FT-990 Switching power supply failure and repair

[ffranzini]

Hi
I want to share my experience about the failure on my FT-990 power supply, the failure of the power supply has been done because i forgot to switch the voltage selection to 200V from 100V  , after i powered on the equipment in few seconds i heard a loud bang.....
After initial inspection i found fuses to replace and, most important C8, C9 (please see attached schematic), R1, R2  and Q1 (Triac) blown... what shocked me was the status of the capacitors, i found them with the external can pierced by some inside arching (my guess).
After the replacement of the components, including the bridge and the IGBT's with equivalent type the power supply started to work fine but randomly R1 burned up again only during power on  , i scratched my head to understand why because i didn't found anything wrong, everything was working right, i replaced R1 couple of times with no luck, randomly R1 burned up again during power on  .
Examining the schematic my understanding of the R1 function is to limit the inrush current during power on and my best guess on R1 failure is related to the very low ESR of the new C8 and C9, right now i am using for R1 38 ohm instead of 10 ohm and (so far) everything seems to work.

Please note that the schematic have some error on it, for example the gate of T1 is connected to C7/R2 and T1 of Q1 is connected to R1

I very appreciate any advice on my findings or for a better fix.

Thank you

Francesco

[Mick B]

Perhaps you could put an NTC Thermistor at the input to suppress the the inrush they work really well and will save your bacon
Mouser part number 954-102D-18LC this one is 10ohm 6A for instance. $2.50 USD   

[dl6lr]

My service manual has the same schematic with the weird connection of Q1.
As soon as the converter "sings", the resistor is bridged by Q1, so the value of the resistor limits the inrush current only. If it burns during inrush, Q1 will not help (if the converter does not start, the current is nearly zero after the caps are charged).
I assume you are already using a wirewound type? If it works with the higher resistor value, I would leave it as is. Otherwise only a higher rating of the resistor could help.

[ffranzini]

Thank you both for your advice.
Regarding the R1 type i tried both wirewound and metal oxide resistor, i had both type blown by current spike during power on, this has been confirmed by the "autopsy" of the wirewound resistor, i have found no trace of heat at all but melted wire on the connection point to the endcap, on the metal oxide type i got a "speck" on the outrside and the resistor interrupted.
Since i am not able to find any information on the R1 mfg code A53K10J-10 described as fuse resistor i assumed that it was a normal resistor, may be it was an inrush limiter instead, then  NTC thermistor may be a  good option.
In the meantime i keep crossing my fingers every time i push the button to power on my FT-990.

[ffranzini]

To close the issue

The solution to increase the value of R1 to reduce inrush current during power-on worked out without any downside.

Thank you

Francesco

[m3vuv]

its another crappy yaesu,what do you expect?,yaesu rigs always seem underbuilt as far as im concerned.

[ffranzini]

The FT-990 is the only Yaesu rig i have, from my limited experience i have the same feeling, quality of design and construction are not very good, I can tell you that, on this rig, to make the tuner working on all band i had to modify the original circuit, is really weird how commercial rig are built today (last 30 years).

Thank you

IK2BIY Francesco

[m3vuv]

ive had a couple of yaesu rigs,first was an ft-290,agc issues caused by cheap conductive paper phenolic pcb,2nd was an ft-840,that blew the pa,looked at the specs of the finals and saw there was no margin,my kenwood finals are rated at %200 of the output power,i would never buy another yaesu rig,poor quality crap as far as i am concerned!,73 de m3vuv.