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NEC ProSpeed 386SX

TheLastNinja

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Apr 27, 2023
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Good evening,
I have an issue with a PSU's and I feel I have gone as far as I can without direction or help from someone more experienced.

I'm learning as I go (with electronics) since starting collecting vintage machines, I began with a C64 with a black screen, have done some moding here and there and enjoy making PCB projects for them etc so I have acquired some knowledge but I'm still learning as I come across issues.


I'll try and give important details that may assist in a resolution.

The machine has a detachable PSU block that can be swapped with a battery.

To charge the battery, you remove the PSU from the machine and plug into the wall and connect a small AC cable from PSU to Battery.

The PSU takes AC100-240V and outputs DC20V 2A

When I tested the PSU, the output was only DC .06

The computer appears fine. I have a voltage generator and set it to 20V DC 2A, plugged it into the battery AC cable instead of the PSU and it charged the battery, and the computer turned on.


There appeared to be capacitor residue, no damage to anything but I changed those (330uf 35v 2x 390uf 35v) but they didn't solve the issue unfortunately.

I ordered a replacement NEC AC08F as a guess.

Nothing looks damaged or any signs other than the residue. The owner said it worked the last time they checked not long ago for what that's worth.

Any ideas why it's outputting a tiny DC voltage? fixable?

*** not an important question but if someone has this machine, on boot it says something about CMOS parameter's missing and time before the DOS prompt. When I opened the computer looking for clues on the PSU issue I noticed there was an empty slot for a square chip. Is that just for expansion or should there be a chip there?


Thank you
 

oldpcguy

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Sep 23, 2021
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My guess would be that it needs the capacitors replaced. When things work and then don't work after going into storage that's my first thought. Careful as there are potentially lethal voltages in the power supply.
 

TheLastNinja

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Apr 27, 2023
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25
Hi, that would be a nice easy fix if so (although those 330 and 390 capacitors took some finding). I'll start logging the capacitors and find replacements thank you!
 

TheLastNinja

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I took some close up pictures. Is it fair to say even if a capacitor looks fine it can still be bad?
* these are the 3 I replaced that had residue underneath
# I think the discoloration is from the grey case top. I removed and tested with a TC1 tester and it was ok.

PSU Q2.pngPSU Q3.pngPSU Q4.pngSide Board.pngPSU Q1.png
 

modem7

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Good evening,
Welcome to these forums.

When I tested the PSU, the output was only DC .06
Any ideas why it's outputting a tiny DC voltage?
Some power supplies of this vintage require an adequate load to be connected, otherwise they shut down.
Was there an adequate load connected when you did the measurement?
Or do you know from an information source that adequate loading is not required for this power supply?
 

TheLastNinja

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Hi, thank you!
Unfortuantly no manuals and can't find a service manual online.

Little confused on "load", by load do you mean the input voltage? or that the PSU was connected to the laptop to have a reason to supply power?

For the input I used the supplied cable which is one of those PC/Monitor 3 prong cables to the wall outlet. I tried another cable rated the same incase the cable was faulty and nothing.

If the PSU need to be connected to something. I did try to get it to charge the backup battery but nothing after many hours. The set up to charge the backup battery is WALL OUTLET ->PSU ->MINI AC CABLE -> BATTERY
 

modem7

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Little confused on "load", by load do you mean the input voltage? or that the PSU was connected to the laptop to have a reason to supply power?
"Load" is the technical term for 'something connected up to the output'.

And for power supplies that require a load, often, not just any load will do. For example, the fictional power supply model FREDDIE 55 might have a requirement that for operation, a load must be connected that draws at least 4 amps from the +5V line.

So this is where a technical manual hopefully helps, informing one of how to test bits; the testing setup, and expected results.

The PSU takes AC100-240V and outputs DC20V 2A
The "2A" there will be 'up to 2A maximum'.

I have a voltage generator and set it to 20V DC 2A
The "2A" there is a current limit. The voltage generator will supply up to 2A at the selected output voltage. If the 'load' tries to 'draw' more than 2A, the voltage generator will do something to stop that, e.g. by reducing the voltage.

