Help finding a short - Page 1
Forum section: electronics / beginners

author: Sanxion
author_rank: Contributor
timestamp: October 06, 2023, 07:10:41 pm
content: Hello all,I have spent literally months restoring my Amiga 2000 and have managed to find what I believe is the final problem...There is continuity between pins 5, 6 & 7 of each RAM IC - which should not happen.So in my attempt to determine the cause. I have done the following:1. Desoldered all 32 RAM ICs (a very painful task)2. Desoldered RP5033. Desoldered RP5004. Desoldered U540All to no avail...The short still exists. I am not sure what else to do or where to look. I have attached a section of the schematic which will hopefully clarify why I removed the above components.Please help...someone.

author: kripton2035
author_rank: Super Contributor
timestamp: October 06, 2023, 07:14:56 pm
content: easy to build tool to find shorts on a pcb :http://kripton2035.free.fr/Projects/shorty-display.html

author: Sanxion
author_rank: Contributor
timestamp: October 06, 2023, 07:46:22 pm
content: I can see that tool would be very useful if I had several components and didn't know which was was causing the short...but in my case, I believe I have removed all the components that could be causing the problem.

author: kripton2035
author_rank: Super Contributor
timestamp: October 06, 2023, 08:57:41 pm
content: so the short can be on the pcb directly, some soldier bridge for example, and the shorty tool is still useful to find it.

author: Sanxion
author_rank: Contributor
timestamp: October 07, 2023, 06:19:42 am
content: Is it possible to buy this from somewhere?

author: u666sa
author_rank: Frequent Contributor
timestamp: October 07, 2023, 06:50:45 am
content: You need a freezer in a spray can.You apply voltage, start with 2V 500 mAh, increase as needed, go as far as 1 amp and as high as 5 volts. At that point you should be able to see the short. Idea here is to find the heat spot.Apply voltage. Spray. See where there is heat.Method #2 is to use isopropyl alcohol, the IPA. You apply voltage until you feel with your fingers heat spot, then with a cotton swap apply IPA to the spot where it is hot, whichever part evaporates first is the culprit.Mthoed #3 is to use thermal imaging. You apply voltage and see where it gets hot.//////////////////Often times you see chips get hot.This does not mean the chip is bad, it is almost always, more than 50% of a time, means cap or something around the chip is causing the short.

author: kripton2035
author_rank: Super Contributor
timestamp: October 07, 2023, 06:56:12 am
content: Quote from: Sanxion on October 07, 2023, 06:19:42 amno sorry you have to build it following the site design instruction

author: u666sa
author_rank: Frequent Contributor
timestamp: October 07, 2023, 07:05:49 am
content: Now, as a side note, to remove these memory chips. By the way we don't remove stuff to find shorts, we use thermal imaging or other mentioned methods.To remove chips like that easily you can not use heat gun on these older boards they are going to burn. What you need a low melt. For example, 25% Sn, 25% Pb, 50% Bi.Or, 42% Sn, 58% BiOr there are Indium based low melts, NorthridgeFix uses indium basedlow melt.These are all equally good low melts and they perform pretty much the same.The one you should not get are Cadmium containing low melts. Also called Wood's metal. Cadmium is toxic, inhaling fumes will raise toxicity in your body.Anyway, you apply low melt to legs from bottom and maybe even from top. You heat with iron and remove. If you have long tip that will help. If iron is not helping you can apply a little bit of heat, perhaps 280C and safely remove them.P.S. we never remove innocent chips to find shorts.

author: dmulligan
author_rank: Regular Contributor
timestamp: October 07, 2023, 07:21:41 am
content: Have you desoldered the RAM chip sockets? Were the resistor packs socketed?I checked out the Shorty tool. If you click around the site, you'll eventually find the original perfboard version. You should be able to build one.

author: Sanxion
author_rank: Contributor
timestamp: October 07, 2023, 07:36:19 am
content: Thank you.As a side note, I only discovered the short after I had removed the RAM. A "green" screen on an Amiga - after eliminating all else - is usually indicative of bad RAM...which is why I desoldered them.

author: CaptDon
author_rank: Super Contributor
timestamp: October 07, 2023, 01:46:47 pm
content: Guessing at this point you have thoroughly killed any hope of repair. 99.99% of the time an I.C. wouldn't fail in such a way as to have a dead short across 3 adjacent pins. Even with static damage the resistance may be around 100 ohms or so and not a dead short. More than likely you have a solder splash somewhere from other work you were doing or a metallic fragment from flash metal from a screw, washer or other source has gotten lodged under the pins of an I.C.

