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CRT BRIGHTNESS

The CRT(tube) uses these power lines
1) 25kv to the eht top cap
2) 2-4kv adjustable (top knob on loptx) focus
3) 300v-800v 1st anode(G2/Screen) Brightness Control (bottom knob on loptx)
4) 170v-210v Video feed to output transistors(or chip).
5) Grid (g1) should be 0v or negative volts (never positive)

1) To test if the 25kv to crt top cap is there, I feel the screen front with the back of my hand to feel the hairs stand on end , I have an eht meter but I'm lazy.
also I place the handle of a neon screwdriver held by the insulated blade against the body of the loptx and it lights if ok


2) I have to use an eht meter here , but its very rare so I leave this test till last and I've usually found the fault somewhere else so it never gets done .
I have only had this line cause no pic and it was a small philips loptx.

3) The A1's , I can use my multimeter here , but what I ususlly do it, first remember what posistion the adjustment knob on the loptx is at, then turn it up/down to see what effect it has on the screen
Warning if you turn up the A1's too high the power protection will/may turn the power off (tripped)

4) If this voltage is low then the brightness will be high, so you must have a good supply here, it feeds the three output transistors via 10k/15k resistors
Usually if its very low the cause is an open circuit safety resistor near the loptx (check the diode also)
if the voltage is low (30v-120v) its usually the electrolytic decoupling capacitor(4-10uf 250v) This line is very unlikely to cause dark screen.

another cause of dark pic is the beam limiter circuit, it usually measures the negative voltage caused by the crt currents returning back up from the chassis to the loptx earth, its usually fedd with a small amount of the HT line to give it a posistive starting value, so look for a high value resistor (120k to 500k) from the HT line (100v) down to the beam linmiter line, which feeds a signal back to the video signal output chip , on the main pcb.
5) This grid voltage is always zero(connected directly to chassis) or has a negative voltage,if any signal is applied to the grid, there is usually a diode to prevent any positive and sometimes a resistor 4k7 which if faulty causes screen brightness to increase after a few mins.


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