Electronics > Repair

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

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dh5wm:
Hi Jens,

The PSU of my FSEA is also broken a few days ago.
I lost the +5.5V on X21.1/2. The PSU switches off after a few seconds.

The capacitors seems to be ok, but I will change all the electrolytics. 
At the moment I'm wondering because I can't follow the 5.5V output trace on the pcb backwards after a larger inductance.

Maybe we can exchange some of our experiences during the repair. That would be helpful.

Regards Markus


sm7ovk:
Hello Markus,

I'll be happy to share my information but I don't think we have exactly the same fault, or mine is more faulty than yours.
Just be careful when you change any components. I think it's a 4-layer PCB and it's easy to break the lines inside the PCB...
I have changed all electrolytics on my board and a check with RLC-meter didn't show any that was really bad. Seen to the risk to damage the board when replacing, it might not be the first thing to do - if there isn't any obvious faulty/thick caps.
The unit is a real pain to get in and out from the chassis...

I get the standby-power of 12V at X23.5 and this is the only voltage that really works. I have 6V to the fans and they spinn when I start.
My 5.2V at X23.1 is only up to 4.1V and then the PSU shots off, (and fans don't spinn). If I load with 2.5ohm I only get 0.5V. (Load with 100ohm at the same time on 15V at X21.5.)

Hard to follow the wires... check the voltage on UC3845 at pin 8. There are 3 of them and pin 8 should be 5V. (a voltage-reference and one of my ICs was bad)

I have now changed a double diode, (with the PCB on the table and fans pointing from you, it's the one at the right hand side and the outerside of the small cooler that the right fan is blowing through.). I thought I've checked it earlier, but perhaps not.
I now have one more that might be faulty. Could you check the doubble diode, (on my attached picture it's the second from the main transformer on the left hand side of the board (towards the cooling). It's a BYV72EW-200 and with a ohm meter I get about 7.95ohm between the pins and with diode-test more or less 0V. Could be a transfomer but could also be another faulty shottkydiode...?
Please note that I don't have any schematic so the marked things in the attached picture is what I assume/think is right.

Regards Jens

dh5wm:
Hello Jens,

My power supply is working again!
It wasn't a big thing. I changed the LM358 on the little plug-in pcb as shown in the picture from OH2LIY in the first post here.
Now the psu starts and does not switch off after a few seconds. All voltages are back.

However, I still have strong ripple on the +/- 15V lines. I will change the 220uF as Martin reported above. Thank you for your advice, I will only swap the necessary capacitors.

I checked the double diode BYV72: With diode-test I measured about 0.32V in forward direction and "OL" in reverse direction. The outer pins are connected together. Probably the DC output.
You should replace the diode first!

Thank for you picture with the labels! I know it is reverse engineering but it's very useful for further repairs.
While I'm waiting for the capacitors, I can do some more measurements for you. Let me know if you need anything.

Best regards
Markus


sm7ovk:
Congratulations Markus!

I've been out traveling and will not have any time for this repair until mid February. I'll order some BYV72 and test to exchange it. Many thanks for your information.
What voltage do you have to the fans when you start the PSU? You can easily measure this on the upside-down PCB at the connector.
Do you load it in some way when you start up the PSU, as adviced earlier in this thread?

73 Jens

av500:
When suddenly the fan noise in the lab goes down, you know something is wrong. My FSEB just shut down and at first I thought it overheated since it sits in a corner.... so I blew a large fan at it for a while and tried to restart, but nada. It turns on, starts to boot and then shuts down 5 seconds later. Top and bottom covers were off anyway since I'm hunting another error, so I just flipped in on the side and started measuring the voltage rails on the bottom main board. All were good except the 5.5V analog which starts at 5.5V and then rapidly drops to some ~3V when the supervisor circuit has enough and shuts down.

So, out comes the PSU which by now I can do in the dark with my hands tied behind my back :)

Taking off the top cover I do smell something "electrical", but nothing is obvious. Thus I hooked up my PSU test jig, preloaded the rails as per the service manual and tried again - same thing, the analog 5.5V drops and it shuts down.

Since this was before I found this thread I had to do my own staring at the PCB and following the traces but eventually I found out that the little add-on PCB is the DC/DC for the 5.5V analog. And it did smell a bit "burned", mainly the inductor which showed some signs on overheating. At first I thought this was a transformer, but it has only 2 connections and measures as ~30uH, so I am hoping it is still good. Next I desoldered the MOSFET and the dual diode, but these also seem OK. The inductor that got too hot is right after the diode rectifier, so if it got hot something must be drawing too much current to ground. So out came the three 1000uF/10V caps and lo and behold what is there on the PCB? 

After some scraping and cleaning, the resistance from 5.5V to GND went back from the Ohm range to MOhms. The PCB is pretty much eaten away around the via for the positive cap lead. Luckily ground is on one side and the 5.5V is on the other, so a repair should be fairly easy. Ordered new caps and UV curable solder mask, I'll be back with hopefully good new when these get here...

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