Hantarex MTC 9110 lost brightness

[MartyMcFly78]

Hi
I have got a Hantarex MTC 9110.
Some days ago it lost the bright. If I turn RV7 on, nothing happens.
It just gets a little better if I turn on the “Screen potentiometer” on the TH2.

Then I checked:
- The UPS250 output is 130V
- TP10 voltage is 111v
- SP3 voltage is 118v
- SP4 voltage is 111v
So I followed this tutorial

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3. Check the 130V. The B+ voltage is best measured from black wire link in the scan coil plug. The scan coils are the ones round the neck of the tube. Place the red meter probe on the black link and the black probe to metal chassis. Alternatively use test point 10 (TP10). This should read 130V DC and should not vary by more than a few volts. If this is well below the 130V, three things could be at fault.
A. C34, a 22MFD 160V capacitor. 2. C53, a 470MFD capacitor. You will have B+ at 60-70Volts.
B. TR20, the TIPL762 (BU508) power transistor mounted on the chassis by the 330 ohm resistor is open circuit. If you want to check it with a meter, remove it first.
C. Check R110, a 33K ohm resistor at the top right corner. Lift one end and measure it. If it needs replacing, use a 1/2 watt or higher resistor in place of the 1/4 watt one fitted.
D. If the B+ voltage is to high and you have a wavy picture, TR20 may be short circuit. Another cause of wavy screens is C53.
E. Another reason for a missing B+ 130V is the activation of the X-Ray protection circuit. A reference voltage is produced from the LOPT and is fed via a resistive divider to pin 8 of IC2 (TDA 2595). The voltage here is compared with an accurate reference voltage and if the EHT exceeds 28KV, the voltage at pin 8 operates a trigger circuit which inhibits the oscillator and shuts the monitor down. This can be noticeable when the monitor comes on briefly then shuts down. Temporally disable this circuit by lifting one end of D10. Now apply power to the monitor. If the monitor now gives you a picture, the X-ray circuit is activating. The problem could be in the voltage regulator circuit. Never leave a monitor running with the X-ray circuit disabled. Check C36, C37, C40 and C41.

I replaced C34, C53, TR20, RV7 and R110.
When I turned on the monitor, the R109 burned out.
Any ideas?

Thanks.

[grantspain]

perhaps r109 was bad all along causing the pow b+, replace it
you should check all the resistors in the regulation circuit including the 330r load resistor and the bu801
make sure your tr20 insulation sheet is not missing or damaged

[MartyMcFly78]

Hi grantspain,

I will check BU801 and R105.
I replaced the TR20 with a BU508AF. Is it the right one?

Thanks.

[grantspain]

yes it will work and i think the af is isolated package in any case so no insulation sheet required

[MartyMcFly78]

Hi,

I want to replace several components, including the R105 resistor (330 ohm, 30 watts).
But I can't find any like it. Could I replace it with one like this?

"330 ohm wirewound aluminum housed resistor 50 watts"

Thanks.

[grantspain]

you can use a 25w metal case type

[MartyMcFly78]

Hi again,

I replaced R1, R109, TR19.
Instead of the R80 there was a variable resistance(100k) that was wrong. I replaced it with a 270K.

I could only find a resistance(R105) '330ohm 50w metal case type'.

Now the voltage at SP4 is 120v but the voltage at TP10 is only 75v.
What can be wrong? Maybe the R105 is not the correct one?


Thanks.