Re: My sick 2/10

From: Macmoni <macmoni_at_email.domain.hidden>
Date: Sun, 28 Nov 2004 20:35:59 +0200

Hi Jason,

we have here in europe sunday 8 p.m. now and I have a little more time to type in some hints for your non-working switching power-supply.

You wrote:
> The +5v wire has 0.7v
> The +12v wire has 7.2v
> The -12v line has -4.3v
If you already opened the PSU and checked the voltage with the "trick" to get the PSU working without the Lisa AND there are the same results, than it's definitely the PSU itself. If the voltage is between the normal parameters, you have a shortcut in your Lisa (if, then very likely somewhere in a +5V circuit, but there are many of them in a Lisa...)
> ... my meter takes a few moments to display the voltage on the screen...
okay, doesn't matter.
Let's go on solving the problem now with "engineeric proceeding". Look at the values of the Voltage above. What would you say is the main problem?
Yes, of course, it's the +5V. The value is only 15% of the nominal. The rest is half or 1/3 of the nominal and only caused by a let's say the "shortcut" in the +5V circuit.
So let's put our focus on the +5V Output of the PSU.

0.7 V is exact the voltage, which is decreasing when you send voltage through a Silicium-diode, in a rectifier e.g. :-))) Take a look at your schematic. There is such a rectifier in the low-voltage secondary part of the PSU. It's CR 20, mostly a NSD 30400, SD 231 or an equivalent type. You find that double-diode in front left side of your PSU with output-side looking to you. Have you found it? It's mounted on a 2"x2"x2" cooling Aluminium unit and looks like a power transistor as the both in the middle behind at the primary circuit of the PSU. But CR20 is not a power transistor it's a double-diode in one housing.

This diode is very likely defect. It has a shortcut.

What will be if ONE diode of those two in one housing has a shortcut? The high frequency low level AC voltage from the secondary side of the main coil transformer T3 has to be rectified by CR 20 before it passes coil L4 and is finally smoothened by C24 4700myFarad 6,3V.

BUT one of your diodes has a shortcut. The AC is rectified and immediately shortcut - so you only have 0,7V. This is the (I don't know the technical word in english) we call it retranslated: "diode forward lost voltage"

So take your meter, and make some measures at Ohm-Range. PIN 1 to housing or PIN 2 to housing of CR 20 should be different if there's an shortcut. One measure should lead to 0,00 Ohm, the other measure should lead to a much higher value, in best case "infinite", but very likely something about 1 kOhm. If you measure 0,00 Ohm...

... Then solder the CR 20 out and remake your measures. If the same result, then replace it, or with an equivalent double diode or with two single rectifier diodes with following specific data: +50V minimum 8A minimum. It's not bad, if you take a 10A type, but what ever you install take care of good cooling !

I hope your PSU and at last your Lisa now works again :-)

> Yes, I realize the dangers of high voltages. I do refocus monitors from time
> to time. But I try to avoid high voltage whenever possible ;)
Just give us all in the List a short note - that your Lisa works and you're still alive :-))), okay?

greetings TOM from Bavaria

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Received on 2004-11-28 11:47:55

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