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Old 29th Nov 2022, 8:51 pm   #1
IKC2E51R8
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Default TX98 Blown up power supply

Hi All

I've gotten hold of a 36K3 using TX98 chassis, It was sold as working however it arrived and had a blown up power supply, the fuse, the bridge, chopper transistor and TDA4600 were all blown, ive replaced them all and I believe the primary is now operating correctly however the set is still behaving totally dead.

after turning my attention to the secondary side, all the pins on the secondary of the chopper have continuity which explains why its still dead but i cant figure out which part/s are causing it, if i unsolder the pins the chopper seems to be ok but the traces themselves all still have continuity.

has anyone seen this before or Could anyone point me in the right direction here?
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Old 29th Nov 2022, 10:05 pm   #2
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Default Re: TX98 Blown up power supply

Don't know this chassis, what year is it from? I have worked on TX92 and TX805 sets, these were I think made between 1995 and 2000.

In any case I would start by checking the secondary loads one by one (being careful not to remove any that are part of a feedback or voltage regulation system). Isolate HT line first, see if you can get it to standby. If you post a close up of the secondary area of the cct diagram we can be more specific.
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Old 29th Nov 2022, 10:16 pm   #3
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Default Re: TX98 Blown up power supply

Think the chassis was launched in 1990.

here is what i have for the TX98 Power Supply, CCRt diagram and board layout
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Old 29th Nov 2022, 10:17 pm   #4
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Default Re: TX98 Blown up power supply

better view of circuit
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Old 29th Nov 2022, 10:40 pm   #5
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Default Re: TX98 Blown up power supply

Not that many outputs, quite a few come via IC11 so that's definitely suspect along with the diode and cap before it, but check the 115v line first
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Old 29th Nov 2022, 11:43 pm   #6
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Default Re: TX98 Blown up power supply

TX98 repair tips, here. Should come in handy..
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Old 29th Nov 2022, 11:49 pm   #7
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Default Re: TX98 Blown up power supply

cap C88, the posistor and the mains switch seem to have led to PSU blowups according to that list.
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Old 30th Nov 2022, 1:21 am   #8
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Default Re: TX98 Blown up power supply

C88 tests good, mains switch seems to be working ok and the Positor is working correctly (at least its degaussing when the power button is pressed).

I think something was spilled on this set (some dried residues on the board) so I've a feeling Im dealing with a very non stock fault on this one
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Old 30th Nov 2022, 12:14 pm   #9
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Default Re: TX98 Blown up power supply

Try checking D12, C87, R89 and R91 as anything wrong with these could prevent the main chip from working properly.

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Old 30th Nov 2022, 2:13 pm   #10
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Default Re: TX98 Blown up power supply

Ben's right - back in the day we changed loads of those IC11s (TDA 8138). In fact they were the main cause of failure in these TVs and comes up first in the link on post #6.

If you have a primary fault check the 270k and 330k resistors.
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Old 30th Nov 2022, 8:47 pm   #11
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Default Re: TX98 Blown up power supply

Ive just discovered on power on Resistor R83 is getting very hot very quickly (too hot to touch) but im not sure why,

does anyone have any ideas why this resistor would get so hot so quick?

I've marked it out with a red circle seen below to make it a bit easier to see
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Old 30th Nov 2022, 8:56 pm   #12
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Default Re: TX98 Blown up power supply

It is running hot because the PSU is not running normally, ie. it is attempting to run from the start up supply.

The TDA4600 PSU is very well documented and has several known stock faults.

The earlier TX100 used the same Thorn derived SCR start up circuit as this TX98.
This PSU is very similar to the TX100 in respect of its design, look at the circuit descriptions of the TX100, TX99 and TX98 as they were all very similar.
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Old 30th Nov 2022, 9:18 pm   #13
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Default Re: TX98 Blown up power supply

R91 (270K) to pin 4 of the TDA4600 can go OC and cause a blow-up of the power supply.

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Old 30th Nov 2022, 9:40 pm   #14
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Default Re: TX98 Blown up power supply

R91 tested good out of circuit.

Ive got 325v making its way to the chopper transformer, nothing on the secondary, all pins are connected to chassis on the secondary, im getting 0v on every pin
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Old 30th Nov 2022, 10:00 pm   #15
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Default Re: TX98 Blown up power supply

Information for the TDA4600: This is for the -3version.
https://www.datasheets360.com/pdf/-4320547024499770993

From the Radiomuseum: https://www.radiomuseum.org/tubes/tube_tda4600.html

Described as a self-oscillating converter IC. It might be possible test the device with the base drive to the chopper transistor disconnected.

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Old 30th Nov 2022, 10:02 pm   #16
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Default Re: TX98 Blown up power supply

With any TDA4600 based PSU if it's gone bang always replace the capacitor feeding the chopper transistor base which in this case C88, What usually happens is it goes low capacitance and distorts the drive waveform to the chopper transistor leading to it's demise.

Check all the other components back from the base of the chopper to the IC. In really bad blow ups it's not unknown to find L17, R88 and R97 o/c.

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Old 30th Nov 2022, 10:02 pm   #17
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Default Re: TX98 Blown up power supply

It could be a fault on the secondary side stopping the chopper from working, ie. a shorted lopt or a short on one of the rails, this usually results in tripping in this type of supply but not always!
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Old 1st Dec 2022, 11:04 am   #18
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Default Re: TX98 Blown up power supply

I agree - have a look for a short downstream of the PSU. I think I've had the rectifier feeding IC11 go short and the line output transistor of course.

I'd change R91, C88 and the degaussing posistor anyway - also the mains switch if it's a brown and white Lorlin type.

I think this was the last British chassis from Thorn. Not sure which dress yours is wearing, but most 21" versions (51P5?) used a similar cabinet to the ICC5. Always a pleasant surprise to take the back off and see one of these friendly faces rather than that inscrutable ICC5!
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Old 1st Dec 2022, 5:26 pm   #19
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Default Re: TX98 Blown up power supply

I haven't changed any other parts in it yet however I took some voltage measurements on the primary side and they're not correct at all.

I've 39v feeding the SCR which should be 125v and at the supply pin 9 of the TDA I've got 9v instead of 14.1v, I know (from the doc posted by David - Thanks a mill for that) that the TDA wont be in its 'on' state unless this pin reaches a minimum of 12.3v so aside from whatever short is happening on the secondary side (which I still have to hunt down) I've still got problems on the primary I need to address

I haven't replaced C87 yet cause i need to look through my parts bin for one but capacity wise it measures ok but the ESR on it is way up at 2.3ohms
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Last edited by IKC2E51R8; 1st Dec 2022 at 5:47 pm.
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Old 2nd Dec 2022, 1:09 am   #20
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Default Re: TX98 Blown up power supply

Replaced C88,C87, R84 R85 and R83 and same results, I've no 270k resistors to replace R91.

Voltages are a little different after changing those parts but still incorrect, I've have 44v going to the SCR now and 10v going to pin 9 of the TDA so not much of a difference and still not enough for the primary to be running properly.

Anyone any ideas of what else could be wrong?

R91 tests the correct value out of circuit so it may be ok any other suggestions as to why the voltages are off? I've 325v on the mains smoother C86 which is about correct (marked 315v on the board)
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