Atari SC1224 vertical deflection fault

[SainT]

I'm after some help troubleshooting a goldstar SC1224 with no vertical deflection. I just get a single horizontal line.

 

So far I've found R731 was blown, I've replaced it and its blown again. So something is drawing a high current further down the line. D708 seems ok, which was the other component mentioned in the service manual relating to the 24v line.

 

Looking at the schematic I'm fairly clueless as where to look next other than perhaps the la7830? If anyone has any experience with common faults with this monitor, that would be appreciated!

[+Stephen Moss]

From what little I remember about monitors from Tech that sounds like what is known as field or frame collapse (usually indicated by a single horizontal line in the middle of the screen in which you can usually make out colour changes as the image changes) so you might want to try googling those terms.

 

I believe there are two timebases (oscillators) running one for Line (Horizontal scan) and one for Frame (Vertical scan) so you might want to start there, i.e. is the oscillator running and if so tracing the signal through to the deflection coils.

Depending on the design the oscillator may or may not be running from a low voltage supply so you might want to establish which before poking around, but IIRC by the time it reaches the CRT the signal can be high voltage.

[SainT]

Sorted it! I managed to get the wrong fusible resistor to replace the blown one, I got 0.25w instead of 0.5w. I only noticed last night after looking through the schematics for ages trying to figure out what else could be wrong.

 

I replaced the blown one with a 0.5w FR2R2 as it should be and we're working again.

 

The issue was basically there was no power getting the the vertical deflection circuitry which is powered from the 24v line. That particular resistor was the fuse for the 24v line coming from the FBT.

 

It looks like a case of PCB component gunk becoming conductive over time and causing a short and blowing the fuse, as the blown fuse was well gunked from a nearby cap which covered a diode as well.

 

Another monitor saved from the tip!