Hello! I bought a very nice TZ. It has the fuse 115/116/12v error on start up. From talking to the previous owner, it sounds like it was a simple problem, but in the process of "fixing" it, it was made a big issue. The guy who repaired it thought the opto board needed new 339 chip. For the life of me, I cant understand why, but he told the owner he was going to put better chips in and replaced them with 7408 chips. Yeah, no joke! I have put socketed 339 chips in and have tested all components and traces on the opto board. Its fine now.

I have tested literally everything. The 12V levels are all fine up to the new 339 chips. I replaced the ribbon cable. Reseated things. I did the switch test at the MPU headers. Ive even done the switch test at the opto board header, and it works perfectly.

I suspect that the 7408 mishap may have fried the opto LEDs. Do you think this may be possible? Is is safe to jump the leads on the opto reciever to simulate the trigger of the circuit? This would prove that its fried if I could.

On another odd note, I have the Ingo clock board. It has test LEDs and indicated that all optos are working on the clock, yet nothing is being seen at the MPU. I find this odd as Im not sure they go through the opto board. I need to look.

Maybe the 7408 sparked back in the 12v circuit and damaged multiple items?

Figured it out...the capacitor on the opto board was in spec, but I decided to replace it anyway. Underneath it, the trace to the Vcc has corroded and was not being consistent. Hope that helps someone! Replace it no matter what!