Electronics > Repair

Fixing Rohde & Schwarz FSIG (FSEA, FSIQ...) Ascom power supply

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Harry_22:
Hi Andi555!

I corrected the drawing.

The similar SMD capacitor on the bottom left has 0.15uF.
Use the one suggested by Jarek.

Could you make a photo of unsoldered soft start resistor to find a replacement.

Andi555:
Hi Harry,
thats the charge resistor on the primary side of PFC. No visual burn marks, but "open".

Harry_22:
This is Vitrohm fusable resistor.
https://www.vitrohm.com/content/files/vitrohm-ktf-wirewound-fusible-resistor-datasheet.pdf

You can use the following A5MC
https://www.uchihashi.co.jp/en/fuse/a5mc/

Andi555:
These special resistors seem to be very hard to buy from a "normal" supplier for low quantities.
 Its a bit risky to replace it with a regular 5W ceramic power resistor and rely on the mains fuse...

Harry_22:
Don't be afraid to install a 5W resistor without a fuse.
Your resistor was burned out abnormally. Its jumper didn't unsolder during long heating as it was intended, but the coil instantly burned out due to a short circuit.
This is because it has thermal protection against breakage of the IGBT transistor. In this case, it turns out to be connected in series with the PS and a lot of power begins to be released on it.

Below you can see the diagram of this unit from another R&S serial power supply. There a resistor without protection is used. It burns out when the PFC key is broken.
For fun I disassembled both resistors.

You can find fused resistor on Aliexpress. It is original.

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