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TP48D50 convergence / blows pico fuse

4505 Views 10 Replies 2 Participants Last post by  lcaillo
Hi Guys,

My 21 year old poor college kid son got a Toshiba TP48D50 given to him. It was free because it has a convergence issue. I did some research and found it's normally caused by the stk chips and I even found a Toshiba Service Tip sheet to follow.

The TV info:
Toshiba 48", Model TP48D50, Chassis 9491, Mfg Date 1994

The problem:
Convergence. When you use the test button to set the convergence you could not adjust either the horizontal or vertical lines.

What the Toshiba Service Tip sheet said to do:

*) Check the pico fuses F701 & F702. If F701 is open circuit then Q702 is blown. If F702 is open circuit then Q701 is blown.

*) Check the six 2.7 ohm 1/2 watt resistors to insure they are 2.7 ohms. They are R713, R720, R725,
R732, R739 and R744.

*) Also it is advisable to resolder QJ01 and QJ02 the +5v and -5v regulators.

What I found:

*) F702 was open circuit, the corresponding STK had a heat sink full of dust and the board was visibly brown where the heat sink mounts to the circuit board.

*) The 2.7 ohm resistors all check out OK (I de-soldered one end and lifted them out of circuit to make sure). The other larger resistors all looked OK and had readings corresponding to their markings.

*) I could see no bad capacitors.

What I did:

*) I replaced both of the STK chips. They were orginally STK 392-020 chips, I replaced them with STK 392-040 chips.

*) I replaced the pico fuses (both, even though only one was bad)

*) I could not find the QJ01 and QJ02 regulators. There are a couple transistors mounted to a couple small heat sinks, they are Q701 and Q702 I believe (board isn't handy right now).

What happened:

*) When we turned it on and entered the test mode to adjust the convergence we could only adjust the horizontal lines, we could not move the vertical ones.

*) After a couple minutes the horizontal lines snapped back out of convergence and we couldn't adjust them at all again - it acted just like it did before started the repair.

*) I pulled the board and found the F702 pico fuse was blown again.

What I've done:

*) Not much now, I'm stumped! I checked and confirmed the F702 was blown, but the other one is OK. I pulled the power supply board that was to the right of the convergence board and took a quick look for the QJ01 & 02 regulators I couldn't find earlier but I'm not spotting them on that board either. I'll clean all the dust off of it tomorrow and take a better look.

HELP!!! :scratch: Does anyone have any ideas on what to do? BTW, I tried to find a service manual for this unit but they seem hard time find as I looked hard for a couple days and can only find the owner's manual and the Tech Tip sheet.

Any help would be much appreciated!!!


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You could have a bad convergence yoke or a tube that is arcing. You could also have a situation where someone tried to adjust the convergence and got it way off, causing the chips to be overdriven. You could have gotten poor quality parts. You could have botcherd the soldering. Lots of possibilities.

Read the first ten posts of the convergence repair sticky very carefully:

Wow! Thanks so much for the quick reply!

> You could have a bad convergence yoke or a tube that is arcing.

Any way to tell with a volt/ohm meter and soldering iron for tools?

> You could also have a situation where someone tried to adjust
> the convergence and got it way off, causing the chips to be overdriven.

There's a panel you flip down and then a switch you flip to TEST and then some pots youturn to adjust the convergence. After the convergence went out it seems about everyone had to turn those pots to confirm they had no effect. I put the pots at their middle adjustment for a starting point when I did the repair. Beyond that, if there is another way to do convergence I'm sure it wasn't done. This tv was in a small town bar & grill. It ran since 1994 with no problems (which is kind of amazing with the 392-020 chips, isn't it?). They were doing karoke with it when it went funky. They tried adjusting the convergence then and several times afterwards. They ran it for a week or two with the messed up convergence and after calling a couple repair shops for ball park estimates they decided it was just better to replace it with an LCD so no repair shop ever touched it and the owners were not techincal people and I'm sure did not remove the back panel.

> You could have gotten poor quality parts.

I got the parts online from electronic-repair-kits (I can't post links due to my low posting count, but you know what to add to the front and back), which were well reviewed on a couple other web sites I ran across before I ran across this one so I hope they were good parts. I'm leaning towards they were since I'm in the same boat afterwards - same fuse blown to the same chip as in the original problem. Kinda makes me lean towards a problem in that chip's circuit.

> You could have botcherd the soldering.

That's always a possibility, but I've done a fair amount of soldering - years ago I was a ham radio operator and built a lot of stuff (homebrew, Heath Kits, etc.) and after that I was an airborne avionics guidance and weaponry control systems specialist in the Air Force. It's been 20 years since I regularly picked up a soldering iron, but I still do it occasionally and usually I don't have any problems. This board was actually quite easy to work on too. It's not overly dense in components so it was very easy to work on (which was quite refreshing - it reminded me of something made in the '50s rather than the '90s!).

