Understanding TV power supplies

[tab a]

I'm trying to learn more about TV power supplies in general, and this one, which may not be working correctly, in particular. I've done a fair amount of reading about them on this site, and others but obviously still have much to learn.

1. Is this PS for this TV a SMPS? I'm guessing Q801 Volt. Reg.--STRS5041T --is sending chopped DC to T862, but may be way off base? The only datasheet I could find for the STR VR is in Japanese and doesn't tell ME much!

2. My measurements on the Q801 pins are all very close to the listed values, except I'm measuring .64v at pin 4 (should be 0?) and -5.1v at pin 6 (should be -6.9?). However, the schematic shown is NOT exactly the same as the TV in question. Both Toshibas, of similar vintage, and power supplies VERY similar but not identical. I drew a schematic for this TV by hand and found the only difference to be small caps between pin 5 and 6, and 2 and 3, on this set, Toshiba TP4688J. I'd like to understand more about what the datasheet calls "drive control"--pins 4 through 6. Can someone explain or give me a good link?

Thanks. (We'll see if the attachments make it!?)

 

[Nigel Goodwin]

Yes, it's switch-mode - check R862, which looks to be the start-up resistor.

 

[Brian Hoskins]

You're quite correct, the power supply for your T.V. is a switch-mode type (as are most if not all T.V. power supplies).

When these go faulty, your first task is to find out if you've got a primary side problem, or a secondary side problem. I'll need to know a bit more about the symptom you're getting to be honest, but if you're certain it's the power supply try the following;

1. First do a visual inspection of the power supply side of the T.V. Look for damaged components (has the power supply had a blow up?) and also pay attension to the electrolytic capacitors. Check for leakage from the bottom of the caps, and also check for bulging at the top. Check for resistors with burned bands around them that might indicate stress.

2. If no obvious problems are found, you need to determine what kind of switch-mode power supply problem you've got. There's a few directions you can go here, but the first thing I like to do is check the secondary voltages. If some are present but others are missing, that would indicate a problem on the secondary side. If none are present, you could have a problem on either the primary or the secondary side (the feedback is usually taken from the H.T. rail so if this one is down the power supply won't function at all - check this one first).

Once you've got to this point, let me know what the results are and then we can make some decisions about what to do next. Others on here might advise you differently - that's ok, and feel free to follow their advice as well because everyone has their own preferences when it comes to switch-mode-PSU servicing.

Brian

 

[HD ELEKTRONİK]

Please check R862 and C863.
(R862:0,5W or 1W use).

 

[tab a]

Well, on the one hand, I learned a lot since yesterday. On the other hand, I'm going to have to learn a whole lot more!

I now think I don't have a power supply problem but rather a start up problem. Prior to posting, I had checked all the resistors and electrolytic caps mentioned and had found them to be OK. I did think an SCR (near the bottom on the attached pdf) was bad, and replaced it. I was testing the powere supply board with it unhooked and removed from the TV. With it back in the TV, when plugged in, I hear a steady whine (deflection?) and every 5-10 seconds the relay clicks. Pushing the power button also causes the relay to click. 0v measuring across the relay. No picture or sound.

On the attached schematic (the other half of the first schematic but as noted, from a different model) I have -30v and 5v supplies from the lower transformer. This TV, only has one secondary from the upper transformer (T862), which I measure as 5v, UNLESS I remove F802, and I then get 115v as apparently I should. So, presumably the problem is downstream of that fuse.

That 115v source is supposed to feed (?) the SCR I replaced, which of course makes me wonder about that?? It also goes to the ''400"s numbered components, and I don't have a schematic for them. I've read about SCRs but am not experienced enough to really understand how this one is being turned on and off. It will not latch in the circuit when tested with my DMM--though it would latch when tested prior to installing it. Not sure if that's significant or not.

Any explanation of how that SCR is supposed to be turned on and off, or where to look further would be greatly appreciated.

Thanks again.