Mitsumi SR151 Overheating, almost burning?

[ziggy]

Hi there,
after reading on this forum for some months and getting a lot of help from existing threads I now need to post a problem of my own
I'm repairing some Mega STs and one of them has quite a few problems, starting with the PSU, the aforementioned Mitsumi SR151.

While trying to diagnose the other hardware I noticed a strong, burning plastic smell and immediately checked the PSU. The resistors seemed to get very hot. After closer inspection I noticed the PCB had started to darken quite a bit. I desoldered the switching regulator to get a better picture. The the thing is, I can't really localize the culprit, as the darkening stretches over a wider area around the regulator and the resistors.
I measured the resistors, all values are fine. Not sure about the regulator though. Voltages were still 5/12, don't know about eventual ripple (No Oscilloscope right now, it's in my closed workshop)
Do you have any tips on which component could be the main issue? I' trying to order a new regulator and found a 800V/15A one: 15A10G, should it suffice?
Greetings

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[Tomswork]

This link has some info on the psu

https://www.exxosforum.co.uk/atari/last/psu/index.htm

I would check the rectifier. For a starting place

The solder looks dull in the picture and maybe suspect
And it looks like the emi socket has be replaced with a basic socket

Tom

[ziggy]

Hi Tom,
thanks for your reply. I already read Exxos' part on this specific PSU (otherwise I wouldn't dare to post here ) His Photo doesn't show any darkening of the PCB whatsoever, so something's definitely wrong with mine.

In which aspect do you think the rectifier could be damaged? I'll desolder it and measure the diodes.
Are you talking about the soldering being dull in general, or specific solder joints?

I haven't even noticed the emi socket is a basic one... I wonder if it is stock.

[Icky]

@ziggy - I had a similar burning smell and black PCB problem with a ASP34-2 https://www.exxosforum.co.uk/forum/viewt ... f=9&t=1369. Fortunately it looked like it was one of the resistors so not only recapped the PSU I also replaced all the metal film resistors.

Looking at your pics the blackness does seem to be round the rectifier and near the metal film resistors. Are the resistors under those ok or black?

[Tomswork]

I have other switching power supplies were the rectifier is letting alot of ac though. It shows signs of the pcb getting cooked like that and it causes thermal damage to the primary side capacitors take the worst of it. The resistor are still in specification so I would start at the rectifier and inspect the capacitors or replace them. The emi filter is missing as best as I can tell. The socket combo emi has the leads coming out of the tin in a different manner

More like this

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Tom

[ziggy]

@ziggy - I had a similar burning smell and black PCB problem with a ASP34-2

I read your topic just before posting my problem the SR98 seems to be the predecessor to the SR151.
It is really hard to pinpoint the hotspot on the PCB, quite literally It seems to stretch from the regulator south towards the resistors. The smaller resistors underneath seem fine optically. I measured the large ones, all perfect. But they were HOT when I tested the PSU, so definitely absorbing too much energy and transferring it unto the PCB.

The Emi filter is missing

Yes, thank you for pointing it out. I completely missed it, don't know why.

For good measure I ordered the following:

EMI socket
Regulator
Rectifier

I hope that'll resolve this issue. Electrolytics seem fine optically, no bulbing or leaking. I'm aware that their capacity has most likely degraded, but recapping is another chapter. First, I want to adress the overheating issues.

Any further tests I should run? I'll desolder the rectifier and measure it, also measure the smaller resistors. Anything else?

The Mega ST itself has some major issues as well (Even Exxos' diagnostic cartridge only boots 1 out of 20 times, and with a faulty screen).

[exxos]

They do run hot and can burn the PCB. I would change the main switching transistor while you have it out anyway.also looks like there is a primary pin on the transformer not soldered.

[ziggy]

They do run hot and can burn the PCB. I would change the main switching transistor while you have it out anyway.also looks like there is a primary pin on the transformer not soldered.

Hey Exxos, thanks for the input I'll let the board rest until all parts (rectifier, regulator, emi socket) have arrived. In the meantime, here's a screenshot of the (rare) outpot of your Diagnostic card. Just a preview, I'll start another topic about the Mega ST these days in the right section.
Don't know what to make of E9

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[exxos]

just a preview, I'll start another topic about the Mega ST these days in the right section.
Don't know what to make of E9

Yeah, please open up a new topic about that error, likely "bad instruction fetch".

[ziggy]

Hi again,
replacement parts arrived and I soldered them in . EMI socket works fine, so does the new rectifier.
But I think I made a mistake with the switching regulator, the 2SC3459. I think I bought the wrong part, which isn't a switching regulator at all: https://pdf1.alldatasheet.com/datasheet ... 5A10G.html Can anyone confirm this?

No wonder no voltage arrived at the 12/5V Pins
What would be a good replacement for the original 2SC3459? They seem to be out of production.

Greetings