ATARI STF Power Supply problem

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demonlg
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ATARI STF Power Supply problem

Post by demonlg »

Hi at all,

i have read on this forum more discussion of this problem but i don't have to resolve my problem.
I have the ATARI 1040 STF with power supply model MITSUMI SR98.
The problem same ro be present when system is hot, when i turn on computer the power output not start to max output voltage but stay at 1v on 5 volt rail and 3 volt on 12 volt rail and little ultrasonic noise, leds very low lighting.
If i disconnect the motherboard from psu and power on it without charge, i have 5v and 12 volt correctly.
At first time i have checked this i found a big input power electrolytic capacitor with top very deformed and i have replaced it first, then i have replaced all capacitor on it, replaced the diode bridge, and then replaced the output diode transistor with a big one specs.

But the problem is the same! Then my ultimate test is to put into 1040 the power supply from my 520, the computer start perfectly forever, in this case the power supply is different model, it is a ASP34 rev6.
If i put the 1040 PSU on a 520 i have the same first problem.

What my 1040 psu have?

Thanks

Emanuel
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exxos
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Re: ATARI STF Power Supply problem

Post by exxos »

Sounds more like your motherboard is dead. You should test the PSU with some load like a lamp or resistors.

Also check resistance on the 5v rail on the motherboard and report back its value.
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demonlg
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Re: ATARI STF Power Supply problem

Post by demonlg »

Hi exxos, with the 520ST power supply the motherboard works perfectly, without any problem, boots, read from original floppy recapped, all ok.

Only with your original psu recapped and revised the motherboard boot two, three times then present the described problems in post, and this psu installed in a 520 have the same problem.

MB not have any short, or apparently not, probably a capacitor on motherboard and with original psu the problem is visible and with other different psu not?

The motherboard are not recapped.

Thanks for reply

Emanuel
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Re: ATARI STF Power Supply problem

Post by exxos »

I would suggest you re-solder your SR98 as if it sometimes works and fails when warm it indicates a bad solder joint somewhere.
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
demonlg
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Re: ATARI STF Power Supply problem

Post by demonlg »

Nothing, checked now, all solder point are ok, i have resoldered all, but nothing! The problem still present after and before recapping psu,

A questions for the first high voltage capacitor: i have replaced it with other one with same value but not very new, it is desoldered from another device year ago, if possible the problem is from this first input source voltage?

I have a capacimeter but only to max 100 uf, the capacitor is a 47uf 400v and my capacimeter sign 40uF at test.

All other capacitor on psu are new bought 4 days ago.

Emanuel
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Re: ATARI STF Power Supply problem

Post by rubber_jonnie »

First off, when you are testing the PSU out of a machine on its own, I'd really advise using an artificial load (exxos store sells suitable resistors) or you might get inconsistent results.

For example, it may not give 5/12v when under load like yours, but gives good voltages when off load disguising a possible issue. You definitely don't want to use a known working ST as a load.

I note you've changed one of the 2 transistors as well as recapped and replaced the bridge rectifier, but I'd suggest you need to replace both of the transistors, it sounds like the PSU isn't starting. If replacing the 2nd transistor too makes no difference, then look at the PSU schematics to look at the switching circuit, see what other components may have died.

Also, there is a large white component at the front left edge of the high voltage side, it's a fusible resistor, you should do an off power continuity test to see if it has failed. They don't usually fail but they can.

As a safety note, be aware that the 2 heatsinks on the SR98 can carry high voltage and give you a very nasty shock.
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demonlg
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Re: ATARI STF Power Supply problem

Post by demonlg »

Not, i have tested psu out of ST without charge on output, what is the value of ceramic resistor to use for charge?

The SR98 with problem not have a big white fuse, but normal glass fuse.

I have not changed the primary mosfet transistor, do you have a part number to replace the original mosfet?

For the new bridge i have used a KBL04 type 4A max on specs, original is 1,5 ampere max specs.

All new capacitor have the same uF value, only max voltage are higher, 50v max for all on secondary and 400v for primary.

Emanuel
demonlg
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Re: ATARI STF Power Supply problem

Post by demonlg »

Argh sorry, yes front of big capacitor on primary, is a ceramic resistor 8,2 ohm value, i try to post photo of my psu. Substantially the PSU won't start if have a charge, but start without it.

But in some cases it starts and functions perfectly until the next shutdown and then power on again.

Emanuel

sr98.jpg
sr98.jpg (161.47 KiB) Viewed 1934 times
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rubber_jonnie
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Re: ATARI STF Power Supply problem

Post by rubber_jonnie »

These are the resistors I use for loading the PSU: https://www.exxosforum.co.uk/atari/store2/#0070

Beware, they do get very hot.

I presume by charge you mean load, so if it doesn't start under load/charge then there is likely a problem with one of the two transistors, specifically the switching transistor. I'd replace both with new ones of the same type or specs.

That's the one in front of the large capacitor, just be sure it isn't open and can be read with an ohmmeter.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
demonlg
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Joined: Wed Dec 16, 2020 9:38 am

Re: ATARI STF Power Supply problem

Post by demonlg »

Yes sorry for my english, i'm write from Italy, charge (carico) in effect is load....sorry

Yes the resistor seem to be ok, 8,2 ohm on multimeter.

The original transistors are simple to find on line?

This night i try to make a load with resistor and test it.

Thanks

Emanuel
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