Repairing a ZX Spectrum 128K +2 (serial no U-043583) - Part 4

In part 3, I checked the inputs to the TEA2000 composite video chip and could not see any obvious issues other than the low voltage on the 12V input.

I decided to check and replace some components on the onboard powersupply to see if they were causing the low 10V voltage measured on the +12V supply.

The onboard powersupply uses a ZTX650 transistor TR9 which looked okay but I decided to replace it with a ZTX651 which is an acceptable replacement for the 650 and I also replaced electrolytic caps C41 (100uF) and C23 (22uF) in case they were starting to fail.

Measuring the voltages I still get similar results so TR9 and the caps do not seem to be the cause of the low voltage on the 12V supply.

I measured

• -12.34 (-12V)
•  10.98 (+12V)
• -5.03 (-5V)
• 4.96V (+5V)

I posted on world of spectrum and it was suggested I check resistors R54(1K) and R55(1K8) which form the sense/feedback resistor network that drives TR8. Both resistors checked out okay and I noticed that the two resistors form a voltage divider so I decided to experiment. Replacing R55 with a 2K2 resistor results in 12.3V on the +12V supply. This does not fix the display problem but I have decided to leave it in for now.

Finally, I have decided to order a replacement TEA2000 to see if the IC is faulty.