can you put some kind of dummy load on it and re measure?

When you say goes up and down over what time frame is that? To me it sounds like a voltage reference is going all over the place so a faulty ground trace or a zener on it's way out.

maxtherabbit wrote on 2020-06-04, 18:41:

can you put some kind of dummy load on it and re measure?

Like attaching a Hard Drive ?? done that.. same result

evasive wrote on 2020-06-04, 19:58:

When you say goes up and down over what time frame is that? To me it sounds like a voltage reference is going all over the place so a faulty ground trace or a zener on it's way out.

it's lihe less than half a second.. jumping up down... I can see and ear the power supply fan accelarating and desacelarranting when the voltage is low or high

nuno14272 wrote on 2020-06-04, 20:38:

it's lihe less than half a second.. jumping up down... I can see and ear the power supply fan accelarating and desacelarranting when the voltage is low or high

Could be simply not enough load, or some dried out caps on the secondary side and/or the PWM controller supply. Attach a car 21W 12V bulb to the PSU - along with the HDD so that something loads the 5V rail as well. If that doesn't help then move the bulb to the 5V rail. Or attach 2 bulbs, one on each rail in place of that HDD so it doesn't get fried in the process.

What brand/series capacitors did you use for replacement?

evasive wrote on 2020-06-05, 10:16:

What brand/series capacitors did you use for replacement?

For the X2 filter (0,47uf)
https://mauser.pt/catalog/product_info.php?cP … cts_id=004-3798

For the y2 supressor (6,8nf)
https://mauser.pt/catalog/product_info.php?cP … cts_id=004-0793

The power transistor - exchange a ST F444 --> BU508AW
Bulk cap - exchange a 47uf 385v --> 100uf 400v

But the oscilation of the values, was present before i made the changes... the changes where made to see if the values would be stable..

I'm getting really frustated with this...

I now that without a proper load, the power supply doesn't work well, and i expect some oscillations, but not of this magnitude..

I was watching a two part video on youtube about the restoration of a olivetti 286 pcs, and in the video, that guy couldn't even start the power supply with a hd conected. but without any load, the voltages where somewhat stable.. not the kinf of values i'm having...

i'm reluctant in conecting the motherboard and fry out everything.

I0m willing to adapt another power supply... just need to figure it out , without any doughts the pinout.

Change capacitors all around (electrolytics) including in hot side. This oscillation comes from feedback circuit not stable, regulator controller PWM, moving around to not regulate the outputs.

Cheers,

It looks like that new transistor is not a HF switching one, only high-voltage.

You did not yet change the capacitors on the (low-voltage) secondary side then?

evasive wrote on 2020-06-05, 15:56:

It looks like that new transistor is not a HF switching one, only high-voltage.

You did not yet change the capacitors on the (low-voltage) secondary side then?

I've changed all the eletrolitic capacitors in the psu... (almost everything is new on the power supply

maybe I can check the 12v and -12v transistors as well???

about the power transistor.. i used the one that the person in those youtube videos used.. seem to be a guy that knows what is doing... and is true that i have seen other ol psu with the bu508D

pentiumspeed wrote on 2020-06-05, 14:36:

Change capacitors all around (electrolytics) including in hot side. This oscillation comes from feedback circuit not stable, regulator controller PWM, moving around to not regulate the outputs.

Cheers,

what this means ? is the TDA chip ic that controls the power transistor that is bad ?

BU508D is for horizontal deflection transistor which is 15.7KHz, PSU runs higher than that and completely different waveform, and driving circuit is different as H.O.T. base pin had to be driven HARD, very clean sharp square drive driving into this transformer driver, using a small transformer driver that is designed to have high impedance to very very low impedance turns to drive the base, and I have not seen PSU use one even the one transistor drive, but BU508D is commonly used in early TV's then later on TVs used very different transistor that is best replaced with exact replacement when I was working at that electronics repair shop.

Second, BU508D is not meant to be driven at high watts rating as it were designed for around 130-140W using hot chassis design via flyback transformer as power supply sources instead of separate power supply.

Need to use a proper compatible switching power supply transistor, look up on your original transistor and search for substitute.

Cheers,

nuno14272 wrote on 2020-06-05, 12:38:

I'm getting really frustated with this...