author: Buriedcode
author_rank: Super Contributor
timestamp: October 07, 2023, 02:40:51 pm
content: Whats the resistance of the short? What is the resistance of these shorted pins to the local GND? Is there diode continuity between these pins and ground/VDD?

author: Sanxion
author_rank: Contributor
timestamp: October 07, 2023, 04:12:11 pm
content: Quote from: Buriedcode on October 07, 2023, 02:40:51 pmOK. I have continuity in three separate places, all around the 32 RAM ICs.1. the RAM ICs:With my meter set to 200 ohms:Resistance of the short:pins 5 to 6, 6 to 7, 5 to 7: it seems to vary between 17 and 50 and slowly increasing (?!). Sometimes it stays around 18.Resistance to the local GND: (I assume you mean the GND on the IC) there is no continuity on any of the three pins to GND.Is there diode continuity: no.2. RP500. There is continuity between pins 1,3 and 5 around 20 Ohms and sometimes it slowly increases. There is no continuity to GND.3. RP503: Continuity between pins 8, 9 & 10 around 74 Ohms. No continuity to GND.

author: kripton2035
author_rank: Super Contributor
timestamp: October 07, 2023, 05:25:47 pm
content: the shorty tool will not work above 2-3 ohms... sorry it will not help find the fault on your pcb.

author: dmulligan
author_rank: Regular Contributor
timestamp: October 07, 2023, 05:53:52 pm
content: Are your reported measurements with the chips in circuit or of the board with the chips and resistor packs removed?

author: Sanxion
author_rank: Contributor
timestamp: October 07, 2023, 07:50:27 pm
content: All the RAM ICs have been removed and the 2 resistor packs have been removed too.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 07, 2023, 08:59:49 pm
content: This schematic looks a bit better (SRAM: Page 255)https://support.retrorewind.ca/_media/commodore/amiga/commodore_a500-a2000_technical_reference_manual.pdfBy removing all the SRAMs, RP500, RP501, RP502 and RP503, the SRAM address and control buses become completely isolated from the rest of the circuit, so you should measure open circuit in every posssible combination.If you're still getting something, it must be a defect in the board. Was the board damaged by battery acid?

author: Buriedcode
author_rank: Super Contributor
timestamp: October 08, 2023, 12:19:56 am
content: Quote from: Sanxion on October 07, 2023, 04:12:11 pmThis is interesting. So, you've removed all the RAM chips, the pullup resistor pack/array (470R, RP503), and the series resistor pack RP500, as David mentioned - effectively isolating the troublesome/shorted address lines (A0, A1, and A2) from the rest of the circuit.But RP503 pins 8, 9 and 10 are not those same troublesome address lines - according to your schem, those are adderess lines A6 - A8.Could this be a tin wiskers thing? I would have thought a true short such as a solder bridge would have a much lower resistance. As you appear to have isolated these traces - no active components connected to those lines - I would be tempted to start using power to narrow it down, as suggested above - using a current limted supply between pairs of shorted pins, and isopropyl alcohol to show where the heat builds. 17-50 ohms is a nice resistance to find as it won't require much power - 150 - 250mA should create 1W of heat in a small area enough to evapourate the iso easily.However!!This is assuming that there really isn't another short somewhere connecting these short pins to something else they shouldn't be connected to - thats why I suggested a resistance check to ground -even high resistance, since many things have a path to ground somewhere, be it via protection diodes in an IC or pull up/down resistors.Edit: Cool PCB explorer for looking at the traces:https://locator.reamiga.info/locator.php?project=A2000Edit 2: Scrub that - looks like its a clone that doens't have the exact same traces!

author: Sanxion
author_rank: Contributor
timestamp: October 08, 2023, 08:15:01 am
content: Thanks.I have only removed the RAM, RP503 and RP500. RP501 & RP502 remain. Should I remove them? They don't seem to be exhibiting any strange behavior. The other connection to this area is via 15 tracks from the Paula IC.What do you mean by "measure open circuit in ever possible combination"? Could you clarify please?Yes, there was battery damage until I cleaned it. I then repaired sever broken tracks. Before that, there was no activity from the CPU at all.

author: Sanxion
author_rank: Contributor
timestamp: October 08, 2023, 08:18:53 am
content: I agree - in my novice opinion - that an errant blob of solder would not result in a short that produces almost 20ohm's of resistance? Also, across three pins?Anyway...how would you suggest I apply power to this board in incremental amounts when it is powered by an almost standard PC AT power connector?

