Hi wolfman. I already had replaced all capacitors as mentioned in my first post:
“All new quality caps properly installed and tested. Bizarre problem remains.
VR501 factory set to output 4.75v at the primary voltage regulator (good).”
Regardless, I had a crazy idea. Decided to do away with aluminum electrolytics entirely. I replaced all caps again...this time with solid tantalums. The new aluminum caps I pulled still test just fine, but curiously, the contrast problem is now completely gone.
The added benefits: This TE will likely never require a capacitor replacement ever again, no nasty electrolyte to leak and corrode pads/traces/ICs, extremely low ESR, great performance in a wide temperature range (they don’t mind the cold); -Much more efficient than all other capacitor types resulting in...increased battery life, brighter screen, great contrast, and even louder volume.
Possible risks: If a short circuit or power surge were to hit the TE’s mainboard. Tantalums are very sensitive to electrical faults. Failure mode at best is some smoke similar to a blown diode. Worst case scenario is actual flame or capacitor detonation. But consider that a major short circuit or surge would likely result in an unsalvagable unit anyhow. Very unlikely events regardless.
Tantalums get a lot of negative attention from fear mongers and those that misunderstand them. The reality is that they are simply the best capacitors available, still preferred by many mainboard and cell phone manufacturers...not to mention NASA. If sufficiently voltage derated and installed properly, any inherent risks are almost completely nullified.
I considered using ceramics, but their capacitance value must be derated to an insane amount to achieve acceptable performance. Also thought of dry polymer types and other hybrids, but their expected service life isn’t long enough. Mica film caps are too expensive and overkill.
Topic: strange problem...need Turbo Express / PC Engine GT service manual (Read 552 times)