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ST-ranger

Atari 520STfm PSU component replacement

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'Morning all,

 

Currently in the process of repairing a poorly 520STfm. This is a UK 520, and it has found itself in sunny Australia, where the voltage is the same (220-240v).

Board is nice and clean, no issues with burst caps or any other visible damage. The main symptom has been no power.

I am replacing the PSU caps based on the Techwiki Astec ASP34 (Mine is ASP34-2).

After removing all of them, I note most of the PSU caps have leaked at some point, so I have cleaned up the bottom & top of the board. Please see the photos.

 

One of them has explosively decompressed out the side ( C7 ) . A couple of components also look burnt and/or damaged.

 

Could anyone please assist in identifying the components in the pictures so I can replace them? I have pointed to them the best my limited paint skills allow.

 

20210510-183043.jpg

 

20210510-183100.jpg

 

20210510-183132.jpg

 

Thanks in advance all.

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4 hours ago, MasterMotorola said:

ST-ranger, take a look at this page... https://www.exxoshost.co.uk/atari/last/psu/index.htm#ASP34

 

Also, it may be cheaper and better in the long run to replace the entire PSU with a new one made by Mean Well like in this video...

 

MasterMotorola, thanks for your quick reply, much appreciated.

I've watched the video now, glad to know there is a drop-in replacement.

They are readily available in Australia - can I use these to replace the supply in my 1040STfm and STe? If so, I might do that, and continue to repair the current one.

 

I have been through the schematics - it looks as if it's in another language to me. Here are the values I've come up with for things that appear damaged:

 

R5 - 51 ohms 2 W

R4 - 330 ohms 2W

R6 - 3.3K ohms, watts not specified

R7 - 15 ohms, watts not specified

C8 - 1 uF 16v

 

Is it okay to use any wattage for the above without a W rating?

I might be able to get these from the local electronics supplier.

 

 

There are also two components which I'm totally stumped by:

 

Q1 - ? SC1384

DB1 - No value - another part of the site shows it as a "bridge recitifier". On the original part is has printed on top: "G1 W10M 8741"

 

Where could I source these components? There are specialty stores I've used for ZX Spectrums, Amigas, etc., however haven't found anything which sells these components for STs.

 

Thanks again

 

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The DB1 is a W10M full wave bridge rectifier it's rated at 1.5A, so reasonably any mains voltage bridge rectifier

with at least 1.5A rating or above will do the job. MUST BE MAIN VOLTAGE RATED. else smoke ensues :(

Here's the datasheet.

 

The SC1384 is an NPN transistor, again, here's the datasheet, seems to be a power transistor with a max 

current rating of 1A.

 

Had a quick look on "FleaBay" :) and they are still available in the UK, worth checking.

datasheet.pdf 2SC1384_UTC.pdf

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Posted (edited)
13 hours ago, TGB1718 said:

The DB1 is a W10M full wave bridge rectifier it's rated at 1.5A, so reasonably any mains voltage bridge rectifier

with at least 1.5A rating or above will do the job. MUST BE MAIN VOLTAGE RATED. else smoke ensues :(

Here's the datasheet.

 

The SC1384 is an NPN transistor, again, here's the datasheet, seems to be a power transistor with a max 

current rating of 1A.

 

Had a quick look on "FleaBay" :) and they are still available in the UK, worth checking.

datasheet.pdf 102.08 kB · 3 downloads 2SC1384_UTC.pdf 282.03 kB · 3 downloads

This is fantastic, thank you so much.

 

I've ordered the SC1384 TO-90L from China - the wait is only a month or so, and they were incredibly cheap.

Sadly I'll have 49 of these leftover, feels like a waste, perhaps time to do the other Ataris while at it.

 

The W10M, have found a source over here - how do I tell if it's mains rated? Here is the blurb from "fleaBay":

 

>"The 1.5 Amp Full Wave Bridge Rectifier (W10M) is a single phase Bridge Rectifier. Voltage Range is 50 to 1000 Volts and Current is 1.5 Amperes."

 

Because my voltage range lies within this (220v to - 240v) is it safe to assume that it is mains rated?

 

22 hours ago, Chri O. said:

R6 and R7 are 1/2 Watt 0.5W Resistors.

 

Thank you! All ordered.

 

-edit- when choosing resistors, is the resistance non-negotiable & the power flexible, similar to capacitors?

 

I.e. if I need a 51 ohm resistor at 2W, can I choose a 51 ohm resistor at 3W, or perhaps below at 0.6W? Which is better? Thanks.

Edited by ST-ranger
-question about resistance-

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9 hours ago, ST-ranger said:

Voltage Range is 50 to 1000 Volts and Current is 1.5 Amperes."

