Amiga 500 Power Supply Repair Attempt

hi it's John beta and today we are going to take a look at the Amiga 500 power supply from the previous episode that didn't quite work as expected after recapping and refurbishing it [Music] so here's the power supply in question that I tried to do some retro writing on in the previous episode and I also replaced all the electrolytic capacitors in here this is a November 1987 Amiga 500 power supply that allegedly came with the early Amiga 500 that I've worked on in previous videos this is one of the early models of these which has 2.5 amps on the 5 volt rail it's a kind of a hybrid between a linear power supply for the 12 volt rails plus and minus 12 volt and there is a very simple switching power supply rail for the 5 volts to provide the higher output that is needed on that rail I've looked at this power supply some more off camera and I actually tried it with this Crystal Clear Amiga 500 which has a revision 6 board in it and while it didn't power the Amiga 5 hundred with the revision three board look at this this just starts up fine without any issues with the power supply that wasn't able to power the rav3 so uh my assumption kind of is that the newer board revisions require a bit less power actually and this is enough to power this I've also tried a revision 5 board that was powered fine as well the only thing that isn't powered correctly is the rav3 so we're going to have to take a look at this power supply I guess I think there's something marginal in here yeah maybe this is a good opportunity to take a look at these power supplies because they are quite interesting old school switching power supplies with a linear portion so I dug out a schematic for this which somebody provides on GitHub thankfully for these early power supplies I have some spare parts from my own old power supply that I salvaged that we can try in this one we're going to try to figure out what the trouble is with this power supply let me take a couple of seconds to thank the sponsor for this video which is PC CB way my favorite manufacturer of prototype pcbs they also do CNC Machining sheet metal fabrication 3D printing you name it all the products a tinkerer needs produced they can offer the prices are very reasonable delivery is fast they are friendly people to work with I highly recommend checking out the link in the video description back to our power supply I now hooked up my ref5 board which is actually broken it displays a yellow screen last time I tried this to this power supply and turned it upside down so we have access to these points where the power connector connects to measure the actual voltages that come from the power supply under load so this is basically just a glorified load because it doesn't work it's not a working Amiga 500 board that's something I want to fix in another video at some point but yeah this should power up because I tried and it bought up to the yellow screen that the sport produces with the power supply that didn't power the Amiga 500 ref 3 at all it didn't even light up the power LED or anything like that so something is pretty marginal with this one we are going to figure that out so this one should fire up to a yellow screen at least that's what it did yeah something like that it's a uh I don't know cyan colored screen in this case this isn't working but the power LED is lit this board doesn't quite power up correctly because it is a broken board but we are going to have some load on the power rails here so that's the main purpose of this exercise okay let's get some voltages I'm just using this ground here next to this and this is our 12 volts rail that's 11.6 that is a bit low but it's going to be okay that's our -12 volts rail that's pretty much spot on under load we had a minus 16 of there abouts reading without a load on this power supply yeah this is the five volt Rail and it's at 4.95 which is technically okay for powering an Amiga but if the voltage if the current draw on the Amiga 500 ref 3 is a bit more this will probably go lower we measured 5.1 volts on the connector without a load and this these are our three voltages on the load yeah so the 12 volt rail is pretty much okay plus 12 is a tiny bit low but that's still okay the five volt rail I suspect the five volt Ray something's wrong with this five volt rail yeah the five volt rail is the most important one by far and that's the one that has the switching power supply portion in this power supply to provide uh enough current to actually power an Amiga I think the 4.99 thereabouts we got on this rav5 board is uh actually in Spec so that is going to power any Amiga fine but it is a bit low considering that we had 5.1 volts on the connector without a loads so it should be pretty much straight up five volts and it might be that the ref 3 board draws enough current to bring that down to 4.8 or something and the Amiga is not going to be able to start up the five volt rail Powers all the chips uh the only thing the 12 volts is used for basically is expansions and the disk drive motor and also the sound amplifier there's a differential amplifier chip in the audio circuitry in these Amigas and that uses the -12 for one half of the differential amplification of the plus 12 for the other half so that those voltages are not really needed to power up the system the five volt rail is the one that is powering everything basically including the power LED so uh that is the one we are going to have to take a look at I guess I unplugged this and I'm going to have to open it up again the Amiga 500 rav3 uh worked fine with another power supply so maybe something's wrong with this even though it Powers the other Amigas I tried fine there we go it's all nice and clean at this point because because I cleaned it in the previous video I'm going to take the whole board out to be able to work on it that