Repair of PSU of dumpster dive PABX :-)

[on4top]

Today i got a nice PABX from Ascom/Astel.
Doesn't power up .. checked fuse blowen ofcourse .
Replace fuse = instantly blowen again  .

Removed the mainboard ..unfourtuanly  PSU is integraded in PABX itself . ( so had to remove the complete board .. )

After some testing came to following conclusions :

1- Mains caps are ok  ( 2x 220uf   200V in series = 100uf  - tested esr & capacitance all ok )
2 -Bridige rectifier seems fine with diode testing on my fluke 177 .
3 - lots of voltage suppresor diodes are on the board ( i think GHR printed on them ) .. found 1 piece that measured 3 ohm in both directions..( removed it from board ).
4 - the main power transitor didn't measure anything .. removed the headsink ..it fell appart in 2 halfs ..
      It's  TOP224Y -     http://www.mouser.com/ds/2/328/power%20integrations_top221-227-327916.pdf
      Removed it from the board - have no replacement .. but replacing has no use for now .. fuse still blows ( and i guess a new top224y also ) .
5  the opocoupler driving the TOP224Y seems also ok ( no shorts )   type = CNY-17-4

Don't know where te look further in to it now    Can't seem the find any service manual online ...

[nctnico]

Download the application note for the Topswitch device and you'll have a schematic for the power supply. These kind of power supplies are not very difficult to repair. I did come across one which had a bad optocoupler as well. I guess the rectifier diodes are fried and you will want to replace that suppressor diode as well. It is likely this is there to dissipate the high voltage spike which comes back from the transformer.

[on4top]

Hi thanks for your repsonse .

I indeed downloaded the datasheet of the TOP224 now .
Powered the board now with a light bulb in series of 80W and it doesn't glow and the fuse doesn"t blow .
Measured 238V on the ac side of bridge  and +300V on the dc side .. alnormal i guess .
Then i measured on the Drain & Source pad of the PCB also the DC voltage .

After these tests i powered w/o the light bulb .. and seems to work now w/o blowing the fuse .. ( weird ..after i removed the top224 and tvd diode it still blowed fuse )

Guess i wil order a new TOP224Y and TVS diode on monday .. and maybe i replace the opcoupler and bridge rectifier also ( altough i think they are fine ).
This device must have had some overvoltage i guess .. i hope nothing is broken on the secundary side .

 

[Gyro]

In Topswitch supplies, drying out of the auto-restart/compensation capacitor (C5 in their application circuit) is often the cause of not starting up. It's a small can device and normally placed very close to the hot switcher so it tends to dry out over time. Replace with a 125'C rated part.

P.S. In your photo it looks like an smd tantalum. Worth replacing anyway.

[SeanB]

Not likely the secondary side is damaged, unless it was hit with lightning, in which case you might as well strip it for scrap, as it will have all the input and output semiconductors ( diodes as well, not only the line interface chips) either blown short or open, or very leaky.

[on4top]

yes there is  47uF tantalium close to the TOP124Y .

But after further investigation .. i found a short on the SMPS transformer .
A diode ( i think ) that measures shorts in both ways .. doesn't look good ..
I could not measure the Capacitors in the secondary . ( 2x330uf in series ) ..
Remove that diod ande now i can measure them +/- 600uf  looks ok .

The diode is from ST Micro and has written on  D20 ( no idea what kind of diode it  is ? )
After removing the diode i also noticed  is noticed a cracked transitor ( i'm not sure .. packages looks like a transitor sot23 )
Cant read the type on it .. but it looks like others on the board somethink JSS2 of  starting with JS ... any ideas  ?  ( wish i had a schematic or service manual )

So i'm now thinking is it worth the effort trying to get i working i again  ?..  i don't mind spending  +/-10-15 euro for parts with the risk it's not repairable .
And my time ... it's a hobby ..

Could indeed be hit by a lightning  maybe .. ( i have no idea )

[on4top]

Edit :  i think the sot23 is a swichtging diode ..

http://www.s-manuals.com/smd/js

should be  BAS21LT1 i think ..

[on4top]

anyone knows what diode this is ?

is an ST Microelctronics with D20 written on it . and  SOD123 package i think ?

could it be a skotchky ?

http://be.farnell.com/stmicroelectronics/stps3h100uf/schottky-rectifier-3a-100v-do/dp/2353679

This site says zener ?

http://www.s-manuals.com/smd/d2

[on4top]

Ok finaly found the type of the diode ..

https://octopart.com/smbyw04-200-stmicroelectronics-324289

Seems to be a  SMBYW04-200 rather old type .. farnell does not sell them ..

But found a similar type that has same specs :

http://be.farnell.com/stmicroelectronics/stth4r02s/diode-rectifier-ultrafast-4a-smc/dp/1697525

[EPTech]

Hi There,

Also check the varistors (MOVs). If there has been a serious over voltage. They may have shorted out, still causing the fuse to blow even though you have interrupted the power path by removing the TOPSwitch, which is not a power transistor but an IC by the way. The MOV's should have "infinite" resistance.

Good luck.

[on4top]

Thanks for the tip ..

I actually desolderd al the "moves" and mesured them .. all good .

Progress so far  :

Replaced  :   

Top224  ( broken )
3 TVS diodes ( SMCJ170A )
1 bridge rectifier ( was still ok i guess )
1 47uf Tantalium ( close the top224 )  wasn't dead
1 BAS21LT3G   ( was broken in secundairy .. cracked )
1 CNY17-4 ( opcoupler ..)
1 BYW4200B ( diode accros secundairy caps .. shorted  ! )

Now it's partitaly alive .. the 3V rail seems ok .. other rails are to low .. and the sound of the inductor or smsp  sounds not 'normal"

If i measure before smsp is measure 80volts .. on the out put ( across capacitors ) only  16 volts dc .. ( capacitors are rated 63 volt and in series .. so i think voltage should be higher .. )
I think this is to low ..other voltages required higher ..

VCC3
VCC5
VCC20
VEE65
VEE40

It's getting compilcated now    don't know where to look now.
Especially w/o schematic ..

[EPTech]

Hi there,

The 80V you are measuring is not DC. It is the average DC voltage of the PWM that the TOP switch is putting on the transformer primary. Some audible noise is possible but excessive sound may be due to the SMPS doing too much effort. The voltages on the secondary side being too low and one voltage being correct is also an indicator that the supply working but is being overloaded by the subsequent circuits. My guess is the 3V is the regulated voltage, which is fed back to the TOP switch via the optocoupler. So you did a great job so far. If the supply is overloaded there are bound to be components that er getting hot. That is the next thing to check. Like you said, it becomes more complicated from here because everybody on the secondary side is now a suspect, which is... all the rest of the board. 

Good luck.

[on4top]

thanks for the input ..makes indeed sense ..

I'm more famillair with Linear power supplys ... easyer to debug

I probably would see more with the scope ..but don't  want to blow it up using it on mains power rails