TEAC TV CT-M805SV picture width/pincushion problem

Hi all,

I've been attempting to repair my first TV, a Teac CT-M805SV, I've made the problem better, but its still there so I'm hoping for some pointers.

The problem its exhibiting is a lack of picture width (partial horizontal collapse?) with a fairly bad pincushion problem, which I suspect will sort itself out when I fix up the picture width problem.

Anyway, I've replaced a number of caps around the vertical section since that sounded like a good idea, and also fixed a few cold solder joints in the PSU and around some diodes near the LOPT/flyback. I've also tested all transistors in the area (as best I can - a go/no-go type test), and checked all the diodes on the main board.

So far all my efforts have achieved is to make the problem about 50% better than it was. I noticed when I was testing it after replacing caps in the vertical area that the picture was creeping inwards (getting narrower) within about 5 seconds of turning on the set, so I promptly turned it back off.

I replaced as a precaution a couple of diodes that were running hot, and one capacitor that was adjacent to one of those diodes as it had been scorched by the diode beside it. The diodes were still functional, however.

Here's what I've done to date:

Replaced D403 and C431 (D403 was hot, scorched C431)

Replaced D401 (damper diode) as it had been running hot and had blackened the PCB underneath it. It also had dry solder joints at both of its pads.

Replaced C419 (4.7uf 50v Bi-Polar), C401 (1uf 50v), C408 (100uF 35v), C410 (330uF 50v) and as mentioned above, C431 as well.

I don't know where I should actually be looking for this problem on the PCB/schematic, so if anyone can point me in the right direction that would be great! :) I've been contemplating testing the voltages of the PSU while its running which will probably be my next step if I don't any better suggestions.

Thanks.

Reply to
Robert Davidson
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You need a schematic with voltage test points. You can't put the cart before the horse and need to start verifying voltages at the PSU before moving on. If the PSU is within its tolerances then look at the other PSU, the horizontal out. Some scan derived voltages there will be highly influenced by the performance of that section.

Make sure you use extreme caution when testing live circuits with lethal voltages present. One little slip up, worn insulation on a test probe or tool etc... could garner you a nasty shock or worse.

Reply to
Meat Plow

Sounds like a problem with the E-W modulator. Usually a chunky diode, but you really need a schematic to find out where it is on that particular model.

Arfa

Reply to
Arfa Daily

On Sun, 24 May 2009 17:17:21 +1000, Robert Davidson put finger to keyboard and composed:

ISTR working on this set several times. IIRC the small electrolytic capacitors in the PSU should be replaced. Your set may have other problems, though.

Can you upload the schematic to a file sharing site? Alternatively you could email it me and I'll post it on my web space.

- Franc Zabkar

--
Please remove one \'i\' from my address when replying by email.
Reply to
Franc Zabkar

Fair enough. I've checked the power supply voltages (primary supply) as best I can. There is one output I'm unsure of, since the marking on the PCB says its a 5v output, but the schematic clearly shows it going to a

2W resistor and then into an LM7805.

+145v is at +143v

+18v is at +22v +Unknown is at +12v (pin 4 of the PSU connector)

I'm a little concerned about the +18v being at +22v, and I'm currently looking over the schematic to see what impact that might have on things, but so far I've only found one area that its used and its going through a couple more resistors and being clamped to 7.5v by a zener (around the horizontal drive area of IC201).

I've done some poking around the secondary power supplies derived from the flyback transformer but still need to do more poking in that area. There is a +33v output from that area that I've managed to verify is running at +32.57v, so that seems fine, but there are at least a couple others I still need to check.

I've sent it to you via e-mail, if you could post it up on your web space and provide a link to it for anyone else who is interested that would be good. I tried to find the original place I downloaded the service manual from but I didn't have any luck :(

- Rob.

Reply to
Robert Davidson

Those others checked fine... (+8v, +12v)

I went and found the E/W section of the board, and I found a track there that was cracked, so I've fixed that and now the width is better, just too wide now. So whatever caused me to 1/2 fix the width earlier probably caused that problem.

Apart from that the pincushion problem still seems to be there, the OSD text is all in italics at the top of the screen, and about right in the middle (though I can't see the sides of the picture anymore to see how bad it is).

I guess I'll try to find a way of changing the width now, then to tackle the pincushion issue.

- Rob.

