Sinclair ZX80 / ZX81 / Z88 Forums

U1 (µA78S40), Q1 (D432C1), CR1 (1N5821), the inductor L2 and associated components form a buck converter (step-down DC-to-DC converter) which decreases and regulates the output voltage to a set amount. It has the limitation that often a fault with it will result in nearly the full input voltage bein...

Hello and welcome to our forum Diegorand!



I hope you enjoy this forum

Mark

No problem

Good to know that you fixed it. Thank you also for reporting your findings

Mark

Oh, and the voltage source has to be a constant steady voltage, otherwise you are wasting your time.

Mark

Well, you could try using the voltage gradient testing to try to narrow down the area where the fault is. Every conductor (outside of superconductors) has a small resistance, including PCB tracks. Hence if there is a short circuit, there will be a gradual voltage loss (drop) between the source of th...

Shorts on the +5V rail are rather tricky to find, because almost everything is connected to it...

How good is your multimeter?
How many digits and what is the maximum count?
What is the lowest DC voltage range?

Mark

Which power rail? Do you mean the (nominal) +9V input?

It could be something simple like an item of metal or a solder splash. If a visual inspection under a good light does not find the problem, then if you mean the +9V, remove the 7805 and TR9. Then retest.

Mark

They are only fitted on models sold in the USA. They are to reduce the level of EMI “leaking” out. They are not used on UK models. They do not affect the operation of the computer (well unless broken).

I don’t know the value of them.

Mark

ZTX651 and ZTX653 are suitable replacements for the ZTX650 normally found in TR4 position and when I last checked earlier this year, were still in production.

TR4 normally only dies if there is a short circuit or other overload on one of the outputs of the DC/DC converter.

Mark

The colour encoder chip is not involved with the RGB output.

Are the +5V and +12V supplies for the video section okay?

Mark