Reply 60 of 137, by PC@LIVE
snufkin wrote on 2021-11-28, 23:32:
What's your digital meter? You shouldn't get a big difference between the analog and digital meters. […]PC@LIVE wrote on 2021-11-28, 20:59:
Today I tried to rearrange the ideas about the failure of the 6ABX2V, I tried to understand what works and where the failure is, i.e. in which line, according to the measurements made with three different testers, I concluded that the measurements of the analog meter actually show what is in short, with the digital one there is a less reliable indication of short, for comparison I used the Atlas which measures resistances up to 0.01OHm, but some readings can put the doubt (especially if the trace is long?).
What's your digital meter? You shouldn't get a big difference between the analog and digital meters.
I don't think your Atlas can measure shorts, I think the resistance it shows is the ESR, which is the effective resistance at a high frequency. At a high enough frequency an ideal capacitor should measure as a short, but at DC it should (eventually) measure as open circuit. The problem here seems to be a DC short. If you've got some resistors around would you mind using your analog and digital meters to measure their resistance and make sure that your readings match the value given by the colour bands? That will tell you if the meters are working ok. Then use the meters to recheck the resistance from pin 2 of the regulator to Gnd (use a Gnd pin on the power connector).
Some power lines on a motherboard can be low resistance (my KA7 measures around 5 ohms on the CPU regulator output), so low doesn't necessarily mean there's a problem. At the start of this I think you said that a POST card showed the 3.3V supply as ok. We know from measurements that you took before that the 1.5V regulator has 5V going to it, and that the resistance from the 1.5V line to Gnd was about 0.1ohm (or less). That means the regulator would have to put out about 15A to maintain a 1.5V output, which is much too high. So there's a definite problem somewhere between that regulator and Ground. I would concentrate on finding the cause of that problem before worrying about things that might not be a problem (like the 3.3V supply).
If you're uncomfortable with having to keep removing SMD components until you find the one causing the problem then maybe the best thing is to go back to checking to see if any component is getting hot. The simple way (although not as accurate as a thermal camera) is just applying power to the board, then start putting your finger on various components until you find one that's hot. Hopefully something will heat up enough for you to find it before the 1.5V regulator overheats and the over-temperature protection cuts in. If you can't find anything, then it's back to removing components until you find the problem.
As a digital meter I use a small one (see photo), it is quite full of functions, something is missing, but for that I have another bigger one which is pretty similar.
Unfortunately I have a broken one with the capacitance meter, it has a stupid fault, the ignition switch is broken, I bought a new one but it has contacts on one side backwards, I have looked for one that is the same but there seems to be none made like this, those who sell are not good, I keep it aside because maybe I should modify the circuit to adapt it, and get that tester back working.
But you know I'm not a technician and I don't know how the components work, but I can apply a control technique precise enough to tell me if it's okay or not. I also use a comparison technique, that is by measuring a line (+) I note the measured values OHmx1, if I find strange values there must be a problem, if instead the values are lower there is usually something like mosfet or faulty diode, of usually these are shorted, the capacitors also thought, but now I understand that the resistance increases.
That was why I thought the + 3.3V had a problem, but if you tell me 1.2OHm can be fine, OK.
Yes, the regulator is connected to PIN3 directly at + 5V, in fact the measured value is + 4.97V, and the POST CARD has the + 3.3V led on, a sign that the circuit is working, then I don't know if the PCI slot voltage arrives direct from the + 3.3V (orange wire), for this I will do a check on the PCI pins.
Anyway okay I concentrate on the 1085 circuit, I try to remove the other SMDs and see if I find a faulty one, I hope it is one of those in the vicinity, maybe I also try to power up the MB (reassembling the one I removed first), and see if there is anything hot enough.
Apart from that, I worked in parallel on other MBs, one in particular I thought I could easily retrieve it, it is an ECS P4M890T-M2 with Core 2 Duo 6320, it had a swollen capacitor near the chipset, The ESR was 27.6OHm, I changed it with another one that was the same but of a different brand, the removed one was an OST, I changed it with a Rubycon.
Unfortunately, starting it goes into protection, I think I understand what it is, but for the moment I will keep it aside, I will return to take care of this and other MBs at a later time.
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