Reply to
Robert Davidson

There is a limit to the amount of correction that the pincushion modulator can apply. If the width is now excessive, you may well be beyond that limit, and not actually seeing a pincushion correction fault per se, at all ...

Arfa

Reply to
Arfa Daily

No probs, I'll keep that in mind. After I ran the set for about 15-20 mins after I fixed its cracked track, the thing decided to turn itself off and not come back on.

I'm suspecting a short circuit somewhere as the PSU is just making a faint ticking noise every 1/2 second or so, so now I've gotta fix that as well. I don't think it'll be too difficult to track it down.

If you or anyone else wishes to look at the schematic for this thing at all, Franc Zabkar has put a copy of it up on his web space here:

formatting link

- Rob.

Reply to
Robert Davidson

Sure enough, it was easy to track down. The HOT blew and went short-circuit across all pins.

I've replaced the HOT and briefly tested it. The HOT that was in the TV originally is now back in the TV, the one that went S/C was a replacement one I bought, not the original. The reason I replaced it was because a transistor tester said it was dead, but that was rather misguided.

The HOT that blew did it when I was flicking through channels (about 5 flicks, one every 2 seconds or so). Does that sort of thing likely to put strain the HOT?

I've also done some reading around the place and it seems that one HOT is not necessarily equal to another, even if its stamped with the same number. I've also read that it probably isn't a problem in the drive circuity for the HOT since problems there don't normally kill HOTs.

I've run the set for about a minute, keeping an infrared thermometer pointed at the back of the heatsink where the HOT is mounted, it doesn't seem to be getting hot, luke warm at best. I did a similar thing last time after the TV shut down due to the HOT going S/C, the max temperature I read at that time was about 25c (room temperature was about 18c), however I was a bit slow getting out the thermometer so it would have cooled down a bit in that time.

What would be the likelyhood that I have a faulty flyback/LOPT?

Could a faulty flyback/LOPT be causing the picture width problems?

- Rob.

Reply to
Robert Davidson

A lot of things could be at fault. Multiple things in the HOT circuit in general things with low grid numbers like the damper D402. And other components that tune the circuit.

Have access to a decent scope where you can look for deformity in the HOT's output (primary side)?

Haven't seen a seperate damper in awhile mainly because I don't work on this stuff now but did enough Teac/GE/Panasonic warranty back in the day to last a lifetime. These circuits aren't generally too forgiving where the HOT is concerned. Go astray from the tolerances too much and you better have a bin full of them.

Reply to
Meat Plow

'Dead' in what way ? It is very very rare for transistors, particularly HOP transistors, to fail in any obscure way that would lead to any of your width related problems. Power transistors go short circuit, open circuit, and leaky. Small signal transistors sometimes go noisy, and occasionally low gain, as well as the other DC failures. HOP transistors are fussy little devils. The correct one for the chassis must be used, and you must pay attention to suffixes, as sometimes, this can mean the difference between say the transistor having an internal clamper diode, or not.

No, not on a normally working TV

I would have to dispute that little pearl of wisdom. Some of the commonest HOP transistor killers, are bad joints on the driver transformer where one is used, and out of spec base coupling caps, which cause the transistor to be driven with a distorted waveform, which leads to slow switching, over-dissipation, and ultimate failure.

I'd say the chances are pretty slim. You need to determine how the width adjustment on that chassis is supposed to work, and then look into why it isn't. For instance, do VR402 and VR403 have any influence on the width and pincushion ? ( see service man pdf page 9)

Arfa

Reply to
Arfa Daily

Possibly, but probably not easily.

Hrmph. Fair enough.

So with regard to the excessive width of the picture, could that be stressing the HOT at all?

- Rob.

Reply to
Robert Davidson

Well if it's pincusioned the width isn't excessive is it? Can you snap a pic of the screen and upload it to say Flickr or Photobucket? I'd love to see how it looks. I don't suppose you have something that would generate color bars or a test pattern so one could see just how the picture is deformed?

Sometimes when you run out of luck as in replacing a couple caps and a HOT and still have issues there is a minimum amount of test equipment you need to properly diagnose. I wouldn't tackle something like this without a schematic with test points depicting the proper waveforms in critical places and a half decent scope. I'd also want to know what the HV was at the anode end of the crt. I'd also want to isolate the chassis with a 1:1 transformer or use a variable AC supply that did that. You see where I'm going with this?

Reply to
Meat Plow

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