I got this TK95 machine in a non working state. If you don’t know the TK95 is a computer clone made in Brazil of the famous Sinclair Spectrum computer.
Upon firing it up it gave me this yellow/black screen. This machine has RF video, so the quality is not the best.
Opening the machine and the RF modulator box I spotted an unsoldered lead, the one thas goes straight into the RF output connector.
After soldering the lead back in place I got video, but something is not right, The best I can tune the image is this:
I know RF video is not the best, but this looks awful.
After inspecting the PCB I found a burnt resistor (R68) and 2 diodes soldered together (D24), maybe it was repaired in the past, this didn’t look factory made.
Replacing R68 with a new resistor didn’t fix the bad image issue, also it was getting too hot. I did some research and found that D24 is supposed to be a 1N4742 diode, that means a 12V 1W zenner diode, the diodes in the PCB are 400mW ones (Maybe that’s why there’s 2 of them together?)
After replacing the diodes with a new one with the correct value the machine has a nice image, also the resistor R68 doesn’t get that hot anymore. Machine fixed.