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

[dl7oab]

Hello all.

I repaired this type of PS of my ZVC 5 years ago.
Back then, I replaced two leaky electrolytic capacitors and cleaned the pcb.
Unfortunately I didn't replace all of them.
Now it starts to fail again sometimes and I want to replace all electrolytic capacitors.
As so often it seems that they are a also weak point in this PS.

Does anybody have a list of electrolytic capacitors with capacity and voltage values, so that I can buy them before take my ZVC out of service and disassembling the PS?

Regards Matthias

[Seiji53]

Hello. - Hello.
Attached is a picture of the electrolytic condenser information mounted on the power supply board.
There are two types of power supply boards. One with an electrolytic condenser of 10uF/50V and the other with 47uF/50V. (The part described on line 2)
We recommend replacing all electrolytic condensers on this board except the 470uF/630V. The total number of capacitors is 40.
The defective LM358 is caused by a leaking electrolytic capacitor (220uF/25V) attached to the sub-board above it. Therefore, if the LM358 is defective, the surrounding components should also be checked and replaced.

There was a mistake in the text, and the following is the corrected content.
condenser => Capacitor
47uF/50V=>47uF/63V

[Andi555]

Hello,

I try to repair this PSU as well. I got a signal analyser with completly dead PSU without any blown mains fuse at the socket.
After disassembly there was only one visual dead capacitor on the PFC side. After removing it the Short disapeared.

Without the SMD capacitor the big mains capacitor mounted sideways only charges up to about 19V at regular line voltage -> nothing happens, -> only very small mains current consumption
When I charge the capacitor with an external DC PSU up to 70V the 12V standby voltage comes alive.
It seems to me that the PFC circuit responsible for charging the main filter cap is dead.

What do you guys suggest to continue measuring. I am a bit lost without a schematic and layout.
Shouldn't the main filter cap always charge to line voltage even the PFC is off? The PFC charges the capacitor only higher?   

I realy appreciate any help,  thank you in advance

[Harry_22]

Hi Andi555!

Next week I will find broken PS and have a look for PFC.
ML4821 is responsible for PFC work.
Please find in attachment the connection diagram for PS standalone test.

73
Harry

[Harry_22]

Hello,

Without the SMD capacitor the big mains capacitor mounted sideways only charges up to about 19V at regular line voltage -> nothing happens, -> only very small mains current consumption

Hi!

Even with inactive PFC, the voltage across the output capacitor should be up to peak rectified voltage. But you have significantly less.
Let's look at the example diagram from datasheet to see what could cause this.

ML4821 operates with the current sence resistor. Most likely it overheated and burned out as a result of SMD capacitor short.

Could you find it on PCB and check.

[Harry_22]

I found PS with a similar malfunction. SMD capacitor also burned out.
Unfortunately the customer tried to fix it himself but did it carelessly.

The circuit is a little more complicated (please see the picture). There is a soft start on the IGBT transistor. Check it and both MOSFETs.

[Andi555]

Thank you very much,
I will test the boards with you info the upcoming days.
There is a IGBT / Mosfet ? beside the rectifier which switches the recified DC (Minus) filter cap to Ground (Rectifier) -> Maybe this is some kind of overvoltage protection ?

[Harry_22]

Hi Andi555!

I added soft start resistor to the diagram above. In my PS it burned out and was replaced with 20 Ohms. Through this resistor a smooth charge of the 470uF capacitor is carried out. Then the transistor opens shunting the resistor.
Most likely this resistor burned out.

Check all the parts circled in red.

[Andi555]

Thank you Harry for your detailed description!
One hint: The diode symbol right beside the 6,3A fuse is drawn in the oposite orientation. I checked all the marked parts and indeed the 20Ohm start resistor is burnt to "open". The soldering joint also looks like it heated up for a long time.
For the test setup I shorted the resistor and turned up my variac slowly .... -> The PSU now works!

The burned capacitor is still not in place, maybe it is not realy necessary. Do you know the value? Otherwise I will leave it open.

[Jarek]

Hello
I soldered one like this.

https://pl.farnell.com/murata/grj55dr72j224kwj1l/cap-0-22-f-630v-10-x7r-2220/dp/2218826

[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.

[Andi555]

Hi,
amazing news -> Analyzer is working !
I changed the resistor to a diy fused one (-;  With low melt solder it should open if the resistor gets red hot. I hope not.
There are two new 100nF 630V capacitors as a replacement soldered on.

At the first start the analyzer found no HDD and the BIOS stopped. At the second try and several times later it was no problem, the analyzer was booting well. Date and time was all right. BIOS Battery seems to be still ok. Next step is to clone the HDD for backup and clean / lubricate all the fans. Some sound a bit rough / noisy.

Thank you for the support!

[Harry_22]

Hi Andi555!

Well done.
Congratulations!