[space_charged]

I have been soak testing the set for a while now, never leaving it unattended though. There has not been any recurrence of the jump in horizontal static convergence.

However while watching it recently, there were two or three sharp cracks with visible flash seen through the top. That was followed by the picture disappearing and a puff of smoke.

Once on the bench and being supplied from a current limited supply set to 12V it became apparent that there was way too high a current flowing, though the front neon lamp was on, showing the line output stage was was working. there was an obvious smell of something getting hot so started to trace what it was.

The excess current and smell turned out to be tracking of the 160V winding. That comes from the LOPT and goes to a fast rectifier diode plus inductor/capacitor smoothing. I had noted tracking there before but I thought I had cured the problem by removing some carbonised PC board.
I cleaned it up more carefully and made a slight change to the layout of the copper to increase the separation of the 120V DC from anything else. I know the board had been very wet in the past. I also heated the board to drive off any moisture on it.

Result; no tracking and the input current returned to normal, well a bit below normal actually. There was also was no picture! Looked like the arching had taken something else out - but what!

Well we all know what causes no raster in a TV. Hmm, just about everything I hear everyone saying.

I rapidly checked there was line output, and frame time base was also OK. So its probably a CRT bias problem. As the LOPT was not in distress and supplies derived from the LOPT were all normal I made the assumption that the EHT was OK. I do have ways to check that, but I decided to take it as read that it was OK.
Checking the RGB video drive transistors (on the CRT base) showed they were ALL high - enough to be cutting the tube off. OK, so look at the drive transistor emitters, which are connected together and driven from the main signal board through Q305. When I first repaired the set the emitter 39 ohm resistor was O/C. The voltage on the video output transistors was correct.
Then checked the base voltages and they were ALL well below correct at about 3V. The base voltages would vary when the appropriate R,G or B level pot was varied, but up to a maximum of only 4V. So all three transistors were biased off.

Next look at the base drive wave form. This is derived from IC 302. The drive wave form was OK, but its static DC level was incorrect, too low. This was true for all three drives.

Flash-overs can cause lots of damage and can take out multiple transistors so got to suspecting maybe all three video output transistors could be damaged. One was removed and the base voltage where the transistor was measured. The thinking being the (failed) transistor might be holding the voltage from the IC down. Not so, it was still low on the base of the removed transistor. The removed transistor was tested and found to be OK.

Has to be something else, but what?

Don't know it I'm "allowed" to do this but this is getting long. So I'm going to continue this with part two and will reveal all...

Love to know what you think I checked next and what the fault was?

C