author: Buriedcode
author_rank: Super Contributor
timestamp: October 08, 2023, 11:18:44 am
content: Quote from: Sanxion on October 08, 2023, 08:18:53 amNo. Don't apply power to the "board" as in - its power connector. You make sure nothing is connected to the board - not even a ground/earth connection, or any accidental conduction from what its sitting on. Then, using acurrent limitedsupply (limited to say 0.25A) - also limit the voltage, to maybe 5V, so any bench supply, connect the probes of this supply between two of the shorted pins.The idea is that the current only flows between these shorted pins, via the traces to where the short actually is. The current is not to "blast" the short out or to do any damage, but to cause local heating in the area of where the short is. Using a volatile liquid such as isopropyl alcohol, it will evapourate fairly quickly at room temperature, but even faster where this local heating is. So if you see a dry spot forming - thats where the short is.I don't particularly like doing this since we're applying a voltage, to almost random points on a PCB that may or may not be connected to IC's. This is why I asked if these troublesome pins/traces are connected toanythingelse. If they're totally isolated, its ok, as current will just flow through the trace. If they are connected to say, VDD, and you connect the negative terminal of your supply to this, andaccidentlytouch, say an IO pin with the other probe, you apply -5V to an IC and possibly damage it.It's a handy way to find shorts, but not without risks. I only suggested it because it seems you've exausted other avenues.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 08, 2023, 11:26:37 am
content: Quote from: Sanxion on October 08, 2023, 08:15:01 amI mean, after removing these resistor packs and srams, all the address lines A0-Ax should measure open circuit, not showing any resistance.Any combination means you should measure open circuit in any address line agains anything, other address lines, pins, vcc, ground...Yeah only RP500/503 seems connected to the problematic lines. But just in case... remove the other packs (Easy job) and make more measurements.

author: Sanxion
author_rank: Contributor
timestamp: October 08, 2023, 11:39:39 am
content: OK...I will do this after lunch...a quick question. I have always been afraid to spray a circuit board with IPA whilst there is current through its tracks as I was concerned about the IPA causing a short?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 08, 2023, 12:40:10 pm
content: Totally the opposite, IPA is the best thing you can use for cleaning PCBs!You can drop in whatever you want, brush until shiny, rinse with more IPA and let it dry for a few minutes.IPA is not conductive, nor corrosive, and evaporates leaving no residues.If you didn't clean the board after all the messy job removing the ICs... well you might have left some solder blob hidden under the flux crust!Also some fluxes can cause this. I remember a JBC-branded flux bottle. Should be top quality stuff, right?Well, after soldering with it, no resistance was measured. Powered the board, some minutes later a small puf of smoke came out of nowhere and suddenly I had a very small resistance between VDD and GND!After more testing, it was clearly the flux, just applying a small drop, leave it for 5 minutes, again the miniature Hiroshima mushroom and VDD short.cleaning with solvent solved it everytime, and no damage was seen. A true mistery.I'm almost 100% sure it was this one. I clearly remember seeing the "Non conductive" tag, later showing the effect to my boss. What the hell is this?

author: Sanxion
author_rank: Contributor
timestamp: October 08, 2023, 02:50:42 pm
content: That didn't quite go to plan...After spraying the suspect area with IPA, I attached the positive and negative of my PSU to two IC pins and the power started beeping and turned it's voltage supply off...presumably because there is a short?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 08, 2023, 02:54:34 pm
content: Can't tell. Maybe it turns itself off because there's no RAM in the system?I don't know how the Amiga power distribution/control works.

author: Sanxion
author_rank: Contributor
timestamp: October 08, 2023, 03:22:06 pm
content: I am not using the Amiga power supply, I am using a bench one as per BuriedCode's instructions.When using the Amiga supply, the machine would only boot to a green screen.Should I perform the same IPA test with the Amiga supply instead?Something else I have noticed after removing RP501:The resistance of the short has dropped significantly:Pins 5 to 6, 6 to 7, 5 to 7: From 17 to 0.4Surely that is indicative of some progress?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 08, 2023, 04:30:28 pm
content: The psu shouldn't power off on OC, but keep the current constant. You might increase it a bit, 500mA should be fine.Which exact pins are you powering? I guess two of the offendign ones (SRAMs pins 5,6,7 ).