is held in with these two screws and there's another screw here and then you can carefully because these boards are thin carefully lifted out that's what I'm going to do and there's no high voltage switching power supply in here so all the capacitors are low voltage capacitors I think the highest voltage capacitor in here is 25 volts so we don't risk getting shocked from those thankfully it's easier to work on than modern switching power supplies which usually use a very high voltage rail to be able to produce the output voltages so yeah as you can see there's not a lot of components in here compared to other modern more modern switching power supplies we basically have a huge Transformer we have of course we have a fuse we have some Mains filtering capacitors that are basically responsible for filtering the noise that the power supply are switching noise that the power supply creates these are filtering that from going back into the mains we have some electrolytic capacitors that are very prone to failure in these especially since they are old and this is a power supply and runs pretty warm we have a switching transistor that is this on this heatsink here and we have two voltage Regulators here's one and there's another one for the plus 12 and -12 volt rails we obviously have some rectifier diodes this is the bridge rectifier in a little package here we have a coil able to filter some noise and we have a couple of auxiliary stuff this chip here is the most important chip for the switching portion of this in conjunction with the transistor this is basically just used as a power amplifier transistor it's a high power npn transistor I believe this is a pwm chip so that is responsible for switching actually which generates the 5 volt Rail and this is a 3524bn chip which is a general purpose pwm controller chip which in this case is used for the switching portion of the power supply this transistor is a d45h11 so these two things are my main suspects at this point because sometimes if these chips get a bit weak they are not going to produce enough current for the power rail to have enough juice and also the transistor of course can break and can be a tiny bit weak and these resistors here which are two 180 ohms resistors in parallel so we measured 90 ohms across both of them resistors in parallel basically you get 90 ohms if you have two 180 ohms resistors in parallel these are obviously as you can see on the back side here they run really warm so I think it's a good idea to replace those and stand them up from the board slightly and I bought some a bit higher wattage I think these are half watt or quarter watt even resistors originally and they are just doubled up to withstand some more current so I'm going to replace these with higher wattage models basically and I think I want to remove the Chip And I want to remove the transistor and have a look at those and see if those work both of which I have replacement parts for from my power supply that I broke a while back those components should still work fine though so my spares will be good replacements for these and to replace the resistors I got uh two watt versions which are much over the top but we should be able to fit these in here fine and I'm going to stand them up a bit so they don't run as hot these are prone to fail obviously because they are hot all the time basically while this is running the bridge rectifier thingy I also have a replacement for that that is also a standard part uh TLS 1074 and I have a similar one I think it's another part number but it it was the same power supply design that I took these parts from so should be good so I'm going to desolder the chip I marked the pin one with the little red dot on the board I'm going to desolder the resistors the chip is my main suspect at this point because these can be responsible for all kinds of issues with these power supplies on the five volt rail and the resistors also are going to have to go possibly I am going to check if this uh chart backside here became conductive in the meantime which also happens if these this basically turns into coal so that would be a carbon and that would become conductive in the worst case so that might happen doesn't look that bad I've seen much worse on these power supplies but in the worst case we're going to have to cut this a bit in the center which is probably what I'm going to do as a precaution anyway Let's uh desolder some stuff here and try if we can make this work again I mean it does work but it's marginal [Music] these are super easy to work on because single-sided boards not very thick so let's see if we actually have conductivity no not really that's good probably still going to cut a bit of this or scrape it off at least because that could very well become a conductive layer in the future I'm just going to scrape some of this off gas that should do good also in the worst case just cut a slice here or drill it out or something because as you can see even underneath the surface there I scraped away the solder mask basically you can see the brownish spot in the middle there inside the circuit board actually that could be conductive but it isn't in this case which is good so we're going to put the new resistors in there so I think first of all I want to put a socket in here and then put the new resistors in after checking the old resistors I guess that would be a good idea so the old resistors should be 180 ohms if I'm not mistaken yeah 179.2 is this one and the other one should be very much the same 180.1 and these are one percent tolerance parts so that is all perfectly in Spec let's see for new ones live up these are also one percent metal oxide okay let's do it more than one percent should still be good enough for the purpose I'm just going to take two of those and put them in there but first let's put a 16 pin dip socket in this position