Saved Old Pc

[Gianni]

I have repaired an old Olivetti PC using a MSI MS-6178 MOBO with Intel Pentium IIIE, 667 MHz .

To turn it on, you had to wait 20 minutes after you plugged the power cable because there was a problem with a signal transistor on PS.
I had to add a 10K resistor between Base and Emitter because the Base was floating and the transistor stayed on for about 20 minutes blocking the PS.
It took me 5 hours to find out the problem but I couldn't give up because this PS is not standard but it is small and I don't have a replacement for it.
In the picture you can see a comparison between it and an Allied ATX PS.

Moreover on the PS there were 3 problems:

1) the ST-BY voltage was out of control at 5.6V, I have changed the feedback zener connected to the integrated power switch, it was a 24V and I have used a 18V in series with 4.7V to lower voltage at 5.1V,
2) a protection zener on 5V ST-BY was overheating because the voltage was out of control; the zener has burnt PCB and 2 caps;
3) some caps were completely opened.

On the MOBO there were some CHHSI caps bulged.
I have replaced all caps on MOBO and PS with Panasonic FM, I have installed 256MB RAM, it was only 64MB, and now the PC is working perfectly.

Here attached you can see the pictures with badcpas and after recapping.
Ok it is not a fantastic PC, it is slow and old, but it is good enough for the person is using it.

Ciao
Gianni

[Per Hansson]

Re: Saved Old Pc

Wow, very impressive work there!
If only I was so good at troubleshooting maybe I could have fixed my Dell 550W PSU.

[zandrax]

Re: Saved Old Pc

Good job, Gianni.
The motherboard was cheap stuff at the time: the i810 was the first of Intel 800 chipset line and had two weakness, the slow and always on integrated graphic and the southbridge with bandwith issues. Not a great product, but if works then keep it running: Fujitsu-Siemens used it in some barebones, you can find the manual on the Tech support page.

Zandrax

[Gianni]

Re: Saved Old Pc

Hi Per Hansson,

I've been working on PS since I was 16teen.
I have to add I have good quality equipment at work and I can use it.
In this case I had to repair this PSU because I don't know if I were able to find a replacement due to the small size; I could save 4 hours trashing the PSU and just take a new CABINET and one of the spare PSU I have repaired but I have preferred to fight til the end... and this time I succeeded.
What I really didn't understand is why they left a transistor base connected to a 1N4148 cathode without any base-emitter resistor, with spikes and noise it was very close to 0.5V and at 0.6 it turned off the PS.
I have seen that after 15~20 minutes the PS was connected to the line, the base voltage decreased at 0.5~0.4V, the transistor went off and the optocoupler left the PS free to work.
The 1N4148 anode was connected to resistor divider and green cable for power on.
I checked the transistor and it was good... so I add a 10K between B-E and the problem was solved.

@ zandrax: this time I was a little be lucky than the time I worked on Via MOBO. Thanks for infos

Ciao
Gianni