Troubleshooting a Proton VT331 SMPS

[olec280]

I am troubleshooting two voltage problems on the SMPS. The B+ voltage is at 62 VDC at the collector of the H.O.T. What I have read leads me to the SMPS. I have 105vac at the input to the rectifier after the transformer. The output is 62vdc should be 117vdc. Also, the 18V output from the smps is at 9.8 vdc but the voltage into that rectifier is 15.6vac. I have replaced all the caps after the rectifier and even the rectifier itself. No help. I am wondering what causes a low B+ voltage? The TV is about 20 years old. I have the service manual. This is a tinkering task for me. Assistance is appreciated.

[Wizard]

Re: Troubleshooting a Proton VT331 SMPS

you have feedback problem, you will need to find the circuit that holds SMPS at 117V DC. What about the caps in the SMPS output?

Cheers, Wizard

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

I did change all the caps on the output side of the rectifiers that are responsible for the two bad voltages. I figured that woud be an easy place to start. They did not have any effect.

[kc8adu]

Re: Troubleshooting a Proton VT331 SMPS

post a pic.

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

More Information:

The TV makes a very rapid tick, tick sound for lack of a better descripton after it is on for several seconds. No picture of course. Did have sound. Using isolation transformer. SMPS is installed during troubleshooting. Paying attention to hot and cold grounds.

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

First of all, could you post a better picture? I'm getting a headache trying to fuse the two parts together. If you have it as a pdf, attach the pdf, please.

This design uses a tertiary winding to provide feedback to the controller. It may also use current sensing, I've given up on tracing the connections.

I'd propose several tests. First of all, there is a connector labeled 'S12 to Main M12' shown on the right side of the second picture. What are the voltages at the 117, 35, and 18 Volt pins? (The voltage that is 'off' by the greatest fraction is the one that has an excessive load).

Next, it would appear normally the majority of the load is drawn from the 117 V supply. Have you tried disconnecting the power supply from the set and using a small incandescent lamp (25 - 40 watts) as a dummy load? I would suggest trying to provide nominal loads to all outputs to verify the power supply is working.

PlainBill

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

I'll scan it with a 11x17 scanner tomorrow.

Voltages at S12 are Pin1 62vdc, Pin 4 41 vdc, and Pin 5 9.8vdc.

I'll try the lamp test tonight.

New images. Hopefully easier to read until I get the full page scan.

There is overlap with the images.

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

Connected the lightbulb and it shined brightly from the 117vdc output, s12 pin1.

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

Connected the lightbulb and it shined brightly from the 117vdc output, s12 pin1.

Very interesting. By any did you measure the actual voltages at pins 1, 4, and 5 with this hookup? If those voltages are within 20% of nominal (without any load) I would suspect the power supply is working properly. Without the complete schematic I could make only a SWAG about the actual load they see. No, I don't think scanning the entire schematic is worth doing. See below

Voltages at S12 are Pin1 62vdc, Pin 4 41 vdc, and Pin 5 9.8vdc.

As Artie Johnson would say "Verrry Interesting". That would indicate something is loading the 117 AND 18 volt outputs, but not the 35 volt output. The clicking sound you mention COULD be the sound of the power supply current limiting.

How important is this set? Is it a family heirloom, or something you picked up and are tinkering with?

Background: At one time repairing TVs was my second job. At that time the typical TV had a couple of dozen vacuum tubes. While general troubleshooting techniques remain the same, some of the practical applications of those techniques have evolved. On a non-solid state set it was safe to isolate sections of the load to determine WHAT was loading down a power line. I'm a little reluctant to suggest you rush ahead and try that, to my mind it is possible that at some point you could see a transistor go up in smoke.

If you are willing to take the risk, the next step would be to look at the schematic and look for points where a single component (probably an IC) draws power from both the 18 and 117 vdc supplies. Alternatively, look for points where you can interrupt the power to sections of the board.

One obvious point would be the horizontal sweep circuit.

It goes without saying that Wizard knows a heck of a lot more about this than I do.

