Oscilloscope CRT problem

[Afzal Cassim]

Re: Oscilloscope CRT problem

Hello,my name is Afzal!

First I need to introduce my self.

I am from Sri Lanka, and i am also a Electronics Tech / Software Programmer for some years. I do repairs on Computers,PSU,Alarm System panels,CRT /LCD monitors ,and access control card readers,and sometimes X-ray machines etc.The company that I work for does installation / repair /service of security-related electronics equipment for 20+ years. I am 34 years old.

I am studying at the open university of Sri Lanka(I am a late starter) and do my own repair work in my spare time.

I need advice and help on some CRT scopes that had been given to me from the company that i work for as they were removing old items and upgrading equipment.I had managed to some what 'nurse' a very old leader electronics 40MHz 1041 scope from 'death' and is working and is managable!

I hope to contribute to the discussions in the future.

Thank you.

[dandare0648]

Re: Oscilloscope CRT problem

Hi everyone,

All sorted, the B&K folk have helped me out.

Thanks to all anyway.

Cheers

[dandare0648]

Re: Oscilloscope CRT problem

Hi everyone,

I've just scored one of these 'scopes on ebay for AUD40 and I got lucky for once. C901 showed no external symptom but once replaced I have a working scope albeit still a little sick. The jpg's in this thread have helped me achieve my success so far.

I'm keen to get this going and I need the schematic. I scoured the net and it seems the one on rapidonline.com has been removed.

Does anyone still have a copy of the file they could email me please.

Many thanks in advance.



BTW the unit is manufactured by Pintek of Taiwan and rebadged by B&K and lots of others. Mine has a Jaytech (Jaycar Australia) badge.

[LightySA]

Re: Oscilloscope CRT problem

Are you sure its the CVT, I was also convinced it was that! however it was a component on the output side. I even built a Shorted turns tester to test it as advised on another electronics forum, and it showed the coil as dead! but it really wasn't.

Remember the slightest "drain" on the HV side causes huge current draws on the LV side.

[monpirri]

Re: Oscilloscope CRT problem

Well ,thanks for your kind offer ,but my problem was a shorted CVT not an open winding as I deduct from your experience.

Now i am checking some HV ballast for LCD Backlight with not a really good results so far; so as my last resort build a HV power supply

[LightySA]

Re: Oscilloscope CRT problem

Let me know if I can help any any way etc. hope you come right!

[monpirri]

Re: Oscilloscope CRT problem

Thanks.

Excellent job ,

I'm still working on mine

[budm]

Re: Oscilloscope CRT problem

Thanks.

[LightySA]

Re: Oscilloscope CRT problem

Ok, just test it and it sits at 20uS per wave, so if my calculations are right this means its runs at 50khz.

[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.

[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

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

[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!

[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.

[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.

[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

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

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 .

[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?

[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.