Yes, that's fine, the original spec

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9 hours ago, ST-ranger said:

 

I.e. if I need a 51 ohm resistor at 2W, can I choose a 51 ohm resistor at 3W, or perhaps below at 0.6W? Which is better? Thanks

With resistors, if it's rated @2W that's the minimum rating, it's ok to go over i.e. 3W assuming it will still fit

in the PCB, but a0.6W would likely fry itself in a nice mushroom cloud :)

 

With capacitors, they are voltage dependant so again if it's 10uF 20V rated, ok to go 10uF 30V, but not 10uF 10V,

there's also tolerances to take into account on both resistors and capacitors.

 

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On 5/12/2021 at 6:24 PM, TGB1718 said:

With resistors, if it's rated @2W that's the minimum rating, it's ok to go over i.e. 3W assuming it will still fit

in the PCB, but a0.6W would likely fry itself in a nice mushroom cloud :)

 

With capacitors, they are voltage dependant so again if it's 10uF 20V rated, ok to go 10uF 30V, but not 10uF 10V,

there's also tolerances to take into account on both resistors and capacitors.

 

Great information, thanks again.

 

Everything is ordered - will post progress! Cheers

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Posted (edited)
On 5/11/2021 at 10:27 AM, Chri O. said:

R6 and R7 are 1/2 Watt 0.5W Resistors.

 

All parts arrived and have been busy fixing things up.

 

R8 is also damaged on the bottom however the schematic lists it as:

 

R8

1 Ohm

 

Is this also a 0.5 W part? It looks much more comparable in size to R5 which is 51 Ohms & 2 W.

 

Thanks again

 

 

Edited by ST-ranger
Clarification

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Quote

Is this also a 0.5 W part? It looks much more comparable in size to R5 which is 51 Ohms & 2 W.

If you are not sure use the highest wattage you got 2W, also check that diode (D1) it does look discolored: YouTube how to test diode link.

 

 

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17 hours ago, Chri O. said:

If you are not sure use the highest wattage you got 2W, also check that diode (D1) it does look discolored: YouTube how to test diode link.

 

 

Thanks - I've only got the 1 ohm resistor in 0.5 W, will that do?

 

Thanks also for the excellent video, learnt a lot. Took the diode out of circuit, it tests ~ 0.520 in forward bias and OL in reverse, so guess it's just fine.

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10 hours ago, ST-ranger said:

Thanks - I've only got the 1 ohm resistor in 0.5 W, will that do?

If the original is similar size to the 2W one, then I suspect that you will need a 2W one for the 1 ohm slot.

Is it the one in the photo by the "C4" writing, if it is, then that looks like a 2W metal film resistor:- see pic below

 

Resistors Physical Size - UK Vintage Radio Repair and Restoration  Discussion Forum

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13 hours ago, TGB1718 said:

If the original is similar size to the 2W one, then I suspect that you will need a 2W one for the 1 ohm slot.

Is it the one in the photo by the "C4" writing, if it is, then that looks like a 2W metal film resistor:- see pic below

 

Resistors Physical Size - UK Vintage Radio Repair and Restoration  Discussion Forum

 

Yes, it definitely looks like the metal film one. Will get to ordering it straight away.

 

For those that are counting, it has cost more than a modern industrial PSU replacement, but has been triple the fun.

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Well, that was eventful!

 

After replacing everything, plugged it in, turned it on... "BANG!" Double blue flash.

 

Looks like the new fuse is cooked, along with the R8 & R5 resistors.

 

 

R8 was replaced with a 1 Ohm 2 W part.

R5 was replaced with a 51 Ohm 2 W part.

 

06.jpg

 

51.jpg

 

 

Totally stumped now. Poor quality resistors? Some other problem? Thanks all.

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From what you say, it looks like Q2 or something in the feedback circuit is not working and hence

the PSU is not starting, it's just turning on Q2 permanently and not oscillating, did you check Q2

to see if it's ok ?

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2 hours ago, TGB1718 said:

From what you say, it looks like Q2 or something in the feedback circuit is not working and hence

the PSU is not starting, it's just turning on Q2 permanently and not oscillating, did you check Q2

to see if it's ok ?

Thanks! Q2 is the big one wrapped in the rubber blanket... stripped it down, looks just fine.

 

There is also a "Q3" which also looks fine. Can't see any other components that look damaged in any way, apart from the recently blown resistors.

 

Is there another test I can do? Or, time to throw in the towel and get a modern PSU?

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26 minutes ago, ST-ranger said:

Thanks! Q2 is the big one wrapped in the rubber blanket... stripped it down, looks just fine.

Did you test it with a meter out of the circuit ?

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Posted (edited)
On 6/18/2021 at 10:15 PM, TGB1718 said:

Did you test it with a meter out of the circuit ?

