Switching power supplies in cheap "throw away" TV's - Videokarma.org TV - Video

radiotvnut · #1 11-25-2008, 01:12 PM

Has anyone ever run into a switching power supply in one of these cheap TV's that was kicking out wwwwaaaayyyy too much B+? I've got a 2004 13" Durabrand (don't laugh) that was given to me by the TV shop. I just thought I'd play with it just to see if I could get it going. When I got the TV, there was an open 2.2 ohm resistor from the filter cap to the raw B+ input of the switching transformer. After checking and not finding any shorted components, I fired the set up and got a sudden burst of HV and then it went dead. The HOT was found to be shorted and a safety resistor in the B+ line opened. Before going any further, I measured the B+ and it's running at a whopping 256 volts! These TV's usually run around 100 volts. I replaced the optocoupler and a 1 uf, 50 v lytic close to the switching transistors' heat sink. Still no change.

During this whole procedure, I'm asking myself "why in the hell didn't they stick to a simple IC voltage regulator system based on something like an STR30130. Those were common in '90's era TV's and the circuit was simple and easy to repair. And, you didn't have to worry about B+ going through the stratosphere!

Kiwick · #2 11-25-2008, 04:13 PM

sometimes this can be due to a bad 1,5 or 2 Kv capacitor on the collector of the switching transistor, usually about 2000 pf.

Otherwise you have a bad PSU controller chip.

radiotvnut · #3 11-25-2008, 05:00 PM

This one does not use a controller chip as far as I can tell. All I see on the primary side is the FET chopper transistor, a small BJT transistor, a four pin optocoupler IC, and a zener diode. I've seen the same basic cookie cutter P/S in many brands of later model "throw away" TV's

I will check for the cap that you mentioned.

mbates14 · #4 11-25-2008, 08:11 PM

the photo is used in the regulation

radiotvnut · #5 11-25-2008, 08:27 PM

Well, I found a shorted transistor and replaced that along with the fusible resistor in the B+ line. The B+ came down to 134 volts and appears to be regulating as I vary the AC input voltage. Now, it makes a light chirp at turn on and shuts down! The 134 volts may still be too much. I was thinking that the B+ runs around 100 for these sets; but, I could be wrong. I can't believe I'm wasting my time on this POS $79.99 Wal-Fart special. BTW, it's a Durabrand DU1301 made by Orion. I'm about ready to see how many feet I can throw it!

MRX37 · #6 11-25-2008, 08:44 PM

Well hell, you went this far on it, you might as well fix it now.

andy · #7 11-25-2008, 08:47 PM

000

radiotvnut · #8 11-25-2008, 08:49 PM

Well, tomorrow's another day. At least I have not got any $ in it. If I can't fix it from my stash of junk boards; it's not going to happen. It probably wouldn't bring over $20; and, I'd have to buy a universal remote to go with it in order for it to bring that!

What I need to be concentrating on are the two 32" Toshiba's that I just obtained. These are real Toshiba sets; not Orion made ones. So, they should probably be worth repairing.

radiotvnut · #9 11-25-2008, 08:52 PM

Quote:

Originally Posted by andy

134 is a reasonable voltage, but who knows what was blown when it went to more than twice the correct voltage. I can't imagine 12v was too good for the micro controller.

That's what I'm thinking. These cheap sets have little to no overvoltage protection in the event of a runaway power supply. If there's a way they can cut corners, they are going to do it. And, the microprocessor is a little surface mount chip with a million legs on it!

radiotvnut · #10 11-25-2008, 09:36 PM

Well, I found the trouble - shorted vertical IC (LA78040N). With the IC removed, it now stays on, has audio, changes channels, and looks to have decent video. I'll look through my junk tomorrow to see if I have one.

ccheath · #11 11-25-2008, 09:42 PM

Quote:

Originally Posted by radiotvnut

....Wal-Fart....

You sound like me. That's what I call them.

radiotvnut · #12 11-25-2008, 10:04 PM

I just got a laugh. I found this exact TV on the walmart website for $19! However, they claim none are available in store or online. That $19 price must have been the final markdown in order to get rid of this model since it's analog only.

mbates14 · #13 11-26-2008, 06:37 AM

I need to find something like that scrapped around here. Need the CRT to replace the 13" CRT thats extremely burned in on my little old video poker machine. hehe.

radiotvnut · #14 11-26-2008, 10:31 PM

OK, now I'm about to get really disgusted. I replaced the vertical IC and it now appears that the tuner is bad. I have video on all channels; but, the picture is snowy. I'm starting to think that lightning or some other electrical disturbance ran in on the cable/antenna wire and put this TV out of service.

I think I've got a tuner on a junk Orion board. If that don't do it, I'm probably washing my hands of this TV.

waltchan · #15 11-27-2008, 01:13 AM

These 13" Orion-made CRT TVs generally have been pretty reliable and many are still in operation today, but radiotvnut probably got a lemon. In terms of built-quality and design-quality, I would take an Orion than Funai at any day, since Orion never used plastic case in their DVD or VCR players, Orion's labor always cost more than Funai, and Orion's product always weigh more than Funai. Went to Best Buy and picked up the last Orion-made Toshiba SD-V295 DVD/VCR combo available. There is a new Toshiba SD-V296 that is now made by Funai and made out of plastic. Toshiba finally canceled their contract with Orion for video player production after 7 years, although LCD TV production still continue. Orion no longer makes DVD/VCR combos for the US market.