Tuesday, August 30, 2016

5LO38I CRT test

Having this CRT for some time, never tested it, so last weekend with the new soldering iron was the right time for it.

Honestly can't remember for which project I got it, in any case it will be useful in the future.. so I hope,

Power supply was based on this diagram:

From this site:

Added some discharge resistors so it unloads faster, even so it would take around 10m to go to 0v.

The brightness and focus control diagram:

From this reference:

Didn't used any circuit for testing the deflection but touching with a long wire is enough to cause deflection, so it's working also.

I might need to tune the controls and power supply since the dot is a little too big, don't know exactly what is the relation between focus voltage differential from the electron gun and the dot size, sure it's related.

The box it's standard soap box in plastic so I'm not tempted to touch the circuit while it's operating. If I finish the project it will be for sure placed in a more attractive enclosure.


Didn't had an isolation transformer 230/230 so used an 230/12 connect to a 15/230v. I had my shares of electric shock's and will no risk touching a rectified point directly to mains.

The controls:
The rectifier board:

upside down on the box so the electrons can drop easily...

And the connection to the CRT pins, this is the best way found since I had no holder for it, no mess with solder and quite isolated:

For reference, additional sites with more information case you want to build  a scope based on this CRT tube:


The technical details of the tube:

Have a nice day!


dimitris K said...

Hi sir. Im into the crt tubes and DIY oscilloscopes lately and i (DE JAVU!!!) have bought an 5Lo38i 2 months ago that i never really tested.
I used your test circuit and guess what. It was a long long time ago that a project i did worked straight with and no problems. Well it worked at once and guess what. Mine dot as well was a bit large and focus pot did almost nothing to fix. Anyway im very happy, i now know my CRT works and it will be used in the future for a DIY oscilloscope.
Be well!!

Ricardo - CT2GQV said...

Hello Dimitris,

Thank you for the comment.
I updated the design, got better (not perfect) results this time regarding the focus.
Hope shortly to create a new post with the info.


dimitris K said...

Well since my intention was to just test the CRT i did not have the time and knowledge to test the focus issue. Just tried the circuit for 10 minutes, wrote some notes about it in paper and disassembled it. I am now studying your links attached and trying to decide for an oscilloscope circuit with solid state components that i will soon start. But there is a lack of complete projects for this CRT except danyk's tube oscilloscope that i try to avoid.
However about the focus issue before cutting the B+ 200V supply i decided to turn off the separate AC transformer feeding the filament. And what i saw was that the dot before vanishing became really sharp and correct for 4-5 seconds (before the filament cooled down and stop emmiting electrons). So this clue maby makes me think that there is a slight possibility that the filament is overdriven with 6.2V RMS (that is the exact voltage i measured on pins 1-11) and we could try lower filament voltages like 4 or 5 vols rms. A friend of mine that is more involved with tubes suggested that i could use DC supply for filament which would be more handy than trying removing transformer winding from the 6.3V transformer or putting power resistor in series with filament since dc allows us to use regulators or even a power diode to drop voltage by 1volt easily.
i am looking forward to see your updated design and maby an oscilloscope project by you.
be well Ricardo!