01-10-2019, 11:09 AM   #30
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Default Re: Leader lbo 508 oscilloscope help

All the more reason to let someone else try it...the room is very large so I'm not worried about sh!t hitting me in the back of the neck Ok, jokes aside, I'll keep you posted.
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Old 01-10-2019, 01:31 PM   #31
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Default Re: Leader lbo 508 oscilloscope help

You could power it up with a variac.
I'd make sure the mains fuse is the right value, so a shorted cap does not damage anything like the power transformer.
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Old 01-10-2019, 02:15 PM   #32
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Default Re: Leader lbo 508 oscilloscope help

"Beam Finder" is a Tektronix feature, you may have to ignore it for other brands and DSOs... For Tek analog scopes, the "Beam Finder" shrinks whatever image that should be on the screen onto the screen, so if you had a waveform that was completely off screen, it would now be on screen.

I suspect other scopes could have this feature named differently but it's definitely something Tek scopes had.
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Old 01-10-2019, 02:19 PM   #33
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Default Re: Leader lbo 508 oscilloscope help

Telquipment had that, so Tek probably aquired it when they purchased telequipment.
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Old 01-10-2019, 02:40 PM   #34
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Default Re: Leader lbo 508 oscilloscope help

That manual I posted seems to be Tektronix related, now that I better look at it.
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Old Today, 07:23 AM   #35
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Default Re: Leader lbo 508 oscilloscope help

Ok, guys, update: the scope appears to work. The old man fired it up today at last and after a bit of fiddling with some of the knobs a line came on screen. He hooked up a probe and touching the end made some ripples on the screen, so the CRT section IS working...I'm not sure how precise the readings are, since he hasn't tried measuring any known reference values on it, but it displays something nonetheless...will see how this goes. He's not very vexed with a scope, clearly, and neither am I - the only difference is that I'm not afraid to admit it
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