Oscilloscope CRT problem

[LightySA]

Re: Oscilloscope CRT problem

Looking at the BXA12529 invertor/driver, it can deliver 10ma. rated min:950V typ:1000V max:1050V

pushing that through the voltager doubler should give us a min of 1900V and max of 2100V, surely that should work? only thing that might be a problem is that if the input and ouput use a common GND, then how are we going to get the -2000V? or am I getting myself confused?

[LightySA]

Re: Oscilloscope CRT problem

From what i've seen on youtube and some dead boards I have it should power up. but I not 100% sure.

[Agent24]

Re: Oscilloscope CRT problem

Might be a good idea if it works, but from what I have read on these forums CCFL inverters expect to be connected to a CCFL which behaves in a certain way.

If they detect something odd with the load they shut down. I'm thinking that trying to power something other than a CCFL could be classified as 'odd'

[LightySA]

Re: Oscilloscope CRT problem

What about using one of these, as discussed above, would they not work?

http://za.rs-online.com/web/p/lcd-di...ories/5350085/

...................... Min - typ - max
Output Voltage - 600 - 700 - 800
Output current - 4.0 - 5.5 - 7.0

Or perhaps this one driving directly?

http://za.rs-online.com/web/p/lcd-di...ories/5350063/

..................... Min - typ - max
Output Voltage - ... -1500 - ....
Output current - 4.5 - 5.0 - 5.5

[LightySA]

Re: Oscilloscope CRT problem

Thanks monpirri,

but I've just noticed, the schematic is slightly different to what I have. My looks like the picture in post 78, which I copied from post 40. I think mine if equivilent to the 2120B, and not the 2120.

Does anyone perhaps have the schematic for the 2120B?

[budm]

Re: Oscilloscope CRT problem

I do not see any positive feedback for the Base circuit of the transistors.
http://en.wikipedia.org/wiki/CCFL_inverter
you also need the tuning caps that was on the board where the transformer comes from that way it will run at the same tuned circuits.

[monpirri]

Re: Oscilloscope CRT problem

The oscillation frequency of the circuits is unknown since we do not know the winding value of the transformer, C954, R951 are part of the positive feedback circuits.
C951/952/HD950/951 are the Voltage doubler rectifier circuits. If you know the urn cout of the primary side then you can figure out the turn count of the secondary side, but you need to good transformer to verify.
The inverter transformer for the backlight usually be putting out anout 500~800v for sustaing the lamp, startup is about 1500v, but it is a tuned circuits (Royer Oscillator)

Good point , in fact i am trying to build a 2kv ps using a back light self running circuit .

[budm]

Re: Oscilloscope CRT problem

The oscillation frequency of the circuits is unknown since we do not know the winding value of the transformer, C954, R951 are part of the positive feedback circuits.
C951/952/HD950/951 are the Voltage doubler rectifier circuits. If you know the urn cout of the primary side then you can figure out the turn count of the secondary side, but you need to good transformer to verify.
The inverter transformer for the backlight usually be putting out anout 500~800v for sustaing the lamp, startup is about 1500v, but it is a tuned circuits (Royer Oscillator)

[monpirri]

Re: Oscilloscope CRT problem

Well ,about rewinding it just made a couple attempts and always got it shorted.

Last resort was just by trying a HV transformer from a broken 15' LcD monitor but seems the voltage was not high enough to get 2kv after doubler ,just a 170 v peak.

Course i am trying to build a 2kv 10ma power supply ,the real problem is getting the freaking transformer with 500 or 600l v out.

I am uploading the Service Manual if is allowed.

[budm]

Re: Oscilloscope CRT problem

You just have to drop the driving voltage on the primary winding.

[Agent24]

Re: Oscilloscope CRT problem

A TV Flyback would be far too high voltage wouldn't it? That's even if you got an old one without the tripler inbuilt.

[LightySA]

Re: Oscilloscope CRT problem

here is the schematic of the exact power unit. I would imagine the same would apply for the "voltage doubler"?

So if I turned a new transformer I would need to calculate its output to be about 700V, perhaps 800V for safety, as the voltage reg system should drop it if required?

Is a TV flyback not an option?

[tom66]

Re: Oscilloscope CRT problem

Heh, B&K seems to be a cheap re-branding company, judging from the build quality and use of cheap caps.

[budm]

Re: Oscilloscope CRT problem

The SCH that MONPIRRI shows is a simple Self Oscillating with the OP Amp driving it to regulate the 2kvdc output, the rectifier section is the Voltage Doubler circuits, so the output voltage from the secondary may be about 700VAC, once it goes through the Voltage doubler circuits and charge up the filter cap to the peak voltage, you will get around 2KVDC output, I estimate the output current may be up to 5ma. You can use the flyback transfromer but you need to regulate the output using the circuits to do that.

[LightySA]

Re: Oscilloscope CRT problem

Hi monpirri

Must be a common thing with these units then.

Could you elaberate a bit more on your attempt to rewind it! as this is what I was planning on doing!

As it appear the transformer only supply the HV to the rest of the circuit and in not cotrolled by any sort of time from the rest of the system, could one not use a small flyback tranformer from a small 5" B&W TV or something? it could be simply driven from a seperate driver? I'm desperate

Would you perhaps have the complete high resolution schematic? I've tried EVERYWHERE to find it?

Thanks

[monpirri]

Re: Oscilloscope CRT problem

Well i had the same problem with a 2120 ,now is a door stopper.
The HV output transformer (CVT T502 :063-016-9-001) got shorted and then it lost its 2kv volts.
Re-wound didn't work as secondary has a very special isolated wire very hard to find .

I sent an email to B&K and they don't give a supplier nor replacement.

Guess the best choice is to find a non working one as a source of parts .

[LightySA]

Re: Oscilloscope CRT problem

Ok, so it appear the transformer is toast!

So now to try have have it re-wound.

[LightySA]

Re: Oscilloscope CRT problem

Thanks Agent24

Thought I would just use the same thread as alot of the info in this thread is relevent as its the same "design" and problem. If I don't get any response I'll take your advice and start a new thread!

Thanks again

[Agent24]

Re: Oscilloscope CRT problem

It might help if you can post some photos of yours in case there is something you have missed or such. Hopefully someone else can help you as I know nothing of this!

Might help too if you start your own thread, with a better title and include photos. That way it is likely to get more attention.

[LightySA]

Re: Oscilloscope CRT problem

Hi All

I'm new to the forum and wonder if you could please help.

I have the same oscilloscope although re-branded as "Toptronic" that is dead, with very similar faults. (mine is the 20mhz version)

I had used the scope quite a bit, it then went on the blink and I managed to get it working by replacing U950 (741) and the D880 for the HV transformer. it worked for a while before dying again. This time after replace the same components it hasn't helped. I've also noticed that it started to arc on the board near the output from the HV transformer, I cleaned it up and sealed the board.

After changing all the components I find that the D880 (HV transformer driver) gets extremely hot, to a point where it actually melts the insulating washer. I assume that the D880 is not oscillolating?

The CRT has a very slight orange glow in a dark room, but I think this could be the heater? as it even glows with the D880 (for the HV transformer) removed off the board.

Any Help would be greatly appreciated! as I don't have money for another scope.

I've also tried to download the schematic from the previous posts, but comes up with an error. I've managed to find the user manual, but it only has a block diagram at the end.

Thanks in advance!

Justin