Mitsumi SR151 Overheating, almost burning?

[Tomswork]

My sr151 with emi filter in a mega file 30

20200620_211929.jpg

Tom

[ziggy]

My sr151 with emi filter in a mega file 30

How hot do the resistors & voltage regulator get? Can you feel the heat radiation? Is the PCB darkened around the regulator?

[Tomswork]

Sorry I do not have a 240v power source to test it. The board shows no signs of over heating. And I far as I can tell the capacitors look fair.

Tom

[ziggy]

Hi,
still didn't make any progress on the PSU and the computer in general. Could someone with a Mega supply me with some Resistance measurements for comparison?

On the mainboard I measured from the power connector (PSU and FDD disconnected) somewhere around 6kOhm on the 12V rail, but only around 100-200 Ohm on the 5V rails :? Seems a little low to me!

Greetings
Ziggy

[Tomswork]

Have you checked all the diodes to see if one has opened up like a fuse or worse turn to a piece of wire in the circuit .

It may help to check the feed back side to see if the coil is oscillator circuit is working

Tom

[ziggy]

I'll check the Diodes, thank you for the tip. Also I measured resistance both ways, they're the same. ( 5V to GND / GND to 5V)
Off to do some Diode testing :geek:

[Tomswork]

Here what in thinking this an excerp from a service manual

RSTART only supplies a small amount of current to start the device, so once the first pulse makes it through the field-effect transistor (FET), current from a third winding on the transformer is used to provide power to run the oscillator. This is what NA and DBIAS are all about. You might not see that third winding, you might just see all the running power being drawn through a higher power dropping resistor in place of RSTART. But, using the third winding improves efficiency a lot.

When the PWM oscillator is running, it sends constant pulses from the output pin. This turns on the big switching FET, QSW, which pulses the current going through the transformer. As this happens, current is induced in the transformer secondary, rectified and filtered by DOUT and COUT, and current flows from the output.

There is a zenier diode and a capacitor used to supply the start pulse for the oscillator

Tom

[ziggy]

@Tomswork

Just to be clear, are we talking about diodes on the Mainboard or the PSU?

[Tomswork]

Yes a small glass like diode.

Tom

[AtariFox]

Could any of you with this supply take some pictures of where the wiring goes please? I am having such a hard time making sure where the wires go.
I have an SR151 , but, There's one wire i don't know where it connects to. Specifically the wire next to this J5 marking.

I also don't know if anything connects to "LED". This SR151 is in a Megafile 30.

All the pictures i find have obscured images. And there's no schematics i can find. I need to see everything wire side, like what i have shown in the pictures, where all the labeled marking, wires go.