The 3BP1 Anderson monitor

[Harry Dalek]

Harry, please, please, read and comprehend my posting above. This is important.

The CRT PSU should be as shown below...no connection between the 6.3V winding and anything else - except the pin 1-2 join on the CRT.

Steve A.

OH whoops! i didn't ground cathode to ground but i got mixed up on the heater part and did ....ok i will fix this .

[Steve Anderson]

I suggest unplugging the CRT and putting it aside until all voltages are confirmed correct. You can't damage it if it's not in the same room!

As the Americans say, "Speak softly and carry a big stick."

I learnt from mistakes, we all do, but if I can help circumvent them I will.

Steve A.

[Harry Dalek]

Looks good now Steve i am just checking voltages again and i will go back in the house and write them up / BTW its focus is pretty good and by lucki have not broken the tube so far

[Steve Anderson]

I'm a bit worried about the lack of brightness control.

I suggest you disconnect the opto-isolator transistor and make sure there's around 75V across the 75V Zener, it's easy to get it the wrong way around. While you're about it check the 5v1 zener voltage too and that there's around 0.7V across the 1N914. If all is OK the CRT should be 'dark' if you must leave the heater on...I prefer you didn't though.

Do this with the CRT cold - please. Be careful, all of this is sitting at over 1kV negative.

Steve A.

[Harry Dalek]

I'm a bit worried about the lack of brightness control.

I suggest you disconnect the opto-isolator transistor and make sure there's around 75V across the 75V Zener, it's easy to get it the wrong way around. While you're about it check the 5v1 zener voltage too and that there's around 0.7V across the 1N914.

Do this with the CRT cold - please. Be careful, all of this is sitting at over 1kV negative.

Steve A.

Don't worry the Heater is disconnected Steve for now ...

Yes no brightness control yet working via the opto coupler , the phosphor dot looks very bright via the camera but yep i would not have it go brighter ,i had the focus out at first when i was checking i don't want a burn either onit its still not strong enough to do that ....the dot is dimmer to the eye but i understand once its working correctly this could happen .

The over -1400 volts always have that in my mind when i am checking voltages i will be careful.

The in914 i replaced with the newer 1n4148 is that a mistake ? i should of asked first !

All the voltages i took today are on your schematic seemed easy to do it this way .
i can see a few are off top say the least,and as for the voltage drop or difference you say on the cathode to control grid of 60 voltages well its about the same..
BTW the below raster dot test was done before i listened to you and unhooked the heater ,i forgot to unhook the opto coupler on the zener tests Rap over the knuckles for me i will do that test
I just checked the zeners again taking the opto coupler out ..still a 5.1 drop on the 5.1 volt zener but 75 volt zener its o volts like a wire ! checked via the high negative voltage again 0 drop across the 75 zener same voltage in as out but on high negative its 5.1 drop as it should be on the 5.1volt zener checking it this way also.
I will have another look if its a wiring mistake i never say never on mistakes make enough of those.

[Steve Anderson]

It would appear there's something wrong around the 75V Zener, either it's short or the wrong way round. Disconnect the junction between the emitters of the opto-transistor (pin 3) and the ZTX558 and try again. Is it possible that what you think is a 75V Zener is actually 7.5V?

Good to see slightly higher voltages than I expected. They will come down a bit when we add the load from the deflection amplifiers, but not by much.

The 1N914 and 1N4148 are virtually identical, certainly no difference in this application.

Steve A.

I moved the whole thread from 'NBTV' to 'Electronic TV' where it really belongs. It's taken all this time to figure out how to do it! As usual it was stupidly obvious!

[Harry Dalek]

It would appear there's something wrong around the 75V Zener, either it's short or the wrong way round. Disconnect the junction between the emitters of the opto-transistor (pin 3) and the ZTX558 and try again. Is it possible that what you think is a 75V Zener is actually 7.5V?

For sure i need to look into this area of the circuit ,i having been thinking today while working perhaps i have the collector emitter on the PNP ztx558 wrong way around in the circuit ,the zener looks the right way around ,i did buy 2 if this one is shot ,i did check the numbering on them using all the powers my old eyes and some glasses ,i even picked the one with the letter numbering i could work out better ...
But yes a mistake here is possible you saying it could be a 7.5 zener ,its voltage to it is 1464 out 1456 which is 8 volts about right for this ,also the lady at jaycar told me the 5.1s were the 75 volt visa versa ....ok some thing also to check /

Good to see slightly higher voltages than I expected. They will come down a bit when we add the load from the deflection amplifiers, but not by much.

OK this is good ,i can say the power supply is doing enough to supply the circuit and tube / i will try and work out how to mount it safe so future eyes and fingers know whats there .

The 1N914 and 1N4148 are virtually identical, certainly no difference in this application.

I was a little worried i didn't ask or missed something due to the problem now but looking like the zener so far .

Steve A.

I moved the whole thread from 'NBTV' to 'Electronic TV' where it really belongs. It's taken all this time to figure out how to do it! As usual it was stupidly obvious!

[/quote]

I was thinking i would of got moved at the start as i forgot and posted in the wrong place as long as every ones happy i'm ok where its put .

OK time to look for that spare Zener diode .