The chip that had the fuse going to it blown had gotten very hot - the board below where the heat sink bolts down to was much darker than the rest of the board.

I pulled the power supply board last night and just dusted it off and gave it a quick once over this morning - nothing really jumping out as a problem. I still can't find the elusive QJ01 & QJ02 referenced in the tech tip sheet, they are simply not on the convergence board or the power supply board. There are two regulators on the convergence board though, Q711 and Q712. I think I'll warm their connections up. In fact I may just warm most of the connections up on the board and give it another shot. If that doesn't fix it then I think I'm done. I have limited tools these days, can't find a service manual, and have limited time so I'm just not up for a prolonged battle with it. It looked like most of these problems were solved by a quick change of the chips and maybe a resistor or two and fuse so I thought it was worth a shot but I'm not too motivated to keep hammering away at it.

If you can think of anything I should check (or where to look for those QJ01 & 02s!) quick before we pitch it I'd appreciate it. I'll retouch all the solder joints I did and maybe a few more and we'll give it one more shot.

Thanks again for the quick response - it's much appreciated, take care,

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Check the supplies to the ICs, check for solder problems or open resistors...the typical stuff that is discussed in the convergence repair thread. These sets were known to have bad yokes that would take out the chips. It is very hard to diagnose these. A ringing test and inductance test on the yokes would be a start.
Hi Leonard,

I checked for solder problems, in fact I reheated every connection on the entire convergence board this morning (it's a small board). I replaced the pico fuse and again we had control over the horizontal lines again for a bit and then it quit again, I'm sure the pico is blown again. Looks like the corresponding stk was still good (though it may not be now).

I can't find a service manual, but there were test points marked on the power supply board with the voltages that should be present and all the glass fuses in the power supply were good and all the test points checked out OK.

I dunno what voltages should be at the chips, but I think we've came to the end of the road. I gave it a good shot, I'm sure what I've done is fine, it's just got a deeper problem than I'm willing to dig into - especially if your experience shows these were known for bad yokes.

I appreciate your help (on a Sunday even!) tremendously, as well as this great site! Thanks!

I know, that's not much of the stick-to-it-iveness most diy'ers have but we're just not that attached to this set so our level of committment just isn't there. It was free, if it were an easy inexpensive fix it would've been great, but it not looking like it is so we'll just cut bait.

Thanks again for everything!

Take Care,

Steve (who's going to explore the rest of this site! Hmmm a diy projection screen . . . there was a projector with a burnt out bulb on Craigslist the other day . . .)
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Your apreciation is appreciated. Sundays are not unusual, as this is really a volunteer job, not my business, which is repairing and calibrating displays.

There is wisdom in knowing when to quit that some of us lack. My gut feeling was not good about this one. Enjoy "The Shack."
Hi again Leonard,

I dunno if I have the wisdom to know when to quit, or if I'm just lazy! :D

I always try to say thank you . . . I believe in volunteerism and I've done my share of volunteering over the years. Nothing annoys me more than when you take your time to help others and then people wont even say thank you, or worse yet -> they complain about what you're doing but they wont help at all! It's really easy to get burnt out! So when a volunteer does something for me I always try to aknowledge it! For all it's pitfalls though, volunteerism is fun!

Reading through the board here you have a lot to be proud of - I lost count of the number of message threads where you walked someone through their problem and got them back running! And I wonder how many have read your basic threads and got all they needed from it but never posted a thank you - I'll bet it's plenty!

I dunno how I've missed this site in the past! Lots of good info! I'm a tv-aholic and I've built a nice home theater pc system here in the house - 2tb of storage, 3 analog and 1 digital tuner in the main htpc, a client upstairs, and two eGreat M34 network media tanks (nmt's are so cheap I'll never build another client again!). I had been running older crt tvs until recently, just upgraded to an lcd upstairs and now I'm thinking a BIG TV for the family room downstairs may be in order. Of course big screens need big sound to go with 'em . . . I can see this site is going to be an asset while I'm trying to research and decide what to do!

Again - thanks to you for all your efforts and to all your cohorts that help keep this site up and going, it is much appreciated. In the past I've had some technical articles published in various magazines, newsletters, online, etc. so maybe somewhere in my home theater adventures I'll be able to contribute something back.

Take Care,

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Hi Leonard,

When I was starting on this project I ordered some 2.7 ohm 1/2 watt 1% tolerance metal film resistors (yes, 1% - didn't know what they were from the Toshiba number and the Tech Tip sheet said it was impotant they be 2.7 ohms).

I turned out ours were OK so I didn't need them, sooooo I have 100 of them and no use for them. It'd be a shame to toss them - could you use them? If so we'll figure out how to get my pm working and you can send me your snail mail and I'll be happy to drop them in the mail to you.

Take Care,

You need more posts to be able to use the PM service.
It looks like 5 is the magic number - I just sent ya an email.

Take Care,

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