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Old 4th Jan 2011, 5:21 pm   #1
Mikey405
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Default National (Panasonic) TC85G

Hi all.

I was wondering whether anyone had any ideas regarding a National TC85G (later version) set which isn't happy at all. The set was working very well until the picture suddenly went very dull and out of focus and shortly afterwards went off.

The "Going off" was traced to R537 (not present on earlier models and schematics). The 4.7R 2W resistor supplies HT to the bottom end of the line output transformer (pin 8) and it had gone kaffut (open circuit). Having replaced the resistor the dark and out-of-focus picture has returned but the resistor gets very hot within 20 seconds-or-so, so obviously I don't run the set for more than is absolutely necessary (a few seconds at a time whilst making measurements).

Interestingly in the darkness of the picture, there are two brighter vertical areas indicating perhaps that the scan is not a proper sawtooth waveform. (I have not checked this theory yet though.) The waveform from the line driver is a very good solid square wave and the line output transistor is not suffering as far as I can tell in the short time I can safely run the set.

Disconnecting the focus, convergence panel and EHT does not make any difference to the overheating resistor. There are still some things I'd like to check, such as the line output tuning capacitor C215 but I wondered whether anyone had any ideas. I'm hoping that it's not the lopty, but I don't have a spare one to try and I can't find anyone that still sells them.

Thanks all.

Kind regards.

From Mike.
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Old 4th Jan 2011, 5:37 pm   #2
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Default Re: National (Panasonic) TC85G

Well, I just measured the line waveform going to the convergence panel. Where I would have expected a fairly-normal looking sawtooth waveform, the waveform looks like a complete jumble of nonsense. Absolutely nothing like a sawtooth, more like a load of swirls and jagged edges. I think I see a shorted-turn here somewhere. Scan coils? Lin coil? Lopty?

Thanks all.

From Mike.
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Old 4th Jan 2011, 6:09 pm   #3
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Default Re: National (Panasonic) TC85G

Not a tv expert, but my first port of call would be the line O/P transistor, then the line drive transformer soldering, and line output transformer soldering, and line drive transistor. I am of course assuming it is a transistor set!
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Old 4th Jan 2011, 6:13 pm   #4
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Default Re: National (Panasonic) TC85G

Line driver transformer failure was common on these sets as is the transistor itself, Try a new driver transistor first. Something in my mind says its a Japanese special and equivalents are not too happy in the circuit.
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Old 4th Jan 2011, 6:43 pm   #5
Brian R Pateman
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Default Re: National (Panasonic) TC85G

Line driver transformers were known to fail.

I'd agree with Trevor, TR503 is the line drive transistor, it's a 2SD198 and the equivalent is BU126. However I seem to remember that the BU126 didn't always work in these sets, or not for long anyway.

I've got the manual if you want to borrow it.
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Old 4th Jan 2011, 7:00 pm   #6
geofy
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Default Re: National (Panasonic) TC85G

Line driver or Line output, if the driver goes it usually takes the output with it? And don't forget those soldered joints.
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Old 4th Jan 2011, 7:04 pm   #7
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Default Re: National (Panasonic) TC85G

Havent seen one of these for ages. Checked my notes and I have a lot of references to c514 causing dark pic
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Old 4th Jan 2011, 8:44 pm   #8
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Default Re: National (Panasonic) TC85G

Thanks everyone for your excellent suggestions. The waveform on the collector of the line driver transistor is exactly as it says it should be in the manual - a nice clean 340vp-p square wave (although with a tiny amount of ringing on the leading edge, but only very small). The expected waveform on the base of the line output transistor is shown in the manual as a bizzare jagged sawtooth with odd random looking jaggy bits here and there, and not, as I would have expected, a nice sharp square wave. The actual waveform that I have on the base of the line output transistor with the transistor in place is a horrible square wave littered with huge amounts of nasty spikey transients all over the place, all equal to the height of the main part of the waveform, but certainly nothing like the waveform in the manual. With the transistor removed, the output from the line driver circuit is, again, a nice clean square wave.

Unfortnately tonight is my last night in Devon for a good few weeks, but I'll take this up again when I next come down for a few days. I will certainly check, as has been suggested, the line driver circuitry, but I think I also need to check any tuning components such as the capacitor across the line output transistor's collector and emitter, and the line output transformer itself of course. I'll make sure I bring my camera down next time for some photos of waveforms etc.

Thanks all again for everyone's help.

Kind regards.

From Mike.
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Old 4th Jan 2011, 8:56 pm   #9
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Default Re: National (Panasonic) TC85G

Mike.
Can you disconnect the +B to the line o/p transformer and load the HT with a 60watt bulb, Then have a look at your drive waveform on the line output transistor, if all clears up the fault is in the output stage, if not its the driver stage at fault.

Don't forget the +B smoothing cap at the feed to the line transformer could be o/c
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Old 4th Jan 2011, 9:04 pm   #10
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Default Re: National (Panasonic) TC85G

Quote:
Don't forget the +B smoothing cap at the feed to the line transformer could be o/c
You're absolutely correct Trevor - it was O/C and I dashed off to Maplin in Barnstaple to buy one this morning.

I don't have a blub to hand this time but I'll certainly give that a try next time. I'm actually in the process of changing the line output transistor to the nearest equivalent I have as my cheap-and-nasty multimeter is telling me that there is leakage from everything to everything on the 2SD350. (In the M-Ohm range, but I don't think I should expect to see it...) I'll report back very shortly.

Thanks Trevor.
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Old 4th Jan 2011, 9:11 pm   #11
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Default Re: National (Panasonic) TC85G

Hmm. A new BU208 made no difference (despite not reading as leaky from anywhere to anywhere). Ha ha. Oh well, time to pack up for an early start tomorrow. I'll give the old girl another going over when I'm down next. I would take her back with me but my car is chocka-block already.

Thanks Trevor and everyone for your help.

Kind regards.

From Mike.
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Old 4th Jan 2011, 9:18 pm   #12
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Default Re: National (Panasonic) TC85G

Tie it to the roof or sit it on your knee!!
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Old 4th Jan 2011, 9:33 pm   #13
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Default Re: National (Panasonic) TC85G

I recall the harmonic tuning cap (can't remember the cct ref) on these causing unhappy LOP stages and dim pictures.
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