[Steve Anderson]

If the ZTX558 is dead run the circuit without it first to ensure you get 75V across the Zener. Add the ZTX558 with no input to the diode, the cathode-grid voltage should be virtually the same. As you increase the current into the opto-coupler LED the grid-cathode volts should decrease to the point where you can almost get 0V, a couple of volts is OK.

Reverse bias the ZTX can withstand 400V base-collector, but base-emitter is only 5V. Check you've got it right. This is almost the standard voltage rating for all transistors base-emitter whatever they may be able to handle on the collector. There are of course exceptions but none of 400V or even 20V.

Steve A.

[Harry Dalek]

If the ZTX558 is dead run the circuit without it first to ensure you get 75V across the Zener. Add the ZTX558 with no input to the diode, the cathode-grid voltage should be virtually the same. As you increase the current into the opto-coupler LED the grid-cathode volts should decrease to the point where you can almost get 0V, a couple of volts is OK.

Reverse bias the ZTX can withstand 400V base-collector, but base-emitter is only 5V. Check you've got it right. This is almost the standard voltage rating for all transistors base-emitter whatever they may be able to handle on the collector. There are of course exceptions but none of 400V or even 20V.

Steve A.

OK Steve i think i have more time tomorrow afternoon , i still have try the Zener test you mentioned removing the emitters on the opto and 558 ,Troy kindly gave me another ztx558 so have a spare if need be but hope i haven't broken it , i will double check this was soldered in correctly as well.

I will report back what i found .

[Harry Dalek]

Just started to look at the problem Steve /// so far transistor is in the right way around /i forgot i did do the test removing the emitters opto trans from ztx558 my opto is in a socket easy to remove results were the same on the 75 volt zener.

I just did a resistance test either side of the 75 volt zener and theres no resistance showing on my 20 meg meter ...does not look good for this Zener it was put in the right way but i must done something it didn't like....i will take it out and compare it to the spare,and i will check my wiring again.

i have taken the 75 one out it was an in4761A resistance check again no resistance i will replace it .

[Steve Anderson]

This is puzzling. There's only two paths the -ve supply can reach the cathode area and then the focus circuit, either via the 75V Zener or the 47k/ZTX558. So something is conducting! It can't be via the 47k as that will limit the cathode volts to a lot less than you're getting and in the process it will surely go up in smoke.

Check your wiring, check for solder splashes, simply check for shorts.

If that goes nowhere remove the ZTX558 altogether so the -ve volts HAS to go via the Zener and see what happens.

Steve A.

[Harry Dalek]

Steve just replaced the zener it start off showing 75.4 this climbed to 75.9 i turned the power off the data sheet says max is 75.75 ...so it was the zener but i am worried it will climb more and kill it ? it is a slow volt climb every few secs ///
yes i will check if i have short or wiring mistake

[Steve Anderson]

Harry, that sounds all OK by me, but just to be sure what datasheet are you referring to? If you have an electronic copy please post it. Zeners are generally +/-5% or +/-3.75V in this case...AND at the specified test current - unlikely to be the same as what we're using, AND at a specified temperature.

The voltage will change as it warms up from cold and there will be small variations due to incoming mains fluctuations. So none of what you mentioned above concerns me unless the thing is getting REALLY hot. It shouldn't, just warm should be about right.

Don't forget meter calibration too...if it was specified as +/-1% when new, one year later it could be out by 3,4,5%...unless it's something really good like a Fluke, nice but expensive. Also make sure the Lo-Batt indicator in the display hasn't come on. When this happens the meter will generally over-read as the reference voltage for the A-D drops.

Steve A.

[Harry Dalek]

Harry, that sounds all OK by me, but just to be sure what datasheet are you referring to? If you have an electronic copy please post it. Zeners are generally +/-5% or +/-3.75V in this case...AND at the specified test current - unlikely to be the same as what we're using, AND at a specified temperature.

I have to blame my eyes here reading a screen without glasses its 78.75 you are sure right ,i always worry when a voltage starts to climb ...i have never used a zener hooked up to a voltage like this ...will it climb to its maximum working voltage i always thought they worked at what they are rated at and stayed at that .

The voltage will change as it warms up from cold and there will be small variations due to incoming mains fluctuations. So none of what you mentioned above concerns me unless the thing is getting REALLY hot. It shouldn't, just warm should be about right.

No it didn't get hot all the parts are cold thank goodness ,its safe to retest i am happy .

Don't forget meter calibration too...if it was specified as +/-1% when new, one year later it could be out by 3,4,5%.

Steve A.

I need my eyes calibrated don't leave the house without glasses is my next procedure !

[Steve Anderson]

Harry, read this through before doing anything...

Yep, same as the datasheet for the 1N4761 I have too. So leave out the ZTX but leave in the 47k collector resistor and the 1k at the grid - make sure there is about 75V between pins 1&2 (now joined I hope) and pin 3 of the CRT (the grid should be negative 75V to the cathode). If this is the case, and only if this is the case, and all other voltages on the CRT seem sensible, reconnect the heater and power-up. You should get NOTHING on the screen.

If all OK replace the ZTX and wire up the opto-coupler and the temporary brilliance control. If all is well you should have full control over the brightness of the spot. The range on the pot will be quite narrow, this is deliberate.

Steve A.

Yep, same here too, without glasses there's no point in even turning on the PC.