Xerox M20i power supply repair - KA7553

[habaso]

I am trying to repair SPMS based on KA7553.
Schematics (not exactly the same, but almost same usage of KA7553)

SPMS cycling, it produce correct output voltage, than goes down.
Red - output voltage
Blue - gate of MOSFET
Please advice what to check else? Datasheet of KA7553 is really not so informing. I have checked both FB and CS of KA7553 and as I understand datasheet, both of these inputs are lower than threshold voltage of overload or overcurrent shutdown.

[CapLeaker]

Re: Xerox M20i power supply repair - KA7553

Is it doing that cycling with or without a load?

[habaso]

Re: Xerox M20i power supply repair - KA7553

Without load.

BUT I have to write as well, SPMS is more complex than schematics, from 5V it has to produce some other voltages, even high for blade etc. When trying optocoupler I have tested 5V "voltage injection" and there was not problem with that, so I hope there is no shortage inside this other following parts of SPMS.

KA7553 was replaced, all capacitors were replaced as well with optocoupler.

Red 5V voltage line
Blue VCC of KA7553


Red 5V voltage line
Blue FB pin 2 of KA7553


Red 5V voltage line
Blue CS pin 8 of KA7553

[R_J]

Re: Xerox M20i power supply repair - KA7553

What are the voltages? and what is the VCC voltage? Also were there any failed components that were replaced?

The vcc shows 12.6v that is too low.

[habaso]

Re: Xerox M20i power supply repair - KA7553

Capacitors were replaced, but I could not find wrong between them.

KA7553 and optocoupler were replace in attempt to solve problem, but SPMS stays in same state, so I think they were good before.

VCC you can see on picture, 1 oscilloscope "click" is 5V, co VCC cycling up and down between 9V and 16V. I do not know if this VCC cycling is OK a what caused it.

Do you think it can bee caused by zener diode?

[R_J]

Re: Xerox M20i power supply repair - KA7553

The vcc voltage explains why it is operating the way it is, when it reaches 16v the ic starts, when the voltage drops to 9v it goes into under voltage shutdown. Check that the feedback (PVD3 diode in schematic) is ok. check that the transformer winding is not open, check the diode and also any resistor in that circuit. Usually the capacitor (47µf) is bad (open) and does not charge to maintain the vcc voltage.

[habaso]

Re: Xerox M20i power supply repair - KA7553

47uF 35V capacitor was replaced.

Both diodes seems to be OK using multimeter diod testing, but I do not know if zener diod working on correct parameters now as new one.

[habaso]

Re: Xerox M20i power supply repair - KA7553

Both PVD3 and zener diode are open one way and closed other and show around 600mV voltage drop open way using multimeter diode test.

Would you like to see some oscilloscope measured signal i.e. from not vcc side of PVD3 diode?

[R_J]

Re: Xerox M20i power supply repair - KA7553

PR11 & PR12 supply the initial VCC voltage from the main filter capacitor. When the ic starts, the VCC is then supplied by PVD3. It is possible PZD2 or PZD3 is leaky and zenering at a lower voltage. ZD2 seems to be a 20v zener.
the zeners can check good as diodes but might start to conduct at 16v shutting down the vcc

[habaso]

Re: Xerox M20i power supply repair - KA7553

My real circuit is little bit different, there is PVD3 and powering part of circuit, but VCC is directly by zener diode (like PZD3, but without PC32, only with PC12) connected to ground.

I had take out this VCC grounding zener and VCC still cycling between 9V and 16V.

[habaso]

Re: Xerox M20i power supply repair - KA7553

I had tried to help with powering KA7553 externally by voltage injection. With such VCC supported with voltage injection it stays OK, so there is no problem in following circuits, only somewhere in VCC part of KA7553 surroundings.

With VCC voltage injection
Red 5V output
Blue VCC of KA7553



Without VCC voltage injection
Red 5V output
Blue VCC of KA7553

[habaso]

Re: Xerox M20i power supply repair - KA7553

Changed PVD3 diode (which is 1A SUPER FAST RECOVERY RECTIFIER SF10-02) and SPMS seems to be OK, problem solved, tomorrow morning I will try in printer and will see.
Thanks for help