1. Home
  2. Electronics Repair
  3. Toshiba CX2178C TV

Toshiba CX2178C TV

Was given this TV in non-working order. It is dead. No response to any of the front-panel buttons (no remote came with it).

There's one obvious relay on the AC circuit, closing this relay by soldering leads together makes the tube come on, green with light-green line horizontal-ish lines, but no power to any of the logic components, no response to any button pushes.

There is an area on the PCB that looks as if it's gotten a bit too hot around it. I de-soldered the IC at the epicenter of the black area and it's a Sanken STR-D1005T "hybrid voltage regulator". According to the datasheet (which, btw, looks as if it was typed on my old 1955 Royal typewriter comlete with white-out blobs) reference circuit, it converts rectified AC at ~140-180VDC to 12VDC in conjunction with a transformer.

I've done a bit of electronic repair and have never run into such a beast, can't make heads or tails of the circuit, but then I never really have understood SMPS's.

Does anybody have a source for this part? I've checked all the usual suspects, i.e. via findchips.com and am striking out. There are a few distributors who'd gladly sell me 1000 or more but that would certainly go a long ways towards negating the value of my "free" television.

Any help greatly appreciated.

Dave

Reply to
Dave
Loading thread data ...

A bit more info:

As an aside, the main filter cap is a 200V 470uF electrolytic. Just to avoid painfully shocking myself I was going to discharge it prior to messing around. Right after powering off the set this cap only read 400mV on my meter! I shorted the cap with a resistor, then tested using my DMM set on

20M resistance scale expecting to see zero resistance slowly rising to infinite resistance. What I got was infinite resistance right away, like an open circuit.

Is this cap likely bad, or should I lift one lead for a better test? If this cap WERE bad, would it keep the set from powering up? I had heard that the filter cap could lose as much of 75% of it's capacity before you'd likely see any problems. Also, anybody know how I might test the STR-D1005T I removed from the circuit?

Reply to
Dave

Leave the regulator alone for now. Not sure if this chassi has it but, look for a 33uf@160 volt cap in the power supply. If that dried out, it'll make the set deader than a stump.

formatting link

Reply to
Tech Data

I'll have a look. It's chassis TAC8807.

Thanks a lot for your help.

Dave

Reply to
Dave

The closest I can come in a TAC8806 which may be close enough. Check C887 which is the filter for the 115 volt B+. This has a feedback loop that monitors the B+. If it's being dragged down, the hot side of the power supply will shutdown also.

formatting link

Reply to
Tech Data

Check the following components: c840 16v330, c841 50v22, q802 str-d1005

If you don't have an ESR meter you may have to replace these 2 caps to be sure that they are good.

Good luck

Reply to
gnk

There is nothing wrong with the strd1005 change the cap from pin 1 to ground and then chenge r840 it has opened do to a shorted cap that str is a very strong regulator Im' working on that set as we speak so if anyone has the delfelction diagram please send it to me at snipped-for-privacy@comcast.net

Reply to
David Naylor

Thanks Dave, I thought as much when I noted that the regulator is rated to

800V. The resistor was likely the source of the smoke.
Reply to
Dave

Can somebody give me the voltage rating of C841 (22uF electrolytic) on an

8805/8806 chassis? Mine seems to have vaporized.
Reply to
Dave

50 volts.

formatting link

Reply to
Tech Data

Well, boys, I must say I am disappointed. Got her all back together last night and fired her up with the series light bulb and... nothing. No click, no lights, no Wheel of Fortune. Exactly the same as before I started. Now I'm into it for $7 so I am damned and determined to make this sucker work. Here's what I did.

Replaced the following:

c822 100 microfarad 16v c841 22 microfarad 50v c820 33 microfarad 200v c842 100 microfarad 6.3v c840 330 microfarad 16v c887 (well, actually I did this one last after I did the others with no success, replaced it with the 33uF 200V that I had removed from C820) r840 910 ohm 1/2-watt flameproof resistor

I don't have an ESR meter but none of the caps tested open or shorted with a DMM (well, except the 22uF 50V which had exploded). What feeds c840 that might have toasted this cap? The 910-ohm resistor tested 860 ohms so it was still okay too.

I tested the regulator out of circuit as per the data sheet diagram of internal components and it seems just fine. Not sure that the regulator is coming in to play here anyway as I'm not even getting a click.

As near as I can see there's a single transformer in the SMPS with two sets of primaries and two sets of secondaries, one of which is center-tapped.. There's another smaller transformer nearby with no markings on it. None of the windings on the big transformer are open or grounded. Dunno' what's what on the other transformer to test. I'm guessing that if I can short the AC relay and get the tube to light up, the SMPS is basically working although I have no idea where or how my B+ is generated... that's what seems to be missing.

Can anybody suggest my next step? I could really use a service manual at this point.

Dave

Reply to
Dave

Sometimes the regulator chip dies when all these parts fail also there is a resistor in series with the b+ to this chip that can open. ( This is the regulator that supplies the low B+ to the upc. ie. the one with no heatsink.) Chuck

Reply to
Chuck

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.