here with the cut out facing in the correct orientation which is this facing away from the rest of the components it's probably not the best idea to have socketed chips in something that vibrates because there's a huge Transformer in there but these turned pin sockets are usually pretty solid so the chips are held in there pretty well okay let's drop in the spare pwm trip I have from my old power supply which I guess might be the issue here we're going to find that out so there's our chip in I'm going to temporarily put it back in the case so I don't risk zapping myself because this is the mains power device and we always want to be super careful with those just double checking the resistors here we should have 90 ohms all their robots at 88.8 that should be close enough okay I plug this back in and I'm going with my ref5 board here again to see if anything blows up I hope it doesn't let's see if we still get a reasonable voltages here this is our 12 volts which is 11.7 now which is okay minus 12 is still minus 12 and that's 5.03 volts that's much better than before actually this might even be enough to power our ref reboard so here's the revision Six Crystal Clear Amiga again let's see if it powers that and indeed it does now I think it's time to see if it also Powers the rav3 Amiga I am going to leave this on for a while though to see if anything runs away or gets overly hot we should probably be good so I actually left this running for a while with this Amiga and nothing gets overly hot or anything no bulging caps I think this might be fixed let's try with the revision three so same perspective but this time with the Amiga ref 3 that this is supposed to power actually let's see if our replacement chip did do something and it's still not powering this one for whatever reason it's still not doing anything with this nothing not even the power LED comes on what's going on is there something that draws so much current that this doesn't power up it is working Amiga power supply from all I can tell maybe the voltage is still too low so yeah I went on and removed the ram expansion from the rav3 we still don't even get the power led to light up with this power supply so something's still rotten there that is uh kind of strange so uh yeah I'm going to take this out again and I think the last resort at least the last thing I can think of is to replace the transistor which should also be responsible for the switching going on there so I'm going to take that out and put the known working one in that was in my original power supply my Amiga 500 power supply maybe that works better we're going to see not sure what else I can do the rectifier could also be an issue obviously oh maybe this diode here hmm that's a bit low of a voltage or you should remove that diode and give that a proper check so this diode here also measures a strange voltage that's a bit too low for my taste and this is a d45h2 whereas the replacement one is a d45h11 and it's also green maybe that helps I'm not sure so I'm just measuring these in my components tester here this is the replacement part from my working power supply it's a PNP transistor actually so let's note down those values I guess and then we are going to check the other one these are riveted to the heatsinks so it's not easy to test them because I don't want to remove the rivets yeah that's basically the same so that should be good actually probably I'm still going to put the green one in that's basically the same measurements here so they are probably both good might still be marginal the component tester basically doesn't measure those under load or anything like that so that may still be an issue there and we're going to retest this nope still nothing as expected because the transistors seemed good that is strange to put it mildly sanity checking with my non-working power supply here yeah that's more like it so the Amiga is okay it's still the power supply for whatever reason it doesn't produce enough juice to power this Amiga maybe so I desoldered this 1n4001 diode that gave me a strange forward voltage in circuit and it should be around 0.7 or 0.6 volts and yeah that's actually good so it was measuring the wrong forward voltage because there's a resistor in parallel with that uh 47 ohms resistor this one here which is very spot on 47 ohms so that's all good I measured some of the smaller components in general and they all seem to be okay so I'm not sure why this doesn't power the rav3 Amiga still so I looked at the schematics this is all diode and it actually specified a n4003 diode which is basically the same as a 4001 but it has a higher uh blocking voltage so I think the 401s are 50 volts and 4003 is 200 volts or something both of which are way over the top for this purpose I'm still going to replace that diode just because I have a stash of four or o3s in my parts bin we're going to take another look at the capacitors maybe maybe the ones I put in there are not the correct ESR value or something like that and it starts oscillating probably going to take a look at the 5 volt rail on the oscilloscope to rule that out other than that we can also replace our little rectifier that's in there or Rectifier module I have that spare not sure what else to do I'm pretty much stumped this is our little rectifier module here basically a diode a glorified dual output diode we're going to take a look at that and then I'm pretty much out of ideas so putting the 4003 diode in there obviously should go into the same polarity as the original one maybe the oscilloscope picture is going to tell us more or maybe the Amiga has an issue still that I didn't find but why would it work with other power supplies I'm not sure it's it's kind of strange yeah let's try with the new diode in nope not at all I'm going to hook up the broken Amiga 500 as a load and have a look at the oscilloscope