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Old 2nd Dec 2022, 9:39 am   #21
Welsh Anorak
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Default Re: TX98 Blown up power supply

It's definitely stuck in startup - i.e. trip mode. You've done basic checks on the primary, so have a look south of the transformer to see if there's a short or leak there. You could disconnect the feed pin to the LOPT to isolate that stage - that won't do any harm.
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Old 2nd Dec 2022, 4:12 pm   #22
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Default Re: TX98 Blown up power supply

The reason I said look at the TX100 manual circuit description for this Psu was because this chassis was the first that used the novel scr fast Thorn start up version of this circuit.
The later TX99, 98 also used it but their manuals already assumed that Dealers were familiar with it by then, and so didn't need such a comprehensive introduction to it in the later manuals, the earlier MK3 TDA4600 TX9 chopper version never used it, the TX100 was the first,

Ps. the TX98 posistor was also used as the surge limiter resistor too (a first for Thorn) similar in operation to some versions of the Philips 2A chassis and all of the later Philips CP110 chassis
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Last edited by Red to black; 2nd Dec 2022 at 4:24 pm. Reason: ps added
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Old 3rd Dec 2022, 12:30 am   #23
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Default Re: TX98 Blown up power supply

I'm giving up on this set, I can't figure out why it's got continuity on all pins on the secondary, if I remove the transformer and test it it's fine, if I test the tracks on the board with it removed the continuity is gone there too so I can't figure it out.

Give me an ICC5 any day over this abomination. I'd still like a TX98 set just not this disaster.
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Old 3rd Dec 2022, 12:42 am   #24
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Default Re: TX98 Blown up power supply

Have you isolated the 115v line?
Have you checked around IC11? And isolated any loads from there?
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Old 3rd Dec 2022, 12:50 am   #25
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Default Re: TX98 Blown up power supply

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Have you isolated the 115v line?
Have you checked around IC11? And isolated any loads from there?
Done all that and same results.
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Old 3rd Dec 2022, 2:39 am   #26
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Default Re: TX98 Blown up power supply

It seems to me the secundaires should measure continuity to eachother when connected to the PCB. They're both low resistance and are both tied to GND on one end.
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Old 3rd Dec 2022, 4:10 am   #27
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Default Re: TX98 Blown up power supply

I'll have one more look at the chassis tomorrow and see if there's anything obvious i might have missed but so far ive replaced the recommended parts and the results are the same
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Old 3rd Dec 2022, 10:13 am   #28
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Default Re: TX98 Blown up power supply

Not really sure what you're getting at with the continuity tests. The secondaries will have continuity on the pins - what we're looking for is a short after the transformer. I very much doubt there's a fault with the transformer - I did have one fail on a TX100 but that's one in hundreds - and that was a primary short.

Other than the thyristor R2B mentions it's a pretty straightforward TDA4601 circuit so I would go over the circuit again to find something you might have missed. Once you have it working it will probably stay that way unlike your ICC5s!
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Old 3rd Dec 2022, 12:39 pm   #29
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Default Re: TX98 Blown up power supply

After I allowed this to frustrate me more than it should have last night I've thought about it more and I was overreacting, I've ordered a bunch of resistors (which I needed anyway) and once they arrive R91 is the first to be replaced as it been hasn't yet and I've a feeling it's one of the culprits for this set not running. R88 may be suspect too and I'll replace that also

As far as shorts go I've tested and tested and tested all the likely suspects and can't find anything, so once I've replaced those resistors the picture might become a bit clearer.

On an ICC5 side note, I've been extremely surprized lately, my 51K5 has been used for about 5-6 hours a day for the last 3 weeks and hasn't had a single (new) fault, the E-W on it is working, that annoying 'wobble' on the picture hasn't re-appeared and the only issue it presently has is the vertical shift is off by about 2cm until the set warms up (that I can live with)

Back to this TX98 I got the set as a little restoration project, is a 36K3 in white which has yellowed a bit and once the chassis is working I want to re-spray it (colour choice pending) and perhaps add composite/Scart input to it too.
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Old 5th Dec 2022, 11:32 pm   #30
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Default Re: TX98 Blown up power supply

Good news bad news....

Good news...
Figured out the problem, so small so stupid, So Even though i replaced C88 I should also have looked at R88...teeny tiny little 0.82ohm resistor, replaced it today and the set came straight back to life....

bad news...
Its dead again... and for a stupid reason, while adjusting something i had the set running, then after 5-10 mins set shut down and has a slight whining sound, quickly figured out that when i was doing all that testing I had taken the HOT out to test it and when i put it back in i never refit the metal heatsink clip to keep it tight against the heatsink, HOT was cooked and even though i believe i have a suitable replacement i cant find that retaining clip, I've no idea where it got to so cant fit a replacement until i either find it or find a suitable clip that will place the same tension. Until then this TX98 remains dead.
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Old 6th Dec 2022, 12:05 am   #31
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Default Re: TX98 Blown up power supply

The 4600 supply is a fairly easily sorted out PSU, it has a known fault history for sure and is fairly easily repaired, not perfect by any stretch but is a known quantity with no strange and obscure feedback loops
The only quirk with the the Thorn versions from the TX100 onwards was the SCR fast start up bit, Some other makes used a mini thermistor here later such as the 'Turkish' Bush model 2020 etc.
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Old 6th Dec 2022, 12:09 am   #32
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Default Re: TX98 Blown up power supply

Question, the HOT is an R4050 which i dont have any off but i have an S2000AF1 would that suffice?
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Old 6th Dec 2022, 12:27 am   #33
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Default Re: TX98 Blown up power supply

I think a BUT11A will fit here, if the heatsink has the insulating pad, if not the AF version (fully insulated version) in this position works as well.

If it is fitted with the larger type of transistor then either a BU508A or S2000A will do, or their respective AF versions, not the D types (internal Diode) of these latter types though.
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Old 6th Dec 2022, 12:59 am   #34
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Default Re: TX98 Blown up power supply

I fitted it and it seems to be working ok, still a few bits that need adjusting but so far it seems to be running ok

If i wanted to add a scart socket to this would there be much involved seen as the chassis already supports it?
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Old 6th Dec 2022, 1:05 am   #35
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Default Re: TX98 Blown up power supply

Quote:
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If i wanted to add a scart socket to this would there be much involved seen as the chassis already supports it?
Good news on the repair! Glad you did not give up.
As for scart, you'd have to double check the PSU and see what isolation there is. Then there's the matter of AV switching, some non remote and non-AV sets used a simpler micro that omitted that part - the Philips CP90 is one example.
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Old 6th Dec 2022, 1:14 am   #36
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Default Re: TX98 Blown up power supply

Quote:
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Quote:
Originally Posted by IKC2E51R8 View Post
If i wanted to add a scart socket to this would there be much involved seen as the chassis already supports it?
Good news on the repair! Glad you did not give up.
As for scart, you'd have to double check the PSU and see what isolation there is. Then there's the matter of AV switching, some non remote and non-AV sets used a simpler micro that omitted that part - the Philips CP90 is one example.
The set has a dedicated AV channel so the Micro it seems is programmed for it. Im assuming the PSU would be fine seeing as the 21" inch version employed a scart using the exact same chassis
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