Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:Harry, I suggest you stop for the day. I'm trying to put together an explanation how this all works, but it may take me some time...
Steve A.
Steve Anderson wrote:Right, here goes...
First we'll remove the opto-coupler, transistor, CRT and R905.
Next simplify the three resistors and one pot at the top of the drawing into a single resistor. You can forget the focus current, it's just a few micro-amps.
From this we can work out the voltages and the chains current draw...some components are at high voltages, but that doesn't matter, it's the voltage the device 'sees' that counts.\
Part 2 next. Any questions so far?
Sorry about the text..nothing I can do about it really...no, I'll start over, but have a look at the above...Text problem fixed! Quickly too!
I've rounded the numbers to integers to remove the rather useless decimals.
Just like resistors, zeners have a tolerance, usually 5% at a specified test current, if the current is different to the test current that adds or subtracts to/from the final value. I always assume 10% for zeners.Harry Dalek wrote: ...the zeners and diode used to drop the voltage so the 75 v zener drops the 1520 by 75 volts or around that i see that in the readings ..
Well zeners are a poor-mans regulator, put it this way - it's far better than a resistor which has no regulation at all. Though doubtless a Chinese company would 'try it on'. Not recommended!MMM yes could you use resistors here ? ..Zener do keep the voltage constant i suppose.
Grid cut-off (no trace at all) is specified in the datasheet at a certain negative voltage measured from the cathode, there is rarely a tolerance quoted, so -75V on the grid is sure to kill the beam current for this type of CRT. There's no point in increasing the -ve voltage, once the beam is cut-off, it's cut-off.Also why the needed 75 volts drop?
It's there to stop the grid going positive relative to the cathode, that's all, it's not obvious - I'll come back to it later on....the diode in914 i mentioned what i thought it might be there for ? i can see you are dropping the voltage a touch to cathode and more by the time it gets to the focus control / understandable to supply the crts needs.
Steve Anderson wrote:Just like resistors, zeners have a tolerance, usually 5% at a specified test current, if the current is different to the test current that adds or subtracts to/from the final value. I always assume 10% for zeners.
Well zeners are a poor-mans regulator, put it this way - it's far better than a resistor which has no regulation at all. Though doubtless a Chinese company would 'try it on'. Not recommended!
Grid cut-off (no trace at all) is specified in the datasheet at a certain negative voltage measured from the cathode, there is rarely a tolerance quoted, so -75V on the grid is sure to kill the beam current for this type of CRT. There's no point in increasing the -ve voltage, once the beam is cut-off, it's cut-off.
It's there to stop the grid going positive relative to the cathode, that's all, it's not obvious - I'll come back to it later on.the diode in914 i mentioned what i thought it might be there for ? i can see you are dropping the voltage a touch to cathode and more by the time it gets to the focus control / understandable to supply the crts needs.
The -1520v will vary with CRT brightness and a few other factors, it's partly due to the voltage doublers used in the power supply. Also if your 240v mains goes up by 2v (someone switching something off in the house) the -1520v will increase to -1532v. It's nothing to worry about. Even the 240v coming into your house will 'wobble' a bit as neighbours switch things on or off.
Part 2 tomorrow...
Steve A.
What's a good current source? A transistor. And in this case a phototransistor, the output part of an opto-coupler.
Think current, voltage is a result of current. Current is the flow of electrons, hence the name electronics...
All of this is a simplification, there are other factors to be considered, but hopefully this is a beginning.
Harry Dalek wrote:Do resistors change current when warmed up ? just wondering if the larger 2w 230 470k could cause this problem with the Zener ...
Harry Dalek wrote:I am trying to understand how you came about the idea for this...
Steve Anderson wrote:Not being 'funny', but it's best to try and find what's wrong with what you have in front of you. Changing things, "Oh, it might be this." without any reasoning is just throwing one more variable into the mix. It's a bit like, "My car won't start - I'll check the brakes." Extreme, ridiculous, but done without reasoning what the problem actually is.
..."what about Parallel Transistors Current Sharing more to see the result ?"...not sure what you're trying to say here, but the fact remains that this worked fine on three previous builds - why not this time? That's the crux of the matter. There's no point in changing things, something is simply wrong here. not as it should be.
[/quote]"I wonder if its worth building this on a board..." I was going to suggest a a new build, using genuine components, on fresh new board, from the circuit diagram, and NOT copying the existing build. i.e. Start from step one, do a layout on paper first, make sure it's correct and build from that.
Steve A.
Steve Anderson wrote:Harry before you do the re-build I'd like you to check one last thing - check the voltage across the photo-transistor, does that behave in a similar manner to the 5v1 zener?
Steve A.
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