author: Sanxion
author_rank: Contributor
timestamp: October 08, 2023, 04:46:24 pm
content: I set the voltage to 5V, attached the positive lead to pin 5 and the negative to pin 6 of the first RAM IC. The bench PSU then started buzzing and I noticed the voltage had dropped to 0V.

author: Buriedcode
author_rank: Super Contributor
timestamp: October 08, 2023, 04:50:53 pm
content: Quote from: Sanxion on October 08, 2023, 04:46:24 pmDoes your power supply have a current limit/constant current setting? Just using a regulated voltage means the current flowing is limited only by the power supply itself - and the resistance of the short. I'm sorry if I wasn't clear, you should use a CC/CV (constant current/constant voltage) supply - one that will limit either the current OR the voltage to what you set it to. This prevents over voltage, AND over current - in the case of dead shorts, it limits the current to something sensible, and will drop the voltage considerably.Show us a pic of your bench PSU. It sounds like it tripped its protection to prevent sparks flying (a good thing).Edit: lots of typosEdit 2: Just thought - if you can get a photo of your bench supply - you can get a photo of the actual PCB you're working on! That way we can see whats been removed, any bridges etc..Don't worry too much about it being close up, resolution on phone cameras these days are good enough that one can see a fair bit of detail even if you're 30cm away from the board - as long as its in focus.

author: Sanxion
author_rank: Contributor
timestamp: October 09, 2023, 06:43:38 am
content: Good morning,I have attached a picture of my "power supply". I suspect you may be slightly underwhelmed by it.I'm afraid that there is little point in attaching an image of the pcb, allow me to explain why...Once I removed the RAM ICs - several weeks ago - I checked to ensure I hadn't broken any horizontal connections between the pin holes connecting the RAM ICs. Pin 1 to pin 1 etc. Once I was satisfied they were all fine, I soldered round pin sockets on to the board. I did thoroughly clean the area before and after soldering the sockets. Unfortunately, what I did not check for is the presence of shorts between the vertical connections.Therefore, all you can now see on the pcb is 32 IC sockets and two empty places for the resistor packs. If you think an image of that would help, I will send it.However, I will say that the symptoms of this issue appear to be exactly the same before I desoldered any of the RAM ICs as they are now. That is, an instant green screen on power up. That behavior has been constant throughout this exercise. Not that I am saying I have not inadvertently caused this, but it just feels as though the behavior of this board after I repaired the initial group of broken traces and before I desoldered the RAM has not changed.

author: Buriedcode
author_rank: Super Contributor
timestamp: October 09, 2023, 03:15:33 pm
content: Quote from: Sanxion on October 09, 2023, 06:43:38 amIt's fine, I'm sure it does the job. Unfortunately it doesn't lok like it has a constant current facility - so it'll display the current that is drawn, but doesn't allow one to set a limit. The good news is, it look like its limited to 2A, which might be limiting for powering things, but is a reasonably sensible current limit for finding shorts - a beefier power supply might have blown entire PCB traces. So that'll do for nowQuote from: Sanxion on October 09, 2023, 06:43:38 am32? Soldering IC sockets in place is a good idea as you can test out different chips, and means you're not constantly stressing the board with soldering/desoldering cycles - plus turned pin sockets are very handy as test points as they fit probe pins easily. But I'm not sure I understand the "32" ? I googled the Amiga 2000 motherboard and only saw 8 RAM chips. I'm clearly missing something (note: I have never worked on an Amiga 2000 before, I have no idea what its going to look like). A picture would still be handy so we know exactly what board we are talking about - it sounds like we're talking about two different boards here.Quote from: Sanxion on October 09, 2023, 06:43:38 amAgain, with the caveat that I'm pretty clueless about Amiga boards (so I don't know what faults cause what issues), are you sure that the RAM chips are the cause of the issue? An instant green screen on power up sounds like it could be any number of things. Very often on the forum someone posts about an issue they have a with a repair, and don't succeed, because they posted for help ontheirsolution they've chosen to a problem which was actually caused by another issue. A bit like an XY problem. I'm not suggesting your focus on the RAM is wrong - those pins shouldn't be shorted I dont' think, just that the problemmightbe elsewhere.So I sitll want a photo of the board - at the very least just to see what version the board actually is, and to see any damage from battery leakages etc..

