Too Much High Voltage, Flyback Problem? -'92 Samsung

[RevennaFox]

I'm working on an old Samsung 19" TV, circa 1992. Model TC9865, chassis K50UC. I've cleaned everything up and replaced most of the electrolytic capacitors, but I'm still not having any luck getting it to stay powered on. When powering on, I hear the relay click on, the tube filament starts to glow, and I start to get a raster but everything shuts down after about 2 or 3 seconds and won't power up again without being unplugged for a few minutes. This is what it was doing before I replaced the caps. I think this symptom is caused by the fail safe circuit detecting excessively high voltages and shutting down the set to prevent damage/overheating.

The first thing I do is start checking voltages. On power up the anode voltage rises to 35kV or so before the fail safe trips and it slowly drops back to zero. Normal operating voltage is supposed to be 27kV. Yikes. At this point I turn my attention to the power supply. According to the schematic I should be seeing 150V at the input of B+ voltage regulator Q801, where I'm actually reading about 170V, and 160V on the output where I should be seeing 125. Cause for concern, but nothing apocalyptic. I checked all of the resistors upstream from Q801: R802, R804, R805, R806, and R807 and everything is within 5%. Diode D802 checks out as well.

Nothing interesting so far. Out of curiosity I try to measure the voltage at the HOT collector, at power-on I get out of range on my 600V voltmeter, and nothing on my high voltage tester, so it's between 600V and a few kV. When the fail-safe kicks in, it drops to 165V. I power the set up without the yoke connected and everything stays powered on, with a bright white out-of-focus dot in the center of the screen. Anode voltage at this point is 20kV. Thinking that one of the deflection coils might be shorted, I measure their resistance, 16Ω for the V coil and 2.2Ω for the H coil, which is perfect.

Something must be wrong with the deflection circuitry. I rig up a test and connect just the V coil, I get full vertical deflection and the set stays powered on with a nice bright line down the middle of the screen. Anode voltage is 20kV, and everything is still blurry. I power up the set with just the H coil connected and the set behaves as before, fail safe kicks in after 2-3 seconds as anode voltage reaches 35kV.

Now I know something is wrong with the horizontal deflection front end. I power the set back up with just the V coil connected while I think about it and get ready to measure some voltages and waveforms. I adjust the focus pot on the flyback to try and get visible scan lines and the fail safe kicks in again. I wait for it to reset and power back up and the fail safe kicks it off instantly this time. Reset again and it does the same thing, move the focus pot back to where it was and still nothing.

It looks like the flyback is the problem. I desolder it and read the resistance of the primary and secondary windings. Actual values followed by specified values in parentheses: pins 2-3: 0.3Ω(0.33), pins 3-6: 0Ω(0.27), pins 6-1 0.2Ω(0.42), pins 1-4: 0Ω(0.23), pins 10-5: 1Ω(1.1), pins 5-7: 0.2Ω(0.48), pins 7-9: 0.3Ω(0.24). I don't have much experience with flybacks so I don't know how far off these values can be, but it looks like some windings have internal shorts, although I'm taking this with a grain of salt because I can't find my good multimeter and am using a spare.

This flyback (FCM-2015) is pretty generic part, and I can get one for a few bucks, but how can I be sure it's the problem? More importantly, how can I be sure it's the only problem and not just a symptom? It would be foolish to replace it and have the new one instantly destroyed by a problem elsewhere in the circuit. Is there anything, like for example a bad HOT or voltage regulator, that could instantly destroy a new flyback, or would the protection circuitry take care of it? Or maybe I'm barking up the wrong tree altogether? Any ideas would be welcome.

Service manual/schematic is attached. I also have the factory manual, but it's not as easy to read as the Sams' diagrams.

[zeno]

Problem will almost always be Q801 regulator IC.
If BEFORE shut down the B+ rises above 125VDC try that first.
If you have a variac run the set at abt 90-100 volts. for abt 5 seconds
& see if the horz output Q402 is getting hot. If so that points to the FBT
almost always.

73 Zeno
LFOD !

[Popester]

I remember on early ‘80’s Sony set they had a high voltage shutdown circuit that turn off the set usually caused by the HV resistor in the red epoxy pack causing internally arcing.

