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Old 29th Sep 2022, 9:08 pm   #1
andrewn
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Default Zanussi 20ZA374 PSU squealing

Hi everyone, I've repaired this set many times and posted about it here a few times before. I'm on the verge of scrapping it as the pcb print is lifting and its had quite a few parts replaced but the set is an ideal size for my workshop room. It now seems to have a temperature related fault with the PSU. From cold it sometimes won't start first time but when it does it makes a squealing noise but only in standby. This set is designed to go into standby on initial power up. In standby the voltage on the HV output is 195V. If I switch the set out of standby it operates correctly with the voltage measuring 118V (should be 116V). After its been on for a while I can switch it to standby and the PSU will be silent. Voltage measures 132V but I don't know if it should do. I thought the fault would be the small caps on the primary side but they test fine and they were replaced a few years ago. I tried replacing the BA157 diodes with no improvement. I attach a scan of the PSU which uses the TDA4601. What area of the circuit should I focus on?
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Old 29th Sep 2022, 10:13 pm   #2
Cruisin Marine
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Default Re: Zanussi 20ZA374 PSU squealing

May I suggest all the capacitors to begin with, especially the higher value one's as starters.
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Old 30th Sep 2022, 2:24 pm   #3
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Default Re: Zanussi 20ZA374 PSU squealing

agreed change the three 100uf,s with 105degree ones and also replace r404 270k ,i wouldnt bother testing them just change them.
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Old 2nd Oct 2022, 12:11 pm   #4
andrewn
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Default Re: Zanussi 20ZA374 PSU squealing

Thanks for the advice. I've replaced those three 100mfd capacitors and that seems to have done the trick. It must be critical that the performance of them is good. I tested the old ones again after removal and they measured slightly higher in capacitance than the replacements but the ESR was low. The 270K resistor is a 2W type which I haven't got in stock but I measured it and it was fine. No signs of overheating. I'll put the TV back in use and see how it goes.
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Old 2nd Oct 2022, 11:11 pm   #5
Red to black
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Default Re: Zanussi 20ZA374 PSU squealing

Don't forget the 1uf C418 as this smooths the negative feedback voltage that sets the HT
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Old 7th Oct 2022, 9:41 pm   #6
andrewn
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Default Re: Zanussi 20ZA374 PSU squealing

I spoke too soon about this! The fault returned. I've now replaced the 1uF, 100uF mains reservoir and the 270K resistor. After doing that it worked perfectly. The following day the fault has returned! It wasn't all wasted time as the picture quality seems to have improved. It used to have a slight rippling interference on vertical lines which has now gone. The main problem with working on it is the pcb pads lifting.
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