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 09, 2023, 04:47:11 pm
content: Quote from: Buriedcode on October 09, 2023, 03:15:33 pmOlder revisions used 32 ram chips (r1-5). R6 switched to 8 chips.

author: Sanxion
author_rank: Contributor
timestamp: October 09, 2023, 05:46:58 pm
content: Thank you for the detailed response David, I really appreciate it.I have attached a picture of my motherboard. As you correctly deduced, mine is an early version with 32 RAM ICs.To answer your question regarding the fault being RAM related, I don't know that it is. However, the short between pins 5,6&7 definitely is not correct. Now, is that short the cause of the problem or a symptom of the problem?I did test each RAM IC when I desoldered them and found 3 to be faulty. So there was some benefit in enduring the pain of desoldering 32 old RAM ICs.My focus upon the RAM only occurred after I had exhausted all other avenues. I had already checked continuity from every pin on the processor, the Kickstart ROM, the main custom ICs, I even replaced the 84pin Agnus socket which was another traumatic experience. I was therefore left with no other option than to focus on the RAM.Despite what another contributor mentioned earlier about a 99.9% probability of having ruined any hope of salvaging this board, I don't believe that. I think there is a component that has failed or I have missed a track or perhaps there is an errant blob of solder somewhere.Isn't the fact that the resistance between the shortened pins on the RAM now being extremely low of any significance?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 09, 2023, 05:57:07 pm
content: Ouch, the usual destruction caused by battery leak!All I can say it to thorughly clean the board. Hard to see if it's battery deposits or just flux.Plenty of IPA and brush hard! Yeah definitely this would have been a lot better before soldering the sockets!SRAM pins 5, 6, 7 go to RP500 2, 4, 6. I see no traces in the top, so check the bottom!

author: Sanxion
author_rank: Contributor
timestamp: October 09, 2023, 07:03:47 pm
content: RAM pin 5 has continuity to pin 1 RP500 (and 3 and 5)RAM pin 6 has continuity to pin 5 RP500 (and 1 & 3)RAM pin 7 has continuity to pin 3 RP500 (and 1 & 5)Remember there is a short between 1, 3 & 5 on RP500.

author: Buriedcode
author_rank: Super Contributor
timestamp: October 09, 2023, 07:22:06 pm
content: Wow, I had no idea they used so many memory chips.. desoldering those must have been .. a task..Because I was assuming (wrongly) it was another board with only 8 RAM chips spaced out - I assumed you could see all the pins on each socket (top of the board, under the sockets). But now I've seen the board I see that you can't really see the pins 1-8 on most of the chips/socket... And given the fact that its consecutive pins that are shorted, I'm going with a solder short across those pins on one of those sockets.Thankfully the sockets are all aligned so you might be able to peek through to see any bridges between pins under the sockets. Perhaps adding white card against the board that you can see under all the sockets to highlight the gaps between pins - and anything bridging/blocking.Hopefully you'll spot something, but if not, you can sometimes clear a solder bridge by applying flux the the suspected bridged pins, and just reflowing the joints with your soldering iron (no excess solder on there, we're not trying to add more). Given that you can't really access the pints under the sockets on most of them, you'll have to heat pins 5/6/7 from the bottom side - with the flux applied with a syringe to the top of those pins.Good luck!Edit: Forgot to add, if you happen to have a really sensitive resistance meter - most multimeters won't be that sensitive but might be worth a try) you could check the resistance between say pins 5/6 on every socket. Assuming the short(s) are on one of those sockets, the one furtherest away will have slightly higher resistance since it'll be the resistance of the traces connecting it. This probably only works when you can read miliohms - but might be worth a try.Edit 2: David above mentioned about cleaning the board more. I just noticed how bad the battery spill was - whilst flux is rare conductive enough to cause <10k shorts, battery leakage certainly is. Flooding IPA under the sockets and perhaps a pipe-cleaner or q-tip between the pins won't hurt. It'll jsut make it smell of IPA.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 09, 2023, 07:29:59 pm
content: RP500 5 too? Doesn't make any sense! Brush like crazy the RP500 area, top and bottom.

author: dmulligan
author_rank: Regular Contributor
timestamp: October 11, 2023, 12:21:23 am
content: Quote from: Sanxion on October 09, 2023, 07:03:47 pmThe pins which David circled were the correct pins according to the schematic. RP500 pin 2 is DRAB0, 4 is DRAB1 and 6 is DRAB2. (DRAB comes from the schematic, it's the DRAM address lines between RP500 and the chips.)If you are seeing a short between RP500 pins 1, 3 and 5 but not 2, 4 and 6 then the problem is between U540 and RP500. The resistors in RP500 are 47 ohm resistors from 1 to 2, 3 to 4 and so on. What is the resistance between 1, 3, and 5 vs 2, 4 and 6 of RP500 with the resistor pack removed?