PlainBill

[i4004]

Re: Troubleshooting a Proton VT331 SMPS

probably short corcuit somehwere in hor. output....as those low voltage rails have less stress...

but finding exact component may not be easy....see if this helps:
https://www.badcaps.net/forum/showth...hlight=philips

i very much doubt some chip will draw both 18 and 117v dc....or any other single component for that matter...
i wouldn't draw too much conclusions from voltage drop on output of smps(esp. given that 117v and 18 are not derived from same secondary winding)...i would just say all voltages will be clamped if one rail is sc-ed...

also, that can be checked: connect light bulb to 117v without disconnecting rest of the voltages, ie just load 117v with light bulb(offcource, disconnect 117v rail from the rest of the (tv) circuit while doing it), leave the rest as is....
that should reveal if it's just 117v rail that's shorted, or something more is fishy.

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

Here is the complete schematic for the smps.

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

Removed wire from S12 connector going to H.O.T from pin 1 for B+ voltage. Attached S12 connector, turned tv on. Had 117 vdc on pin 1 (bulb bright), 35.4vdc on pin 4 and 15.0vdc on pin 5. This tells me I need to look on the next schematic page, cause I think the smps is working fine.

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

Removed wire from S12 connector going to H.O.T from pin 1 for B+ voltage. Attached S12 connector, turned tv on. Had 117 vdc on pin 1 (bulb bright), 35.4vdc on pin 4 and 15.0vdc on pin 5. This tells me I need to look on the next schematic page, cause I think the smps is working fine.

That is a logical conclusion. I can see two approaches. Unfortunately the schematic is so dense I cannot view it.

The first approach would be to look for a component that is getting very warm (or hot!).

The second would be to isolate sections of components on this board until you find the component that is holding the voltage down.

PlainBill

[olec280]

Re: Troubleshooting a Proton VT331 SMPS

I noticed the same thing with the diagram. I can probably blow up an area before converting to jpg, but the diagram is dense. The smps was the only diagram that was large enough to easily, read, copy, and use.

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

One thing I find a little puzzling (or encouraging) is the voltage. Usually when something shorts it becomes a dead short and the power supply voltage will drop to less than a volt. You are seeing about 60 volts. Guessing that the power supply limits at 1/2 amp on the 117 V supply, that would mean something is dissipating over 30 watts. I would expect whatever it is to be getting very hot.

PlainBill

[i4004]

Re: Troubleshooting a Proton VT331 SMPS

nice to see strategy i suggested helped you some.

h. output is on the bottom-right of your image, but that doesn't really help us that much....because all voltages there will be clamped, so there's nothing to measure...that's why short circuit is a bitch to troubleshoot...
it boils down to checking components one by one, and the some components can make short circuit only when loaded...oh dear...
uusual suspects are h.o. transistor, capacitors(cap can lose capacity and then result is blown h.o.t.) and diodes...

as bill says, temperature could be a culprit...but it doesn't need to be...because smps will enter protection mode(that's the ticking you hear) and won't be giving full power on it's output...in protection mode smps is just giving short and small bursts of power that i doubt can heat anything much...

i don't really understand what he means when he says "to isolate sections of components ".

[olec280]

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.

[PlainBill]

Re: Troubleshooting a Proton VT331 SMPS

<SNIP>
i don't really understand what he means when he says "to isolate sections of components ".

Trace the 117V line on the schematic. When the schematic shows a low resistance, high wattage resistor OR an inductor, unsolder one end of it. If the voltage is still limited, the components that get 117V through the resistor or inductor are not the cause of the problem. Of course, if it jumps back to 117V you have reduced the number of components you have to check. This falls under the technique called 'Divide and conquer'.

i4004 is correct, the most likely culprit is in the horizontal output / flyback transformer area. One of the first things I would try is to turn the TV on for 5 minutes, then turn if off, unplug it, and cautiously check the temperature of the horizontal output transistor, and the horizontal output transformer (flyback).

PlainBill