Scope Clock with 3LO1i

[Max DN]

Hello,

I managed to successfully build a scope clock almost entirely based on Cathode Corner design using CRT 3RP1A.

I happen to have a 3LO1i (I know it only lasts for 1000 hours...) and I tried to modify the voltage doublers to use my circuit with this CRT but I'm not too sure about the correct voltages. At the moment I have:

Grid: -480V; HTR2: -380V; HTR1: -384V; HTR1-HRT2: 6.3VDC; CATH: -384V; A1: -248V; A2: +200V

However I cannot get a small green dot, only a wide green shade, picture attached.

I'm sure I'm doing something wrong with the voltage divider. Any suggestions please?

Thank you,

Max

[gregebert]

I have 12 of these tubes for a future project, so I am very interested in seeing this work. Do you have any details which pin numbers on the 3L01I are connected to the circuit ? I think the voltage for focusing needs to be adjusted.

Does the size or shape of the "dot" change when R43 or R44 are varied ?

[Grahame]

Hi Max

https://threeneurons.files.wordpress.com/2015/05/3lo1i.pdf

gives the normal operation acceleration anode A2 voltage as 500V (WRT cathode) and the focus anode A1 as 0 to 50V. Your voltages are higher, especially the focus anode voltage. Hence the green splat rather than a dot.

My experience with this tube and the 6LO1i which is the slightly larger rectangular version is very poor. The phosphor literally turned grey, then black and the light emission faded over a couple of months in use. It wasn't that the phosphor burned with the image but the phosphor greyed uniformly. Most odd. Might have it been a bad batch of tubes? IIRC someone else has seen this effect.

If you want a small tube then look out for a 1CP1 or DH3-91 or CV2302 would be my suggestion.

Grahame

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[Max DN]

@Grahame

Thanks for the suggestion. I didn't know about CV2302 DH3-91, I see Langrex is selling it for £55 on eBay.

Yes, my voltages were off. I have made some changes:

Cathode is now -350V to ground

A1 is +30V above cathode

A2 is +150V above ground, that is +500V above cathode

I'm having some problems with Grid voltage, I'm guessing...

@gregebert 

This is the pinout I'm using:

Pins 1-14 heater

Pin 2 Cathode

Pin 3 Grid G1

Pin 4 A1 (focus anode)

Pin 5 n/c

Pin 6 Grid G2

Pin 7 X1

Pin 8 X2

Pin 9 A2 (acceleration anode)

Pin 10 Y1

Pin 11 Y2

Pin 12 Grid G3

Pin 13 n/c

Pins 3-6-12 are all internally connected, other than on mine 3 is no longer soldered on the grid.

Pin 3 or 6 or 12 (Grid) is connected to -525V (that's 40V above -485V negative voltage doubler in Cathode Corner's schematic), still not good (pic attached). I connected a few resistors in series 500k, then 1M to bring voltage down up to -250V, with not much difference. So it's clear I don't understand how to connect the Grid pin!!

I did find a schematic here (suggests to reduce max negative voltage rail to -300V):

https://www.catahoulatech.com/index.php?product=KIT-0001

https://www.catahoulatech.com/products/KIT-0001/OscilloscopeCrtDriverData.pdf

I also find something else that is exactly what I'm trying to do. It's using different POTs and +350V vs -350V (whereas I'm using +250V and -485V)... Schematic attached.

[Grahame]

Hi

£55 is expensive - you should be able to get them for £30.

Have you tied the deflection plates to A2 for testing purposes or left them floating? They should be tied to A2.

Can you turn the brightness pot down to fully extinguish the dot? If not you need to be able to.

Grahame

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[Max Di Noi]

Ok, I managed to make some improvements by changing some resistors on the voltage divider, I'll post my new layout later.

Much better now, although I need to better understand how to calculate resistor values as at the moment I don't know current draws by each crt pin. 

I get a very bright spot on the clock image, I don't know if that's because I have not connected the blanking circuitry yet or overvoltage on A1 (I'm waiting for an optocoupler, hopefully it arrives this week). 

Picture attached. Getting there.

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[Max DN]

I'm attaching the voltage divider that I am using and the one that I'm going to try using values calculated assuming 0.5mA current draw (which I understand makes sense for this CRT).

Sharing this useful website to calculate resistor values using parameters from the datasheet, it is starting to make sense now!

https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1946/12/page45/index.html

Blanking pin currently disconnected (still waiting for an optocoupler). I wonder whether that will fix the bright spot (of course it will fix the smears on the display).

@ gregebert - I may find this latest update useful

[Grahame]

I would guess that the bright spot is where the beam rests after drawing the face and is waiting for the next refresh trigger.

My own scope clock draws each face once every 20mS (16.66mS in 60Hz land) synchronised to the mains.  If a particular face takes, say, 8mS to draw then the beam is parked somewhere and left blanked for the remaining 12mS (in this example) waiting for the next face refresh trigger. So if the beam is left on for the waiting period then it will appear as a very bright spot at the parked position. You'd have to investigate the software you're using for what triggers the face drawing and what happens in the waiting period.

Grahame

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[Instrument Resources of America]

Be careful not to burn a spot on the screens phosphor coating. High intensity, coupled with length of time in one spot is what determines if a burn occurs.

Ira.

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[Max DN]

Yes, of course. Thank you

[Max DN]

I agree, that bright dot must be the beam resting on the same spot for too long. I'll check the code, where the refresh seems to be driven by the blanking pin, so with no blanking currently connected (still missing a part), the dot is not been managed.

I thought that the same settings as for 3RP1A would work on 3LO1i but I guess not, they must have different physical characteristics. 

[Anders Mikkelsen]

The current draw for CRTs is mainly between the cathode and A2/A4 (main accelerating anode). A majority of this current flows directly between the electrodes, while a small fraction is represented by the actual beam current hitting the phosphor screen. You need to ensure that the impedance driving these electrodes is low enough that the voltages don't move around a lot when adjusting the brightness grid bias, otherwise the focus, astigmatism and brightness controls will interact in ways that make it hard to control, and brightness modulation will also affect focus.

[Max DN]

@ Anders Thanks for that. Yes, I'm sure you are right.

At the moment I can only get it to work well with Brightness and Focus potentiometer in parallel (it's working ok, but I want to change that and have those potentiometers in series as per standard config). Picture attached (CRT came with a center point already a bit burnt, no biggy, it'll likely extinguish itself soon anyway!). I only have 500k and 1M POTs at the moment. I'm waiting for a 100k POT for the Brightness and from my calcs that should help given my power supply (+240V and -450V).

I'll update you when I receive the parts and I'll post my solution for the resistor bleeder.

@Grahame. I bought a DH3/91 - super cute CRT! I cannot wait to try it.

[Grahame]

Hi

The DH3/91 is a nice tube. It has "automatic focus" so there is no external connection to the focus anode. To keep the pin count down to 8 it has asymmetrical Y deflection but it is an easy low voltage tube to use.

 I used one in this project

http://www.sgitheach.org.uk/scope1.html

Grahame

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[Max Di Noi]

I'm happy to share a little success, after so much work!

Colours not to everyone's taste, I'm sure! 

For the resistor divider, I used a 500k pot for intensity (I connected the cathode there) and 1Mega pot for the focus A1 followed by a 2Mega resistor to ground. 

Voltages -450v to +250v.

I hope you enjoy the pictures and thanks to everyone for all suggestions! 

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