Problem with an old Tandy CM-4 monitor

When you say 'The tube doesn't light up' do you mean that the screen is dark, or that there's (also) no glow from the heater in the electron gun at the back of the CRT? And is this crackling noise there all the time the monitor is turned on? The most common problem that's _not_ fixable without a 'spares unit' is a breakdown of the horizontal output transformer (line output transformer over here :-)). This often does result in a continuous crackling (small sparks from the EHT supply to chassis, etc). It'll result in a totally dark screen, it may also remove the heater supply. But it could be a lot of other things. Many faults result in a totally dark screen. 'Crackling noises' may come from ther sections of the monitor nosies are remarkable hard to describe. What I would do is pull the case and check the CRT electrode voltages (watch out for the focus pin which may have around 5kV on it!). Check the EHT at the anode connector if you have a suitable meter (expect around 25kV here on a colour CRT). And work back from there. -tony