I have problems with an ASTEC AA13780 SMPSU from 1988 in an old Mac II, basically the output voltage is a tad low causing the computer to fail, the first clue was the buzzing sound in the audio loudspeakers suggesting the caps are on their way out. So the plan is to replace the caps, easy thing to do, but I wonder if it is possible to adjust the output voltage a tad. There seems not to be a preset (variable resistor) so I assume the circuit uses a zener or some other reference.
I guess the answer is no, but anyone have a circuit diagram? Or know how the PSU gets a reference.
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Re: Bestec....the next generation

Firstly, I'm not sure if this thread is closed etc. anyway. I have a Bestec ATX-300-12Z Rev: CCR which works except, there is noise on the computer screen that seems to be worst at cold startup and when current is being drawn. That is, when the HD gets action, also I can hear "mouse zip" when I move the mouse I can "hear" the data flow through the speakers in the monitor.

Once the computer has been on for 30 mins, all settles down and everything seems OK.

All this sounds like the output caps in the...

Re: AcBel 344W SMPSU repair clues and suggestions

I just remembered, which might be a clue. The PSU goes into "chirp mode" from the instant on plugging into the mains (240V). The "Power On" signal to the PSU is open circuit (floating) as I have the unit on the bench.

Re: The PSU FAQ

Not sure if to mention this here, but it is interesting to note most of the computer SMPSUs come from Taiwan/China. I can see the economics of manufacture of the far east but do they do the design and development too?

I'm in the UK and trying to design/manufacture in this country is usually not a good idea, just the usual costs of.

fried Apple power supply

There's me happily using my old Apple PowerBook 1400, then the next time I come to use it, nothing. So I open up the little brown brick (the official power supply) and was amazed to discover the destruction shown in the pik. I have no idea what caused it....

I'm trying to fix an AcBel AP10PC24 - power supply, 344W from a Mac G4 circa 2002 (has PFC). Basically on power up the thing does the "chirp-chirp" thing as we know, it is trying to startup but its control circuit knows there is a problem. A resistor smokes which looks to be the current monitor for the main MOSFET. I have removed the PSU from the Mac and use a dummy load.

So far, I have tested all the power semis (removal), watty resistors, the caps look OK, so now I'm down to the control chips, ML4800CP (does the PFC and PWM) and a Weltrend WT7515 power monitor chip....