[zeno]

Good old H-stat control. Did many. For test you can pull off the thin red wire.
Many ran hours without diing or arcing.

Originally Posted by Popester

I remember on early ‘80’s Sony set they had a high voltage shutdown circuit that turn off the set usually caused by the HV resistor in the red epoxy pack causing internally arcing.

[JohnCT]

Originally Posted by RevennaFox

.... and 160V on the output where I should be seeing 125. Cause for concern, but nothing apocalyptic.

Yes it is! That is your problem right there. This TV uses the LV regulator as the HV regulator. The HV is proportional to the LV, so anything more than 125VDC and the HV runs away proportionally.

You've got one of two problems: either C457 is open (47/150) or your Q801 (STR30125) is shorted internally.

Change the cap, if that doesn't fix it, look for the STR if you can find one.

If you want to troubleshoot this a bit for fun, unsolder the Horiz output transistor and remove it from the TV. Put a 75W incandescent bulb wired from the Emitter to the Collector points of the horiz output and turn the TV on. The bulb will light - measure the voltage across the bulb.

If the voltage is 125VDC at 120VAC input, then your C457 is open. If the voltage remains at 160, then the STR is bad.

The STR series of self contained series/pass regulators were made by Sanken and are generally reliable, but they do fail. They contain three or four transistors, some resistors, and a zener or two. These are fixed regulators and the part number identifies what the output voltage should be. So if you STR is a 30125, then you should get 125V +/- 1v over the full range of bright to dark scenes. You can put a STR30120 in place of it, but your B+ will now be 120V and your HV a bit lower as well. If that's all you can find, you may have to fudge the retrace cap on the horiz output to get your HV back up. Do not use a STR30130, 135, etc as the HV will be higher. It may not shut down but you'll be stressing the flyback and pushing the CRT filaments beyond 6.3 as well.


John

[RevennaFox]

C457 is one of the caps I replaced, so it should be good. I tried subbing in a new one there anyway and got the same results, about 170V on the input and 160V on the output of Q801. Looks like the voltage regulator is at fault; I've got a couple ordered and will post my results when they arrive.
The cause of the complete shutdown after trying to adjust the focus was the result of blowing R417, a 1Ω fusistor. It's wired between the 125V output of the regulator and the flyback's primary, which would be consistent with the higher current draw caused by higher B+ voltage. I don't have any fusistors, but it should be safe to replace it with a 1Ω resistor in series with a fuse, correct? It doesn't give the current rating in Sams or the factory manual, but I'm assuming it's well below an amp.
I'm still a little confused why the HV only spikes when the horizontal deflection coil is connected, but hopefully that problem will disappear after I reign in the power supply issues.

[JohnCT]

Originally Posted by RevennaFox

I don't have any fusistors, but it should be safe to replace it with a 1Ω resistor in series with a fuse, correct?

Technically yes, as long as the resistor has long enough leads to prevent scorching the PC should the horiz output transistor short. Don't use a carbon in any case. Sony used to use ceramic coated resistors as fusibles. This prevented scorching anything nearby, eliminated the "crack" noise metal films made when they blew out, and made no smoke. If a customer hears a loud crack and smoke comes out, they think the TV is junk at worst or cheaply made at best.

Originally Posted by RevennaFox

I'm still a little confused why the HV only spikes when the horizontal deflection coil is connected, but hopefully that problem will disappear after I reign in the power supply issues.

The HV isn't spiking with the yoke connected, it's acting normally. By disconnecting the horiz windings, you inadvertently lowered the HV which kept it below the safety shutdown point. The horiz windings of the yoke are part of the tuned circuit that includes the flyback, so the TV will only reach maximum (normal) HV output if the horiz windings are attached.

Once you repair the LV regulator, the HV will return to normal with the windings connected. This assumes your B+ measurement is taken while the TV is actually running. Once it shuts down, the B+ would rise anyway because the STR is bypassed by that big white resistor. With the TV in shut down, there would be no load across the resistor and the voltage would rise to the bridge voltage (160ish).

Now if the TV was otherwise running normally and you disconnected the horiz windings, you would get a vertical line down the middle (maybe) and the HV would drop in half at least because you detuned the circuit.