The computer appears fine. I have a voltage generator and set it to 20V DC 2A, plugged it into the battery AC cable instead of the PSU and it charged the battery, and the computer turned on.
The set up to charge the backup battery is WALL OUTLET ->PSU ->MINI AC CABLE -> BATTERY
Odd that a working PSU supplies a DC voltage, but the cable between it and the battery is an "AC" cable. Is that your name for the cable?

And just be careful working in the PSU, even with the AC input disconnected. See [here] for some hazards.
 

modem7

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A possibility? The battery was completely discharged. The PSU is good. With the PSU in, not enough time was given for the battery to be trickle charged to an adequate voltage. The PSU was substituted and that finished off the charging of the battery. The measurement of .06 VDC out of the PSU is a measurement made in an invalid situation.

This is where one hopes to find user manuals, technical manuals, or people who know facts about the particular make-model of computer.
 

modem7

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The machine has a detachable PSU block that can be swapped with a battery.
Okay. I found some photos online at [here].

So a battery is not needed. A good 'PSU block' can be inserted, AC power connected to it, and that will power the computer. Correct ?

The 'PSU block' can be substituted using a battery. Next to the power switch appears to be a connector labelled 'DC IN'. If so, maybe that is another way of charging the battery ???
 

TheLastNinja

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Apr 27, 2023
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Thank you Modem7 for explaining that, that helps a lot!

No AC cable was a mistake on my part, its DC.

That is correct about the blocks. With the PSU block you only need AC connected but can be subbed for a charged battery.

I notice the terminals on the battery differ from the PSU block
PSU
10001
Battery
10110

With your explanation of load, maybe the issue is in the motherboard at that last contact since the battery does not use it? I'll open tonight

Thanks again for your help
 

TheLastNinja

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Apr 27, 2023
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comp.png
DC input and terminals for the battery/PSU



pins.png
PSU and Battery terminals

charging.png

Setup for charging the battery.
My mistake on the AC cable, the battery is charged via that DC in when its connected to the laptop. So the PSU is AC in DC out. DC connected to laptop to charge.
Instead of connecting to the PSU I used another Power source for the 20DC which did allow the computer to boot. So the battery/charging part seems fine.
 

modem7

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With your explanation of load, maybe the issue is in the motherboard at that last contact since the battery does not use it? I'll open tonight
Having a look online, the third contact on the battery may be for the battery informing the charging circuitry in the notebook: "Something is wrong. I am overheating. Stop charging me."
 

modem7

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If we look at the three scenarios below, I am putting my money on the NEC power supply.

An expected valid way of crudely testing the NEC power supply would be to load it using a resistor (or combination of multiple resistors) of an appropriate value. A value that draws 1A (half of the 2A max. rating of the power supply) may be good enough. (20V ÷ 1A = 20 ohms) (1A at 20V = 20W)

Alternatively, hook the opened NEC power supply up per #2 below, and within the power supply, measure the output voltage on the connector providing the +20 Vdc.

1696287055430.png
 

TheLastNinja

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Apr 27, 2023
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I really apricate the detailed suggestions.

I saw your post after taking the laptop apart.

If there are no other testing's I start with yourself and oldpcguy's advice of changing all electro capacitors.

So with the lap top screen off (screen cable, 8 pin power cable and fan cable disconnected) I will hook up as #2 above. Failing anything I'll reattach incase
 
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TheLastNinja

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Joined
Apr 27, 2023
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I attached a multimeter to the DC output poles on the PSU.
I noted its behavior
*started ~ negative 0.094 and stalled for a minute or so
*raised to ~ -0.084..83..82..81.. stalled here too

I wrote the message before this, took maybe a few minutes...

*It was now at 6v and climbing, pretty quickly with a ~.2-.3-.4 up ~.1-.2 down pattern.

*At around 10v a faint hum began.

*climbed to ~20 fairly quick and stayed pretty stable, occasionally dropping to ~18/19 once while dropped to ~16 but quickly climbed back up.

The "AC power" light on the laptop never turned on even at full 20. I'm fairly confident this would light with the battery charging/in use.

Turned off for about 10 minutes

Try #2
started at negative 0.011
-0.012 after 4 minutes
7:25 moved to -0.013
10:20 .. -0.012
12:11 .. -0.011
18:30 .. -0.010

Quite a difference from the first try

EDIT when I turned it off and unplugged it read positive .004 then dropped to .001
 
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