Zenith System 3 Model # SR2571DT

what model?

Since you had been started that fixing,,you may try replacing some electrolytic capacitors near on that vertical IC?..,Dods

It sounds like your lamp is about to die out,this tv lamps are good for 6000-8000 life hours which is a average time of 2-4 years old and with time the picture starts to get darker and darker until one day black screen. Try Encompass .com ; shopjimmy.com

Most likely will need a new lamp soon. your tv uses model # 6912B22010A

It could be that the picture tubes have gone down on emission, there is a prcedure that can be tried to bring up the overall brightness but it is just possible that you will loose some picture quality. If it's a 1994 projo tv i am thinking of the three crt system, a gray scale alignment would probably correct the dark condition in your picture, again depending on how far gone is the emission in your picture tubes, if your projo tv is the dlp system, the bad news is that you could be needing some light engine repair. The color adjustment is possible with the availability of a service manual or in the case of the dlp system, light engine repairs or replacement are available from some on line stores.

the ICs or the capacitors on the main board of the tv which controls the picture quality of the tv.is either gone loose and needs resoldering.or it is going through its last phase so further problem can occur if it gets completely faulty.get it replaced before it gets completely faulty.to avoid the further problems.

cold solders you may need a tech

The more you play set the longer it will last, it sounds like you may have poor connections, cold solder joints try soldering around flyback transformer.

Your HV circuit is defective. Most likely horizontal output transistor is shorted and you may need to replace the flyback. You need to call an expert.