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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 14th Aug 2023, 10:34 pm   #61
agardiner
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Default Re: My Hitachi CP1421R smoked

Yes, but it could be a number of things. I will try and look at the schematic again over the next day or so and advise further. To save me time trawling back through this thread, please can you summarise what you have replaced so far?
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Old 14th Aug 2023, 10:54 pm   #62
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Default Re: My Hitachi CP1421R smoked

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Originally Posted by agardiner View Post
The exact value is not too critical; the fusible element is quite important. If it were me, I would try putting a 1.5 ohm fusible and 1 ohm 0.75W fusible in parallel. These are easy to get values from the likes of Cricklewood.
Not quite true in that position, that resistor is associated with the OCP (over current protection) pin of that IC.

This senses the voltage drop across that resistor to determine if there is an over current problem, you only need a slightly higher volt drop or increase in resistance and the IC will shutdown as the circuit detects a false OCP condition, it all depends on the thresholds and parameters the circuit is operating within.

Similar problems arises in modern IC frame output stages where negative feedback across the 'bottom end' sense resistor raising a fraction of an ohm can have a massive disproportionate effect
on frame scan amplitude.

It is true that when low value fusible resistors are used in power rail feeds then the resistance value is not that critical
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Old 15th Aug 2023, 10:53 am   #63
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Default Re: My Hitachi CP1421R smoked

Quote:
Originally Posted by agardiner View Post
Yes, but it could be a number of things. I will try and look at the schematic again over the next day or so and advise further. To save me time trawling back through this thread, please can you summarise what you have replaced so far?
I replaced the IC, C803, C804, C806, C810 and R806 (also R804 but they are burning, so it is empty right now).
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Old 15th Aug 2023, 11:08 pm   #64
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Default Re: My Hitachi CP1421R smoked

Incidentally This is a Daewoo chassis CP-375 to be exact
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Old 16th Aug 2023, 9:29 am   #65
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Default Re: My Hitachi CP1421R smoked

OK, so in the absence of enough test equipment, a brute force approach may be needed to repair this set now.

Replace: D808, D805, D806, D807. C807, C808, C811. R804, R805, R807, R806, R811, R809. IC801. D811, D812, D813. C823, C818, C821.

A lot of bits, but this should remove any uncertainty of what is causing the switch mode to blow up. Sometimes it is the best approach to a faulty switch mode, otherwise you can keep replacing the IC as you investitagate.
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Old 12th Sep 2023, 3:54 pm   #66
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Default Re: My Hitachi CP1421R smoked

I've replaced everything except R805, R806, R807 and C811. No change, voltage on IC was 320V on pin 1. There was also 1V on some other pins, but the fusibles blew up and they show 0 now. Maybe it's C811, idk. This, and the fact that with the money I spent on parts I could've bought 5 identical working TVs does not give me hope. I've found a guy who can help, so I'll see what he can do about it.
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Old 12th Sep 2023, 7:01 pm   #67
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Default Re: My Hitachi CP1421R smoked

Do let us know how it goes.
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Old 16th Sep 2023, 10:59 am   #68
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Default Re: My Hitachi CP1421R smoked

I was told that the guy is too old and can't do it. So, if you don't have any ideas, I'll go buy a new TV and use this one for parts or something.
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