John

[RevennaFox]

I've swapped Q802 and replaced fusistor R417 with a 500mA fuse. Since the meltdown mentioned above I haven't been able to get anything out of the flyback, no HV, no filament voltage, and no LV. While measuring voltages on the bottom of the board I heard a sound like glass breaking and quickly powered everything down.

I thought maybe the regulator had overheated and the sound was the little mica insulating sheet cracking but it turned out to be the sound of R808, the big 210Ω/20W power resistor, overheating and splitting in half lengthwise. It was so hot I had to wait about 10 minutes before I could remove it. Strangely enough, it still registered close to it's stated value. I don't have any resistors that big laying around so I'll have to wait until I order some parts to replace it.

Bypassing the regulator and R808, I tried lowering the input voltage to about 90V with a variac to get exactly 125V DC going into the flyback's primary through fusistor R417, with the result being to instantly blow that fuse. Would I be correct to assume that I've damaged the flyback while diagnosing the power supply issues?

[RevennaFox]

I figured it out, but it was kind of a circuitous route. The fuse blowing was the result of the HOT shorting out. I unsoldered it and measured the emitter-collector resistance, and it was only about 45Ω, instead of infinite like it should be. I subbed in a new 2SD1555, replaced R808 with a 200Ω brown devil, and replaced the blown R417 with a new fuse and everything came back to life. The HOT must have blown from the exposure to high voltages while I was testing.

At this point I had a stable picture, but there were still a few problems. If I turned the brightness or contrast up, the picture would shrink and/or cut out whenever displaying a bright scene. I measured the output of Q802 and found it was only around 105V, HV was a little deficient at around 22kV. It also seemed like R808 was running hotter than it really should have been.

Now I look at the input of Q802, it's still a healthy 160V, so the rectifier and filters are good and nothing is being loaded down too much. I check pin 2 of Q802, the base drive, where the schematic calls for 125V and find 157V. That's weird, you would expect higher output voltage with a higher base voltage. I check the voltage at the voltage divider feeding the base of Q802, between R805 and R807 and also find a higher than expected voltage. The only thing I can think of is that the base is not connected here, so I test R806, the 47Ω resistor connecting the base of Q802 to R805/807 and find it open.

I replace R806 and everything is PERFECT. I'm getting exactly 125V B+, HV is about 25kV, and I can turn the screen, brightness, sub brightness, and contrast controls over their full range with no problems. My theory is that the original problem was failure of R806, which caused R808 to overheat and change value, which caused the high voltage that lead to the shutdown issue.

I'm attaching some screen shots, this is the first time in about 20 years this set has displayed any kind of a picture.

Thank you so much for your help, I always learn something new on every one of these projects and this has been no exception.

[JohnCT]

I think you transposed Q802 for Q801.

My theory is that the original problem was failure of R806, which caused R808 to overheat and change value, which caused the high voltage that lead to the shutdown issue.

I did literally hundreds of these back then. The sequence was most likely that Q801 STR regulator shorted causing excessive B+ which tripped the shutdown. It also opened R806. Since the regulator was shorted, the open R806 had no effect on anything at that time.

At that point, you have essentially a jumper wire from the B+ filter to the flyback. So instead of having a regulated 125v, you had 160V feeding the flyback.

Once you put in the new STR regulator with the open R806, the short between the B+ and flyback was gone, the regulator was now out of the circuit, and the 20W R808 now had to supply what B+ it could.

That is a series/pass circuit, and the purpose of the 20W resistor is to carry some of the load for the STR to ensure it's reliablilty. By itself (like when the STR is either removed or dead), the 20W resistor can't supply enough current and it overheats.

With a working STR circuit, the STR then carries the rest of the load and tightly regulates it. With the replaced 47 ohm resistor allowing the STR to do its job (carrying about 70 percent of the load and regulating it), the resistor runs cooler.

John

[zeno]

Good repair ! Just think how easy the next one will be.
Tons of makes have the same basic design on lower
priced models. Higher end sets can get much more "interesting"

73 Zeno
LFOD !

[Popester]

First picture looks like Peter Gabriel and bald chick must be Sinead O’Connor.