author: Sanxion
author_rank: Contributor
timestamp: October 11, 2023, 07:36:29 am
content: I was able to check continuity between RP500 and pin 5, 6 & 7 of the RAM on a working A2000 board and I can confirm:RP500 1 - RAM 5RP500 3 - RAM 7RP500 5 - RAM 6(with no continuity between 1, 3 & 5)On my non-working board, there is continuity between RP500 1, 3 & 5 - that is with the resistor removed.The resistance is 0.3 between 1 & 3 and 0.4 between 1 & 5. There is none on 2, 4 & 6.

author: dmulligan
author_rank: Regular Contributor
timestamp: October 16, 2023, 08:31:04 pm
content: Well, if you can't see anything on the bottom of the board, it must be on the top, under the ram sockets. The A2k motherboard was 4 layers with ground and VCC being the inside layers, so the fault has to be on the top of the bottom. The only things left to try are BuriedCode's suggestion of some flux on pins 5,6 and 7 under all sockets and reflow the solder, then wait for the board to cool and flush the area with IPA and apply voltage across those same pins. If rapid IPA evaporation doesn't show up you just may have to remove all of the ram sockets.Earlier you said the resistance was changing. Is that the case here?Another thought. Even though you don't have a microohm meter, if you measure just two pins, maybe 5 and 6 on all 32 sockets, does the resistance change? Start with U501, U508, U509, U516, U517, U524, U525 and U532. Maybe you can narrow down where the shorts might be, the resistance will be lower close to the short.

author: Konkedout
author_rank: Frequent Contributor
timestamp: October 18, 2023, 03:03:44 am
content: I have found shorts on bare pcbs. My method requires a multimeter capable of resolving 100 microvolts or so, and a power supply with either constant current control or a series power resistor capable of limiting current. For example a 5V power supply and a 5 ohm 10W resistor to limit the current. I find that 1 Amp is a good number. Current must be high enough to produce 100 uV in the traces, and low enough to not smoke anything.The approach is really a "poor man's" 4 wire ohm meter but very sensitive (100 micro ohms) and we do not care much about accuracy.So then you apply your ampere or so between the nodes that seem to be improperly shorted. And then, with your sensitive voltmeter, you look for the small voltage drops in the traces that will tell you which way the current is flowing.I had previously tried to burn out a short with high current (without necessarily finding it) but I do not recommend trying this.

author: Sanxion
author_rank: Contributor
timestamp: October 20, 2023, 08:23:42 pm
content: Update:During the last few days I have tried - to the best of my ability - to follow the suggestions above including the reflowing...I have even removed some "suspect" sockets to check under them. Unfortunately, none of my efforts have brought me any closer to finding this dreaded short. I examined the legs of the sockets whilst training a bright light under the board which worked surprisingly well...I must say that I couldn't see any errant solder at all, let alone a patch of solder bridging three pins. In fact, with these rounded IC sockets, the leg sits on top of the hole and the upper part of the leg is larger than the hole which should actually prevent any solder from seeping through...I then decided to do something drastic! So I bought a thermal imaging camera. I surmised this device should surely reveal the short. I proceeded to resolder the three resistor packs I had removed and started the machine. I then took some pictures...As you can see from the images, some of the custom ICs do indeed become warm...but the 32 RAM ICs remain cool...

author: dmulligan
author_rank: Regular Contributor
timestamp: October 20, 2023, 09:17:37 pm
content: At this point you are looking for traces or pads which are shorted, not bad chips. Remove the ram chips and the resistor pack again and inject voltage across the shorted pins and then look for sources of heat both on top and below the board.

author: Sanxion
author_rank: Contributor
timestamp: October 31, 2023, 06:52:59 pm
content: I am pleased - and slightly bemused - to announce that I have finally "fixed" the short...In the end I used the very fine abrasive rod that comes with the desoldering tool (it is used to clear the nozzle of any debris). I inserted it between pins 5 & 6 along each row of IC sockets and gently moved back and forth. I must say that it didn't feel as though anything had been dislodged and I didn't see anything pop out but lo and behold, the short has gone...I do still have the "green" screen on start up but at least I now know the problem is not due to the short!Thank you to all those who offered suggestions.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 31, 2023, 07:17:43 pm
content: Yeah definitely must have been residues from the battery leak, and probably you'll find more.