ST-ranger Posted May 10, 2021 Share Posted May 10, 2021 '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. Thanks in advance all. Quote Link to comment Share on other sites More sharing options...
MasterMotorola Posted May 10, 2021 Share Posted May 10, 2021 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... 1 Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted May 10, 2021 Author Share Posted May 10, 2021 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 Quote Link to comment Share on other sites More sharing options...
Chri O. Posted May 11, 2021 Share Posted May 11, 2021 R6 and R7 are 1/2 Watt 0.5W Resistors. Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted May 11, 2021 Share Posted May 11, 2021 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 Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted May 11, 2021 Author Share Posted May 11, 2021 (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 May 11, 2021 by ST-ranger -question about resistance- Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted May 12, 2021 Share Posted May 12, 2021 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 Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted May 12, 2021 Share Posted May 12, 2021 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. Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted May 19, 2021 Author Share Posted May 19, 2021 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 Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted June 7, 2021 Author Share Posted June 7, 2021 (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 June 7, 2021 by ST-ranger Clarification Quote Link to comment Share on other sites More sharing options...
Chri O. Posted June 7, 2021 Share Posted June 7, 2021 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. Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted June 7, 2021 Author Share Posted June 7, 2021 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. Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted June 8, 2021 Share Posted June 8, 2021 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 Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted June 8, 2021 Author Share Posted June 8, 2021 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 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. Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted June 17, 2021 Author Share Posted June 17, 2021 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. Totally stumped now. Poor quality resistors? Some other problem? Thanks all. Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted June 18, 2021 Share Posted June 18, 2021 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 ? Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted June 18, 2021 Author Share Posted June 18, 2021 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? Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted June 18, 2021 Share Posted June 18, 2021 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 ? Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted June 22, 2021 Author Share Posted June 22, 2021 (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 June 22, 2021 by ST-ranger Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted June 22, 2021 Share Posted June 22, 2021 (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 June 22, 2021 by TGB1718 Update: Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted June 22, 2021 Author Share Posted June 22, 2021 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 Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted June 22, 2021 Share Posted June 22, 2021 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. 1 Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted July 20, 2021 Author Share Posted July 20, 2021 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? Quote Link to comment Share on other sites More sharing options...
TGB1718 Posted July 21, 2021 Share Posted July 21, 2021 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 Quote Link to comment Share on other sites More sharing options...
ST-ranger Posted July 22, 2021 Author Share Posted July 22, 2021 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) R19 on its own: Quote Link to comment Share on other sites More sharing options...
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