I now that without a proper load, the power supply doesn't work well, and i expect some oscillations, but not of this magnitude..

Then why not try lightbulbs as I've suggested? These are easy to come by and cheap, and make a great load since it's purely resistive, and also you have an indicator at the same time.

pentiumspeed wrote on 2020-06-05, 23:23:

BU508D is for horizontal deflection transistor which is 15.7KHz, PSU runs higher than that and completely different waveform, an […]
Show full quote

BU508D is for horizontal deflection transistor which is 15.7KHz, PSU runs higher than that and completely different waveform, and driving circuit is different as H.O.T. base pin had to be driven HARD, very clean sharp square drive driving into this transformer driver, using a small transformer driver that is designed to have high impedance to very very low impedance turns to drive the base, and I have not seen PSU use one even the one transistor drive, but BU508D is commonly used in early TV's then later on TVs used very different transistor that is best replaced with exact replacement when I was working at that electronics repair shop.

Second, BU508D is not meant to be driven at high watts rating as it were designed for around 130-140W using hot chassis design via flyback transformer as power supply sources instead of separate power supply.

Need to use a proper compatible switching power supply transistor, look up on your original transistor and search for substitute.

Cheers,

i', using the BU508AW not the BU508D

at this stage i just want to proper know what's the pinout and install a replacment power supply... that's simple

nuno14272 wrote on 2020-06-07, 11:47:

pentiumspeed wrote on 2020-06-05, 23:23:

BU508D is for horizontal deflection transistor which is 15.7KHz, PSU runs higher than that and completely different waveform, an […]
Show full quote

BU508D is for horizontal deflection transistor which is 15.7KHz, PSU runs higher than that and completely different waveform, and driving circuit is different as H.O.T. base pin had to be driven HARD, very clean sharp square drive driving into this transformer driver, using a small transformer driver that is designed to have high impedance to very very low impedance turns to drive the base, and I have not seen PSU use one even the one transistor drive, but BU508D is commonly used in early TV's then later on TVs used very different transistor that is best replaced with exact replacement when I was working at that electronics repair shop.

Second, BU508D is not meant to be driven at high watts rating as it were designed for around 130-140W using hot chassis design via flyback transformer as power supply sources instead of separate power supply.

Need to use a proper compatible switching power supply transistor, look up on your original transistor and search for substitute.

Cheers,

i', using the BU508AW not the BU508D

They do not differ that much and the BU508AW is still not the right transistor to use here. It is too slow.

So basically what you want to do now is take whatever AT-compatible power supply and rewire the output voltages so it will match what you need for your Olivetti I guess?

evasive wrote on 2020-06-09, 09:01:

nuno14272 wrote on 2020-06-07, 11:47:

pentiumspeed wrote on 2020-06-05, 23:23:

BU508D is for horizontal deflection transistor which is 15.7KHz, PSU runs higher than that and completely different waveform, an […]
Show full quote

BU508D is for horizontal deflection transistor which is 15.7KHz, PSU runs higher than that and completely different waveform, and driving circuit is different as H.O.T. base pin had to be driven HARD, very clean sharp square drive driving into this transformer driver, using a small transformer driver that is designed to have high impedance to very very low impedance turns to drive the base, and I have not seen PSU use one even the one transistor drive, but BU508D is commonly used in early TV's then later on TVs used very different transistor that is best replaced with exact replacement when I was working at that electronics repair shop.

Second, BU508D is not meant to be driven at high watts rating as it were designed for around 130-140W using hot chassis design via flyback transformer as power supply sources instead of separate power supply.

Need to use a proper compatible switching power supply transistor, look up on your original transistor and search for substitute.

Cheers,

i', using the BU508AW not the BU508D

They do not differ that much and the BU508AW is still not the right transistor to use here. It is too slow.

So basically what you want to do now is take whatever AT-compatible power supply and rewire the output voltages so it will match what you need for your Olivetti I guess?

Yes.. exactly... I've done that with a IBM 300GL with a very unique power supply...
So, Not knowing exactly what are the best replacment parts, and unable to get the voltage stable, i want to convert a AT power supply to use the olivetti pinout.

Here's the mainboard and the power conector