Moderators: Dave Moll, Andrew Davie, Steve Anderson
Steve Anderson wrote:Hang on! Something ain't quite right here. Your measurements added to the drawing are 'odd'. The posting of 26th April shows one end of the 220k resistor as -1400V, the other as -1430V, 30V of difference, a current of only 136uA. Whereas the focus pot end of the same resistor shows +388V (I assume relative to the cathode)...so that's about correct at 1.76mA.
Get the tube at a reasonable brightness with the best focus you can and check again. Don't move/bump the controls as you do so.
68k parallel with 1M is about 63k, so no problems there.
Harry Dalek wrote:...and thanks for looking into this for me any advice to make it better is a good thing .
Steve Anderson wrote:Harry Dalek wrote:...and thanks for looking into this for me any advice to make it better is a good thing .
Well, I drew the circuit so I feel it's only fair that I should at least try to help in sorting out any problems with it. Plus we'll both learn from the exercise - others too hopefully.
Steve A.
Steve Anderson wrote:
With the revised voltages above there's 1.7mA through the 220k resistor, somewhat better than yesterday's few hundred microamps. So something has changed, Is it stable across the 5V zener now? Is R904 really 47k?
Harry Dalek wrote:No the 5 volt Zener is still changing with the opto ,and the 47k is showing 47.1K in circuit ...
Would disconnecting the CRT and doing the test work you think ? crt would be the load yes ? with out a load perhaps different result just thinking ..i can change to another crt mid week if need be to see.
The 75 and 5.1v zeners are 1 watt each is that a problem ?
Steve Anderson wrote:There's no harm in disconnecting the CRT, it could be defective, though I don't see how, it;s basically working.
The 1W zeners should be OK.
Steve A.
Harry Dalek wrote:...I can build the circuit as to your design but how the zeners are working with these high voltages my understanding is lacking i just know whats expected in and out sort of with out getting into to much trouble with it. Could you explain not just for me but for any one else interested here from opto slowly light switching its transistor on ,its control of the circuit via its parts .. as in just lighting the CRT part via the CRT circuit
Steve Anderson wrote:You can't use a 78xx type of regulator here as is quiescent current (with no load) is greater (about 3mA) than all the current in the chain, around 2mA. It's not a problem with the zener, it's just showing us that the current isn't enough to reach the zener point. The current is being 'stolen' somewhere and at this distance it's hard to figure out where. Often you need to think in terms of current and voltage is secondary.
This circuit worked fine on your previous build, there's no reason why it shouldn't work here.
I still think it's the ZTX558. Are these from a cheap Chinese batch again?
If so they may be fine with the low volts in the meter when measuring hFE, but perhaps at these higher voltages they give up.
[/quote]I'll get to the other points in the morning. I need to make an important (and probably long) call to the UK in a short while.
Steve A.
Steve Anderson wrote:Harry Dalek wrote:...I can build the circuit as to your design but how the zeners are working with these high voltages my understanding is lacking i just know whats expected in and out sort of with out getting into to much trouble with it. Could you explain not just for me but for any one else interested here from opto slowly light switching its transistor on ,its control of the circuit via its parts .. as in just lighting the CRT part via the CRT circuit
Sorry Harry, not quite sure what you're asking for here...
Steve A.
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