power supply output only 7.7v instead 24v

[buchman]

Hi guys,

Picked up a pretty new printer on the street and trying to solve the output voltage issue. It is showing me only 7.7v instead of 24v.

All capacitors were replaced.
ic131 was replaced.
tested all components on the top side and seems to be ok.

What is the component on the bottom side which is labeled '2x>'?
I have 2 of them on the bottom side and I suspect that one of them is damaged.

Photos attached.

Thanks a lot in advance.

[R_J]

Re: power supply output only 7.7v instead 24v

MMBT4401 sot23 transistor marked 2X
I believe you need to have 3 volts (power supply on) on CN1, pin 2, I think the power supply is in standby with only 8 volts out
Q131 is the transistor that turns on the power supply, taking out of standby

[budm]

Re: power supply output only 7.7v instead 24v

mmbt4401 sot23 transistor marked 2x
i believe you need to have 3 volts (power supply on) on cn1, pin 2, i think the power supply is in standby with only 8 volts out
q131 is the transistor that turns on the power supply, taking out of standby

+1.

[buchman]

Re: power supply output only 7.7v instead 24v

Q131 is the transistor that turns on the power supply, taking out of standby

You mean ic131?
If so, I have replaced it with a new one and it didn't help.
It's a TL431 ic and I replaced it with 431A, the seller told me that it's the same.

[R_J]

Re: power supply output only 7.7v instead 24v

No, I ment Q131, we are lookling at the same picture correct? IC131 IS a tl431 reference ic.
This power supply is not ALWAYS ON, without the power on command it is in STANDBY

[buchman]

Re: power supply output only 7.7v instead 24v

This power supply is not ALWAYS ON, without the power on command it is in STANDBY

How did you know that?
Is it because you are familiar with that kind of SMPS or it's because you figured it out on the pcb layout?

I am asking this because I want to learn to figure it out next time myself.

[Agent24]

Re: power supply output only 7.7v instead 24v

How did you know that?
Is it because you are familiar with that kind of SMPS or it's because you figured it out on the pcb layout?

I am asking this because I want to learn to figure it out next time myself.

I would say by looking at the design. That centre pin on the output connector goes over to the feedback circuitry and seems to act as a crude standby feature, by tricking the PSU into thinking it's running at higher voltage than it is.
Then it reduces the voltage and ends up at 8v instead of 24v. When the printer sends the power-up signal, that standby circuit turns off and the PSU realises it's actually only at 8v, so then begins running at 24v proper.

That's my quick guess anyway, I haven't traced out the circuit. But normally for PSUs with standby like in computers or TV etc, there is a completely separate optoisolator etc for the standby signal, and the primary controller has a dedicated pin for that signal. This PSU only has one opto so it can't be doing that.

[R_J]

Re: power supply output only 7.7v instead 24v

Most power supplies these days operate in a standby mode when the main unit is off, There are power supplies that will operate in burst mode or skip mode, when in standby, basically just giving bursts of the high frequency to the transformer, the result is a much lower voltage but in most cases it's enough to supply a standby regulator for 5 0r 3.3 volts
Then when the unit is turned on,the power supply runs full and develops the higher voltage to run everything else.
You can see that pin 2 of the plug drives the base of a transistor, this transistor turns on the optocoupler telling the primary to run full on.
I believe this power supply uses a DAP008 (DAP08)

[budm]

Re: power supply output only 7.7v instead 24v

The output can be changed to go to higher output Voltage by changing the feedback Voltage that feed the Ref pin of the 431 like the way Philips used in their TV PS, in standby the 13V and 21V are only puts out about half of teh rated Voltage, then P-ON is applied then they go up to 13V and 21V as needed.