Re: 9920U - power supply repair - thyristor

Jack Rubin

Yes, the part is clearly an NTC inrush current (surge) limiter - as Paul notes, it is in series with one leg of the AC input, located between the line fuse and one leg of the line filter. The power supply is a Boschert XL220 but I haven't been able to find specifications for that model online. The closest thing I could find was an XL25, copyright 1979, so very much in the same time frame as this supply, which is marked with a 1982 copyright on the circuit board. Here is the Maintenance Manual for the XL25 - 

The XL25 has a similarly placed thyristor (called out as TR1 on the prints) just after the input fuse and before the mains filter caps. The XL25 is a lower capacity supply (115V/25W) with a 1.5A fuse rather than the 5A fuse in the XL220 (115V/135W). TR1 for the XL25 is rated at 50 ohms, so I wonder if 10 ohms for the XL220 might be too low? Is there a downside to going with a higher value?


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