Electronics > Repair
Fixing Rohde & Schwarz FSIG (FSEA, FSIQ...) Ascom power supply
av500:
looks to me like +15V and +5.2V appear briefly, then go out?
mpcengineer:
Yes but you can see the 15V is heading towards to 20V which is tripping the PSU.
I have remotely power each UC3845 device and checked the reference voltage and they all show 5V.
Any thoughts greatly appreciated.
MPC.
mpcengineer:
My findings so far....
Using the picture attached. The RED circled devices all run from the 12V standby. The yellow circled device runs from a special 25V supply which is developed using the 15V supply with a 10V shunt regulator on top to make 25V. The 2K2 provides the shunt resistance from the main 45Vdc.
The yellow circled device is powered from this 25V supply and also appears to be used to check for under/over voltage on the 15V supply.
For this PSU unit the TL431 has gone open circuit. Not sure that this is causing the 15V over voltage error but I hope to replace the TL431 and test.
Does anyone know if you can run the PSU without the auxiliary boards?
MPC.
mpcengineer:
Finally working .....
In the end the original TL431 was working ok.
The PSU unit is now working. It must have been a dry joint because soldering wire probes onto the PCB to test the voltage/current error amplifier circuit appears to have fixed the issue. The unit started up okay. This would make sense as the PSU would start if left on for several minutes.
For future reference I have update the photo with the circuit blocks identified. Also attached is a partial circuit diagram of the 15V DC/DC converter and the oscillogram of the units start-up.
Green is the 15V Supply
Yellow is the 15V into pin 3 LM358
Purple is pin 7 of the LM358 error amp
Blue is pin 1 of the LM358 error amp
MPC
mpcengineer:
AND finally.....
The unit failed start-up the next morning.
The first oscillogram shows the 15V is overshooting causing the PSU to shutdown.
Green is the 15V Supply
Yellow is the 15V into pin 3 LM358
Pink is pin 7 of the LM358 error amp
Blue is pin 1 of the LM358 error amp
The TL431 10V fail oscillogram shows the green trace 45/50V supply working which is connected to the top end of the 2K2 shunt resistor
The Blue trace is the cathode of the TL431 which should regulate to 10V above the 15 supply. Here you can this is overshooting the 25V and reaching 30V
The Purple/yellow trace is the 15V. The difference between the Blue and Pink should be 10V but this overshoots. This suggests the TL431 shunt is not regulating.
Removing the TL431 and some parts around the TL431 to isolate the reference input track into the TL431 showed a resistance. On the top of the PCB around the TL431 there was a glossy layer of something which looked like lacquer. Also a copper track running close to the TL431 was black.
The PCB photo shows a conductive coating over the PCB around the TL431. The corrosive liquid looks to have leaked underneath the solder resist and caused the copper track to corrode to black. This conductive layer had leaked onto the TL431 refence input and caused the TL431 to fail.
The final photo shows the leak and black copper corroded track.
Cleaning up the top surface and replacing the three caps close to the TL431 has now fixed the issue. The problem could have been solved just trough observation!
TL431 10V fail photo.
Green is the 45/50V Supply
Yellow/Pink 15V supply
Blue is the 25V supply reaching 30V because the TL431 10V shunt has failed.
Interesting link https://electronics.stackexchange.com/questions/102048/what-effects-might-an-electrolyte-leak-have-on-nearby-electronic-components
MPC.
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