Spectrum not booting

[vallis]

Hello there, I'm new here (and a newbie). I have posted also on spectrumcomputing.co.uk, apologies (and please let me know) if I hit the same people.

I'm trying to boot up my 40-year-old Spectrum (48K, issue 3). After the composite mod I'm getting a white border and thick black and white vertical bars, with or without some flashing junk, depending on the attempt. If there's flashing (about twice per second) I can hear a high-pitched buzzing in sync with it.

I read about testing lower ram. I get 12v on all pins 8, and 5v on pins 9, but -0.2v on pins 1. It seems the next steps should be to test tr4/5, but I don't know how to do that. Or should I test (and/or replace) capacitors first?

Thanks!

[1024MAK]

Welcome

How about you compare

The -5V rail is critical and the machine should not be run for more than a handful of seconds and only when needed for testing purposes. The tolerance for the -5V is -5.5V to -4V.

What is the actual voltage of the +12V rail (IC6 etc. pin 8)?
Are you using an original Sinclair UK1400 PSU?
What is the input supply voltage on the PCB mounting barrel connector socket or input pin on the 7805 voltage regulator (be careful you don’t short anything out).

If the input is near, at or above 12V, the DC/DC converter will not always work correctly.

Next, using a board layout, find diode D11 and measure the voltage across it.
Also do the same for diode D16.

Layout here

Mark