I have just done this, as suggested. (-edit- tested removed from PSU)

 

Following this guide: https://www.wikihow.com/Test-a-Transistor

 

I tried the black probe on the end leg, and red on middle and opposite. All beep & read 000.7.

 

Swapped these around, red on end leg, black on middle then opposite. All readings 000.7 & constant beep.

 

Tried with one on middle then opposite probe on one leg then another, same as above.

 

Looking at this datasheet : https://pdf1.alldatasheet.com/datasheet-pdf/view/22038/STMICROELECTRONICS/BU508A.html

 

It seems I should have at least picked up on the correct combination to test it. Does it sound fried?

 

I have looked at replacement transistors and the only reasonably priced ones are in the TO-3P and TO-247 packages. Is it necessary to get the TO-218?

 

Thanks again

Edited by ST-ranger

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Posted (edited)

Looking at the front i.e. not the surface touching the heat sink.

Pin 1 = Base

Pin 2 = Collector

Pin 3 = Emitter

 To test this put meter on ohms range

red lead to pin 1, black to pin 2 = low reading ~0.4 to 0.9

red lead to pin 1, black to pin 3 = low reading ~0.4 to 0.9

 

Black lead to pin 1, red to pin 2 = Out of Limits (no reading) or "High" depending on your meter

Black lead to pin 1, red to pin 3 = Out of Limits (no reading) or "High" depending on your meter

BU508.pdf

 

With regards to the package, unless the specification in terms of voltage/current/frequency are the same and it's

physically going to fit on the heat sink, you will need an original part.

Edited by TGB1718
Update:

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19 minutes ago, TGB1718 said:

Looking at the front i.e. not the surface touching the heat sink.

Pin 1 = Base

Pin 2 = Collector

Pin 3 = Emitter

 To test this put meter on ohms range

red lead to pin 1, black to pin 2 = low reading ~0.4 to 0.9

red lead to pin 1, black to pin 3 = low reading ~0.4 to 0.9

 

Black lead to pin 1, red to pin 2 = Out of Limits (no reading) or "High" depending on your meter

Black lead to pin 1, red to pin 3 = Out of Limits (no reading) or "High" depending on your meter

BU508.pdf 304.32 kB · 1 download

 

With regards to the package, unless the specification in terms of voltage/current/frequency are the same and it's

physically going to fit on the heat sink, you will need an original part.

Thank you. Results of tests:

 

Meter on 200 ohm range

 

red lead to pin 1, black to pin 2 = rapidly drops from 1.2 to 0.3 and stays

red lead to pin 1, black to pin 3 = rapidly drops from 1.2 to 0.5 and stays

 

Black lead to pin 1, red to pin 2 = rapidly drops from 1.2 to 0.3 and stays

Black lead to pin 1, red to pin 3 = rapidly drops from 1.2 to 0.5 and stays

 

If I use the lowest ohm setting (black right arrow with horizontal line through and vertical line intersecting the end of the triangle - pressing "func" changes it from continuity to this mode)

All tests above read 0.002

 

Does this indicate replacement required?

 

The spec sheet was exremely helpful. Found some "BU508A POWER TRANSISTORS(5A,1500V,125W)". Sounds like they would be a perfect fit. Thanks

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Doesn't look good to me, you should only get conductivity in one direction between Base/Emitter and Base/Collector.

 

BTW. when fitting a new unit don't forget to use some heat sink paste between the back of the transistor and the heat sink.

  • Thanks 1

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On 6/22/2021 at 10:37 PM, TGB1718 said:

Doesn't look good to me, you should only get conductivity in one direction between Base/Emitter and Base/Collector.

 

BTW. when fitting a new unit don't forget to use some heat sink paste between the back of the transistor and the heat sink.

Alright, everything has arrived & is fitted.

 

One more piece of the puzzle to go - noticed an "R19" resistor that isn't present in the schematic at all. It's one of the smaller ones. This doesn't measure any capacitance when taken out so it must be replaced too... have looked at all the different revisions of the schematic apart from my ASP34-2 and it doesn't appear present, they all go up to R18 (this is the one in line with the power connection to board). Which value would be best here?

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10 hours ago, ST-ranger said:

One more piece of the puzzle to go - noticed an "R19" resistor that isn't present in the schematic at all. It's one of the smaller ones. This doesn't measure any capacitance when taken out so it must be replaced too

Do you mean doesn't measure any resistance ?, please attach a photo of the device, it could be a very small capacitor

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19 hours ago, TGB1718 said:

Do you mean doesn't measure any resistance ?, please attach a photo of the device, it could be a very small capacitor

Yes, my bad. Doesn't measure any resistance. Neither R19 nor R9 do. Others have been taken out of circuit and appear OK.

 

Both have been taken out in the photo, (R19 laid beside)

 

1.jpg

 

R19 on its own:

 

2.jpg

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