Problem with an old Tandy CM-4 monitor

Tony Duell ard at
Mon May 18 12:47:09 CDT 2009

> A while ago I got a Tandy 1000 system with a CM-4 monitor. The system worked 
> fine for 20 minutes and then the monitor died. So far I havn't seen anything 
> that looks burnt on the inside, but the tube doesn't light up when you 
> provide power and there is a crackling noise going on. Is there a common 

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?

> issue with these old CGA monitors, and is this fixable without gutting a 
> working one?

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.


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