I think so I hooked up the ground to the ground on the power supply and I'm going to measure the separate voltages here and see how much Ripple we have on the voltage rails so yeah that's around 170 millivolts on that rail that is a bit High the Ripple but still should be nothing to worry about you can see the millivolts that's the peak to Peak ideally this would be a line so a straight DC voltage which it isn't because it is a switching power supply and there's a noise and ripple in all those rails usually the Ripple on the good power supply is something below 100 millivolts so these are a tiny bit high but nothing to worry about really and we're still at 5.04 volts on the voltage rail that we are concerned with so yeah that seems to be all good really the Ripple is a tiny bit high but as I said 120 or 170 millivolts or something like that it's nothing to worry too much about usually maybe let's compare this with my known good power supply and see how that works out so here's the power supply the more recent power supply which has a higher frequency switching power supply in it it's a couple of megahertz there and uh we also have a tiny bit lower Ripple that's also not great it's fluctuating quite a bit but uh yeah that's 100 millivolts or thereabouts so maybe that difference is enough to make the Amiga ref 3 not take it that might be the issue here so I think what I want to do is to replace this rectifier package here and maybe also take a look at this capacitor here which is the output filter capacitor maybe we have an issue there I'm just going to replace this I think and then just for Giggles put in the old filter capacitor here maybe they are ESR rating is different on that one and this doesn't filter as effectively not sure [Music] I'm extracting the rectifier package and these are also slightly different part numbers this the one I just extracted this is a TLS 107.4 and this is a tl106.4 so uh yeah slightly different part number should be completely compatible just probably different voltage ratings or something like that which won't matter much because we're at a relatively low voltage here anyway so let's solder this one in and see if that makes any difference yep that should be in yeah and that's pretty much the same result around 180 millivolts to 200 or something and that's definitely not very good so we should try and replace re-replace that output capacitor there I think probably maybe that's not filtering enough there we go let's measure this against the original capacitor I guess so here goes the new capacitor which hasn't used R of 1.6 ohms and it's a bit over its capacitance which is perfectly okay for for a brand new capacitor and see what the other one says yeah that's the same ballpark so not much difference in these at all as I said these are not very low ESR or anything like that this is pretty worn out as well that shouldn't be any different really and this is the 6.3 volts capacitor and this is a 10 volts capacitor I'm still going to put the old one back in just for testing maybe something else is off I'm not sure at this point as you probably noticed and if I'm not mistaken we shouldn't see any significant difference at all yeah that's exactly the same 180 yeah same same I think let's see if it Powers the Amiga it Powers this Amiga as it did all the time basically okay uh rav3 Amiga soaked up again with the old capacitor in there again let's see if that makes a difference no dice I don't know why this Amiga doesn't power up with this power supply maybe the Ripple is too much but I'm not sure how we would get rid of the Ripple the other capacitors are not very high frequency or anything like that so it should be okay with those general purpose caps in there and everything else I replaced except for the voltage regulators and those are not responsible at all for the 5 volt rail so yeah hmm and all the components are measured the smaller ones are perfectly fine so there's not much that can go wrong there plus this Powers other Amigas I'm not sure what's going on it does work with everything but the Amiga we want to power with this that's kind of disheartening and it does work pretty well I don't get any flicker or anything like that which could be caused by a power supply that's not quite on par picture looks excellent crisp and everything not sure yeah at this point I'm pretty much out of ideas this should technically it provides the correct voltages the Ripple as we've seen is a tiny bit high but that shouldn't really be a problem I guess maybe it is for the ref 3 Amiga this rev6 and also the rav5 did power on no problems whatsoever and I had this running for several hours with this Amiga so it definitely Powers illegals just not the one we wanted to power with it if you have any ideas how I should go about trying to figure this out please leave them in the comments I am not particularly a professional with power supplies as some of you who are have probably noticed I don't recommend you do any of the things I did because I'm clearly not the right person to try and fix this still not sure what causes the issue as I said repeatedly now I hope someone in the comments knows a solution to this problem otherwise I'm just going to leave it at that and uh write a little note on the power supply that it works with all Amigas I've tried but not with the ref 3 that it came with hope you enjoyed this anyway uh can't always be a win I guess thank you so much for your support on patreon and on the channel memberships page and on Kofi and also for your subscriptions I recently passed 50k which is kind of amazing uh given what I do here so thank you very very much hope to see you again on this channel I'm Jan beta thanks for watching see you next time bye

Channel: Jan Beta

Views: 14,083

Keywords: Jan Beta

Id: 3MqysMpiQYU

Length: 31min 6sec (1866 seconds)

Published: Wed Apr 19 2023