philips crt hot waveform

[Arthur Ritus]

I am having a couple of problems with a philips em3e-aa.
Attatched is a trace of the h.o.t. collector voltage over the base voltage. I have been unable to find what the trace should be but i was expecting when the transistor is turned on it would be fully on dropping the Vce to virtually zero as opposed to the 80volts in this case. Its also weird to me that at the point its switched on there is not even a slight niggle in the falling voltage.

The original problem was the LOPT was arcing which i tried to bodge with some epoxy. This failed and after replacement the hot had gone taking out many other components inc p/s mosfet. Working through got Vbat back to 141v and after a short while the hot died again.
I have replaced it for a third time and now only allow it to be on for short time to get measurements. I don't know what current the hot should be carrying but at 80v every 1a is 80w which is much more than the 45w spec and would account for short life span. Also is a trace of the voltage across a 5ohm resistor in series with the lopt supply which is also unexpected. I would have expected a larger current going negative as the energy is returned. I have tested all the components around and all check out ok with multimeter (cap, diode, test res tests). It makes me wonder where all the energy is coming from to heat the hot.
If anyone can say the waveform is correct i will let it run and see what happens (presuming the hots faulty?) but if this one dies i am not buying another its the end. A shame after so much work (and cost).

I have also had to replace the video amp (tda6108jf) now the screen won't unblank. The current from pin 5 the black current measurement is around 200uA even with all the outputs disconnected. I have replaced this twice and both are giving this same reading. The spec says max leakage should be 50uA and the tv manual says the hop can manage a maximum leakage of 100uA. I have fitted a pot in this line shunting some of this current and by reducing it unblanks the set (gets retrace lines so not completely happy). I find it hard to belive two faulty components although bought same place same time if there was a bad batch? I am strugling to work on this until i can get some confidence in the hot's survival. I feel i must be missing something and would appreciate any thoughts.

 

[Arthur Ritus]

CRT hot waveform better now

I don't know if anybody has spent any time over my problematic waveform, appologies if so.

I think the home made attenuator might have been a problem. With a standard 10:1 although the peek volts go way off the screen the on voltage is shown right down probabally in the region of 10v. Unexplianably the series resistance on the lopt shows a more expected sawtooth wave going slightly neg peeking at 14v (10r) so indicating a current of 1.4A. Running it for 5 mins the hot was warm but touchable (once power off).
Unless i have fixed a bad joint during the component testing i am running with the idea the two first hot were bad (new ones from different supplier) and i cannot make a scope antenuator. Time will tell but I am going to give it longer runs a try to diagnose whats happening with the blanking.

I would really appreciate any experience any body has had with the tda6108 vid amp especially the current from pin 5. Alll theoris wellcome.

 

[Jotto]

I would repair the HOT circuit first, the video can be dealt with later.

There is a small capacitor in the HOT circuit that if you don't replace it will continue to blow the HOT. This is the component that took the HOT out to begin with. No raster no need to bother with the video problem.

Value of the capacitor varies depending on the unit. If you post the schematic I will point it out to you. It is usually 100mf or under. Look at the first transformer before the HOT, there will be a capacitor on the primary that you need to look at.

http://www.arcade-museum.com/manuals-monitors/Kortec_KT1703NA_Schematics.pdf

Listed above is a drawing for Kortek unit. Look at C421, right next to Q402, that is the capacitor you need to look at. Some units will use a non polarized capaictor, some just a regular capacitor.