Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:Progress indeed Harry...I'll leave you get on with it...
178k is near enough, even 170-190k should be fine. Don't forget that multimeters on the resistance ranges are less accurate than on voltage, plus it may need calibrating - but few do that. A 1% resistor is more than likely to be more accurate than the meter.
Just because things may be 'digital' it doesn't mean they're accurate. e.g. A DS18B20 direct-to-digital thermometer has a resolution of 1/16 of a degree Celcius. If it's saying the temperature is 20.0000 degrees and it gets slightly warmer it outputs 20.0625 degrees. But the accuracy is +/-0.5 degrees. It actually could be anywhere, say 20.4375 degrees.
So when matching components, it's usually not the precise value that's important, but they read as close as required.
Steve A.
Harry Dalek wrote: ...to find the same reading on all meters its always close but never 100% the same .
Steve Anderson wrote:There's an old saying in horology (the study of timekeeping), "A man with one clock always knows what the time is, the man with two clocks is never sure."
Steve A.
Steve Anderson wrote:Yep, as I expected, you'll have to add those two resistors otherwise as you see you've got a bit too much deflection. Let us know what values you end up fitting and I'll add them to the circuits for future reference. They may not be the same value...
Steve A.
Steve Anderson wrote:As a starting point make the resistors equal to the 'size' pot value, than adjust as (if) required.
You may find you'll have to do the same with the 'position' pots too - see how it goes.
Steve A.
Steve Anderson wrote:More progress. The 'position' controls are a little one-sided, but you can centre it so leave it as is for now, you can come back later if you wish and do it.
Now you have the deflection amps working you should be able to get a better focus by adjusting the focus and asitig contols, if not let me know. Remember this is an older design of tube so it's not going to be quite as sharp as the 3BP1, it's also running at a much lower voltage.
[/quote]Adjust the 'bias' pots pots in each deflection amp so the ramp waveforms at the collectors of the output transistors are centred around +180V or thereabouts with the raster centred. They probably won't be exactly equal, a few volts either way shouldn't make any difference. Then twiddle the focus/astig pots. Turn the brightness down a way, these older tubes were not that bright, nowhere near what you may be used to. Setting them too bright leads to de-focusing which can't be fixed. Remember modern 'scope CRT tubes run on many kVs, sometimes up to 15kV - not just 900V.
Steve A.
Steve Anderson wrote:The brightness will affect the focus, I suspect you've got it too high. These are pre-WWII designs and had to be viewed in 'subdued lighting', much like the early TV CRTs. Normal interior household light is far too bright. Pull the curtains, drop the blinds, turn the brightness down and you'll get far better results. How old are these tubes? No surprise really. Think of dimly lit research labs....
Even the comparatively modern (50s?) DG7-32 3" CRT I have used in the past was far better in low ambient light.
Steve A.
Steve Anderson wrote:These early tubes were susceptible trace movement/distortion when you touched the screen. Later tubes have a very thin layer of Aluminium deposited on the inside of the glass. This stops the electrostatic deflection caused by your hand/body which is (or should be) near ground/earth potential.
On some datasheets you may notice a word similar to 'Aluminiumised', that refers to this process that has been applied to the tube. A side effect is that in some tubes it reflects light from the rear of the phosphor forwards making the trace a bit brighter.
Steve A.
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