Troubleshooting a Proton VT331 SMPS

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

Would it be reasonable to think that I can follow the next component in the circuit off the 115vdc and create a circuit gap following that component then check for the voltage. Now that I think about it, I may not be able to it cause of traces. I know the voltage is incorrect at the H.O.T, but I guess what I am hearing is it may be down stream of that point as well. Similar to how I suspected the smps originally. I tried to blow up a section with the H.O.T.

That in essence is what I mean by isolating sections of components. But don't cut traces; they are a bitch to repair properly. Rather lift (unsolder one end and lift it out of the circuit board) jumpers, inductors, or power resistors.

PlainBill

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

I lifted one end of R445 which is tied to the base of the H.O.Transistor through transformer T401. The voltage on the lifted side of the resistor reads 137vdc, also terminal 2 of the flyback on the print reads 137vdc. It is suppose to read 117. Also there was no ticking sound.

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

I lifted one end of R445 which is tied to the base of the H.O.Transistor through transformer T401. The voltage on the lifted side of the resistor reads 137vdc, also terminal 2 of the flyback on the print reads 137vdc. It is suppose to read 117. Also there was no ticking sound.

Good!!! That tells me the Horizontal Output Transistor is not shorted, if it were the 117 Volt supply would still be limiting. You're reading 137 volts because the power supply doesn't have any load. Hook up the light bulb again and it would drop to 117 Volts.

The next step would be to check the Horizontal Driver Transistor. It's the one I've circled (I can't read the numbers on the schematic). It APPEARS you should be able to check the resistance between the leads while it is in circuit (and with R445 lifted). I'd check it with the meter on the diode test scale. Check between two pins of the transistor, then reverse the leads and test again. Then go on to the next two leads and repeat. This will result in a total of 6 measurements. Only two of them should show some resistance.

PlainBill

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

PlainBill,

Did you attach the image with the circled transistor?

Is is the transistor with the zener tied to the base or the one above it?

Thanks for you help.

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

PlainBill,

Did you attach the image with the circled transistor?

Is is the transistor with the zener tied to the base or the one above it?

Thanks for you help.

Ahhhh - No.

PlainBill

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

Under load (lightbulb) the voltage drops from 137vdc to roughly 2vdc.

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

Under load (lightbulb) the voltage drops from 137vdc to roughly 2vdc.

???? That is REALLY puzzling. That suggests there is an major load on the 18V line. ????

I think you need help from someone brighter than I am.

PlainBill

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

This morning I connected the lightbulb to the side of the resistor that is not lifted and it worked. The resistor is cutting voltage down to 2vdc. Is that part of the circuit only suppose to have 2 vdc going through it under normal operation? I am not sure.

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

This morning I connected the lightbulb to the side of the resistor that is not lifted and it worked. The resistor is cutting voltage down to 2vdc. Is that part of the circuit only suppose to have 2 vdc going through it under normal operation? I am not sure.

OK, it is time you learned some basic electronics. I'm going to give you the basic 2 bit explanation, to troubleshoot effectively you should have at least a minimal knowledge of electronics - the $50 course.

The transistor I circled amplifies the horizontal drive signal (presumably from a 'jungle' ic or the horizontal sweep controller). That signal is coupled to the base of the transistor by C422. When the signal turns the transistor on, the transistor draws current from the 118V supply through R445 and the primary of T401. The secondary of T401 drives the horizontal output transistor, which drives T402, the horizontal output transformer.

NOW, if my brain had been working I would have realized R445 is a 1.5K resistor. Even if the driver transistor was shorted there would have been only 79 ma through it, not enough to load the 118 V line.

Therefore, the problem must be with the OTHER load on the 118 V supply, T402 and associated components. By any chance do you have a ring tester?

Use a DMM on the diode range to check D480, the B++ diode. Pending approval from the wiser people, the other alternative would be to lift one end of R447.

PlainBill