oscilloscope

[ljcox]

Page 68.

Let me know if you need any particular part of a page rescanned.

 

[Lighty]

Thanks ljcox

 

[ljcox]

You're welcome.

Please let us know the outcome, once you fix it.

 

[Lighty]

Ok, to through a spanner into the works.

I took the transformer down to a local transfomer manufacturer and he tested it on his Shorted Turns Tester, which was a single probe "device", and it got a clean bill of health.... So back to the drawing board!

It must me something to do with the oscillation driver?

 

[ljcox]

Have you measured the DC voltages?

Do you see any reading on the AC range?

Can you borrow another scope in order to look at the waveforms (if there are any)

EDIT.
Questions re the circuit.
1. Is it identical to your circuit?
2. Where does pin 10 of the TF go?
3. What voltages do you measure on the U950 pins?
4. What voltages do you measure on the Base & Emitter of the transistor?
5. What voltages do you measure at each end of VR963?
6. What is the resistance of your meter?

Obviously U950 is part of the feed back loop & amplifies the signal at the slider of VR 963.

 

[twister]

The problem with horizontal oscillators is that everything in the circuit has to be good in order for it to function. I assume that you have checked the zener diode. The most likely suspects are the high voltage components. Have you checked the high voltage diodes? You might need a 12 or higher voltage to check them. Use a small current thru a resistor. Just change all the high voltage caps if you don't have a esr meter. That's the easiest way I've found.

 

[Lighty]

I have tested DC voltages, they where all correct.

I have not tested for any AC, I will and will report back.

Unfortunately, finding another scope is not easy, thinking I may just try to use my Arduino as a basic scope.

1 - Yes, its identical to the schem I posted in Post7
2 - There is no pin 10 on this model, I think it's previsions for other models, this have been confirmed bu other users of the same scope.
3 - I will recheck this evening, but if I remember correctly, I was geting the +12V and -12V on Pin 7 and 4. approx +12V on 3 and 6.
4 - 24V across base and emmitor, with U950 removed (collector disconnected), Will test with U950 in place, but think it will be 0V and hence its burning? will advise.
5 - Will check and advise.
6 - Will check and advise.

Thats how I understand it too, the U950 acts as a "throttle".

 

[Lighty]

Thanks Twister,

I don't think its the output side of the transformer, as I get 0V. but I will take the advise and replace all HV components.

 

[Lighty]

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!

I would just like to thank EVERYONE who helped me to fault find my way though this repair!

Thanks.... A very happy person!

 

[ljcox]

That's excellent.

I suggest that you measure the voltages now that it is working just in case it fails again.

You could also use it to look at & record (I use a digital camera to take photos of the screen) the waveforms.

It may not be wise to look at the collector waveform as it may be high & thus it may damage your probe.

 

[Lighty]

Good idea!

I did check the base voltage waveform, its at 20uS = 50kHz. will check the others!

 

[twister]

I'm glad you got it fixed! I remember the first time I fixed one. I was the same way, I about fell off my chair!

 

[ljcox]

Good idea!

I did check the base voltage waveform, its at 20uS = 50kHz. will check the others!

Don't forget to record the voltage levels also.

 

[CalebG]

I was looking for a shorted turns tester and came across this thread, but the attached thumbnails dont show up? Does anyone still have that article from 2004 SC?
Thanks

 

[dr pepper]

I've seen a shorted turns tester too, but this was a loop wand type device (probably no good for torroids), I too would be inerested in such a project.

 

[CalebG]

This is the link to the one I found, Supposedly it is able to test smps transformers as well but not with as much certainty as a flyback. I have the parts on order from mouser they should be in later this week. **broken link removed**

 

[dr pepper]

Usefull gizmo, only without experience or another tranny to compare against the results can be inconclucsive.