Imac G5 PSU no power to 3845B

[wibble]

Hi I've got an iMac PSU - it's an Acbel API4PC47 from an iMac G5. It had the old burst capacitors. I've replaced all secondary side capacitors and also a 25v 47uf near the transformer T2, and also transistor Q1.

The power supply is not starting up. With jumper inserted, just the 24V & 5VSB are present. I found with the voltmeter that from pin 5 (GND) to pin 7 (VCC) on the PWM chip (3845B) is only 2V. Whereas I believe the chip needs 5V or 12V to run (going by the spec sheet). Would anyone know how the 3845B part of the circuit works and if there is something like a common fault, or could shed any light on what might be causing this?

Thank you for the consideration.

[Toasty]

Re: Imac G5 PSU no power to 3845B

What brand/series caps did you use?

Was Q1 visibly damaged?

The Emitter of Q1 is tied to pin 7 (Vcc) of M2 (3845B).

Off - Q1: [B]B[/B]= 4 [B]C[/B]= 25 [B]E[/B]= 3.5
On - Q1: [B]B[/B]= 15.9 [B]C[/B]= 17.3 [B]E[/B]= 15.2

If when off, Q1 has the proper (or close) base and collector voltages, and when on it does not, then M2 (3845B)is likely bad. However, check that diode D2 and zener ZD6 are not shorted.

Toast

[wibble]

Re: Imac G5 PSU no power to 3845B

Hi thank you for the response I have just tested the power supply unjumpered, i.e. off and the reading I get for Q1 is 16V at the base, 2.76V at collector, 2.22V at emitter.

Ther don't appear to be any shorts on the diodes or zener diodes, but I am measuring them in circuit. This is a bit strange, could the PWM chip be faulty, or would it be something else up with the voltages.

Regards

[Toasty]

Re: Imac G5 PSU no power to 3845B

You may have damaged Q1 when you did the replacement. The base voltage is very high for the off state.

Just to confirm, top down on Q1 legs:
Lower left is Base
Top is Collector
Lower right is Emitter

Again:
What brand/series caps did you use?
Was Q1 visibly damaged when you did the initial replacement?

Toast

[Toasty]

Re: Imac G5 PSU no power to 3845B

*bump*

[DukeFawks]

Re: Imac G5 PSU no power to 3845B

Off - Q1: [B]B[/B]= 4 [B]C[/B]= 25 [B]E[/B]= 3.5
On - Q1: [B]B[/B]= 15.9 [B]C[/B]= 17.3 [B]E[/B]= 15.2

Toast

I've been messing with these old junkers for a bit. Q1 keeps on dying after a few days/weeks/months. I've done 10 or so of them, all the same issue again.
In your "on" case there is a 2V drop over CE and with 16V at the base that transistor should be saturated. It means that Q1 is dissipating a lot of power and will soon die again.
The 16V zener at the base will limit base voltage, D2 (E to B) will limit emitter voltage at 16V+0.6V. So a voltage higher than 16.5V at the collector will start to heat up Q1 and the zener.
I have replaced all the caps in the stb. circuit and 5V and 20V look nice and stable. I have about 22V coming through D18, but I cannot understand how they want to control this voltage. I also have a few of these PSUs that have a series resistor in with D18 as an attempt for them to drop some voltage. This is a factory mod and it seems they also had issues with this. My feeling says the transformer is actually leaking and causing the voltage to go high because the only way to control this voltage is through the transformer winding ratio.
Any other ideas what is causing the high voltage at D18?

[Toasty]

Re: Imac G5 PSU no power to 3845B

>>It means that Q1 is dissipating a lot of power and will soon die again.<<

I've not found that to be the case. Are you replacing the Zener ZD6?

What part number are you replacing Q1 with?

>>The 16V zener at the base will limit base voltage<<

?? What 16v zener at the base?

Q1 pins - top down view
Lower left = base
Top middle = collector
Lower right = emitter

>>I have replaced all the caps in the stb. circuit and 5V and 20V look nice and stable.<<

5v and 19-21v at standby, 5v and 24v at run, yes?

>>I also have a few of these PSUs that have a series resistor in with D18 as an attempt for them to drop some voltage. <<

I don't believe that's it's purpose. What's the resistor's value?

22v actually seems low. I've seen 25v there.

ALL of the voltage from the transformer is a result of the TOP245Y. You have 3 secondary windings on that transformer. Make sure you are reading voltages from the correct supply. C37 is for the 3845B circuit.

T