Oscilloscope CRT problem

[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

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

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

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.

[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?

[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

[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

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

[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?

[budm]

Re: Oscilloscope CRT problem

The circuits as shown is voltage doubler circuits that will put out Neagtive voltage with ref to ground, the positive output is tied to ground is generate from the Isolated secondary winding of the transformer, if you take the output of the transformer winding that is isolated from the primary side, you can generate any polarity you want with ref to the ground.

[LightySA]

Re: Oscilloscope CRT problem

Ok, as I thought, but I'm now wondering if the CCFL inverters secondary winding is also isolated, or if they share the gound? id it does share, could one not just modified the board (cut the track) to isolate it?

[monpirri]

Re: Oscilloscope CRT problem

Ok, as I thought, but I'm now wondering if the CCFL inverters secondary winding is also isolated, or if they share the gound? id it does share, could one not just modified the board (cut the track) to isolate it?

CCFLs output are normally at HF ( >20 khz ) so i guess if a high speed rectifier( or 3 serial ) and a capacitor in half wave DC can allow - 2kv .

[budm]

Re: Oscilloscope CRT problem

The hard part will be finding High speed Diode with 1000V rating, the secondary side of the Inverter transformer is isolated (of all teh one I repired so far).
The SCH of B&K shows 1N4007 1000 1A Genaral purpose Diode, so the Oscillator frequency must not be very high.

[monpirri]

Re: Oscilloscope CRT problem

Maybe if someone having a good 2120 can check the oscillation freq.

with a 1000VAC CCFL directly to the doubler probably works tho.

[budm]

Re: Oscilloscope CRT problem

Any one has the pictures of this transformer, I wonder if it is Iron core or Ferrite core, that will give us some idea what frequency it is running at.

[LightySA]

Re: Oscilloscope CRT problem

Its a Ferrite core transformer, with a E I configuration, it has a small peice of plastic between the ends of the "E" and the "I" creating about a 1.5mm gap.

[LightySA]

Re: Oscilloscope CRT problem

Its back from the dead !!!!

Don't ask what it was, but here's what I did:

Reassemble the transformer, less one turn on the output side (as I started to strip it...)

Replaced C951, C952 and the Zener.

I cut a "slot" between the tracks on the PCB where the HV had arced before.

Turned it one for 5 seconds and felt Q950, it was cold, so tried it again for 10 seconds, again cold...Mmmm decided to turn it around and look at the screen and I couldn't believe it..... A green dot! Almost fell off my chair!

So perhaps those who have had the same problem could try the same?

Thanks for the input....

A very happy person!

[budm]

Re: Oscilloscope CRT problem

Great, now please read the frequency of this thing for future repair.

[LightySA]

Re: Oscilloscope CRT problem

Great, now please read the frequency of this thing for future repair.

Stupid question, but can I test the frequency use the scope on itself? sure the grounds must be common?

[budm]

Re: Oscilloscope CRT problem

Yes, just measure the primary side of the oscilltor, you may be able to see at the BASE of the transformer driver transistor, I expect it to be aorund 25 (400msec)~30 (333msec)KHz range.