|
|
|
#1
08-03-2019, 09:54 PM
|
|||
|
|||
|
Scott 6T11 no brightness on CRT
Several months ago I restored a Scott 6T11 and it has been working ok until a couple of weeks ago. The HV wire arced over where it enters the optical box causing a periodic arcing but still working 99% of the time. Then a day or two later I turned it on and nothing on the screen but darkness.
I replaced the HV wire. Still nothing. The interesting symptom that I'm not even sure where to start looking for is this. Audio seems fine. No picture, raster or any sign of life on the screen. Advancing the contrast control beyond about 12 o'clock position results in a a muffled popping sound, something arcing, which increases in rate of repetition as the contrast control is advanced. The contrast control changes the bias on the video IF stages. Nothing it does, or should do, seems to be anything that would affect anything with enough voltage to create an arc. Certainly not one so audible. I occurs to me that if the coupling cap between the 4th video IF and the video output were shorted this would essentially be changing the bias on the CRT, since it would then be DC coupled from the 4th IF to the the CRT. The video in this set feeds the grid. Does this seem a reasonable place to start? I'd like to have some idea where I'm going with this thing before removing the chassis, power supply, HV chassis and CRT/yoke and putting it on the bench. Perhaps a shorted coupling cap would keep the CRT biased to cut off regardless of the setting of the contrast pot? Am I going down the wrong path here? Actually, after looking at the schematic again there is a coupling cap between the video output and CRT so I don't have a clue. What could the contrast control be doing that would affect an arc? Last edited by Phil; 08-03-2019 at 10:14 PM. 
|
|
#2
08-03-2019, 11:24 PM
|
||||
|
||||
|
Strange symtoms, I would be incined to start backtracking from where the arc took place
The arcing could have overloaded the horizontal output tube, hv rectifier. Seeing as it has audio, i would imagine that the B+ is alright. Sure hope the high voltage transformer is OK
|
|
#3
08-03-2019, 11:49 PM
|
|||
|
|||
|
Yes, probably the first thing to do once it is set up on the bench is to see if I have HV. On these sets the HV is supplied by a separate chassis and not connected to the horizontal sweep at all. The way they are made it is basically impossible to measure the HV in situ.
|
|
#4
08-04-2019, 12:13 AM
|
||||
|
||||
|
Yeah, after I posted I looked it up and it's a really nice projection set!
I can see how that would complicate the symtoms that you are having while changing the contrast control. It must require quite a high voltage to get enough brightness from the 3NP4.
|
 |
|
|
Linear Mode
