Leader lbo 508 oscilloscope help

[redwire]

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.

[eccerr0r]

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?

[redwire]

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.

[eccerr0r]

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.

[stj]

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!

[redwire]

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.

[Dannyx]

Re: Leader lbo 508 oscilloscope help

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...

[Dannyx]

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

[stj]

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!!

[Dannyx]

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.

[redwire]

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.

[eccerr0r]

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.

[stj]

Re: Leader lbo 508 oscilloscope help

Telquipment had that, so Tek probably aquired it when they purchased telequipment.

[Dannyx]

Re: Leader lbo 508 oscilloscope help

That manual I posted seems to be Tektronix related, now that I better look at it.

[Dannyx]

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

[eccerr0r]

Re: Leader lbo 508 oscilloscope help

I suspect a word substitution of "s/vexed/well versed/" unless you were planning on being annoyed at the scope, that would not be a plan for success!

It may well be working fine. Calibration is always an issue but sometimes it's not needed, just knowing the waveform helps tremendously.

[Dannyx]

Re: Leader lbo 508 oscilloscope help

I didn't get what you meant at first, but yeah - wrong word there....my Ingly is a bit rusty I meant to say he's not a gleaming expert, as he's still finding his way around it by trying out all the knobly bits and buttons first, just like I would do it too, unlike someone who clearly knows exactly what each and every control does and what everything does, even though scopes vary greatly in design.

[eccerr0r]

Re: Leader lbo 508 oscilloscope help

Yeah I figured as much, taking what you wrote at face value, it didn't make sense as being vexed is not exactly a good thing - something that happens infrustration. Alas "well versed" (synonym of "experienced") is somewhat of a colloquialism however, but it makes more sense and thus I suspected that's what you meant.

Oddly enough a lot of post 1960s scopes that have triggered sweeps are all designed fairly similarly. The pre 1960s scopes that don't have triggers are a bit more difficult to use, and of course the 2000s DSOs with menus hidden behind menus can be confusing to use too... The scopes that have dual timebase are the sneaky ones.

I spotted an old HP scope with the Beam Finder button, so I guess even HP has it, at least on some scopes. I just don't have it on my late 1970s (noname) scope.

[Dannyx]

Re: Leader lbo 508 oscilloscope help

Now I just have to get him to quit hogging it so I can have a go...sucks it doesn't actually display any values, so you have to do it "manually" by counting divisions and stuff...it's actually harder to use than I thought, especially for someone who's never used one.

He was measuring something on a tuner for instance (it's not tuning and channels) and the scope showed this squiggly line....yeah, great, so what is that ?! Does it work ? Is it good is it bad ? We're not even sure this thing works right, because the old man apparently failed to noticed the calibration hook on the front of the machine (or didn't know what it's for, having confessed that he didn't actually read the manual and his English is even worse than mine) which you'd use to tune it, so he just rolled with it as it is, resulting in...something, but this something could be anything: noise, random voltages...it's a bit of fun trying to figure this stuff out actually

[stj]

Re: Leader lbo 508 oscilloscope help

that hook is for setting the trimmer in the probe.