| Electronics > Repair |
| miller dynasty what is precharge relay |
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| coppercone2:
I replaced all the DIP chips on that board. It seems to work... better? It does not always turn on, but it seems to stay on. I still wanna replace that controller chip on there. I do recall finding a busted CD series gate on the other board (which was replaced instead of repaired). Maybe those rails took damage,. I also notice, when it fails to start up, the BUS voltage is around 310V, and the +30V and +15V is OK, but -12 (confirmed on another forum that its not -15, but -12) is like -8 V. The relay "pin" is 4V when its on, and 30V when its off. So its like there is a device that is... effecting the -12V rail.. sometimes. Now I don't see any negative -12V regulator on there, but there is a coil. I wonder how the hell its getting negative voltage I found this drawn by john76 https://forum.millerwelds.com/filedata/fetch?photoid=597488 ANy idea what could cause that regulator to act up intermittantly? They are using the NAND gate to make some kind of oscillator to power the transistor? The 30V is there on mine, but I get like 8V from the negative thing. Can someone explain to me WTF is the point of that crazy circuit. They really need all that to get a minus voltage?!?!! |
| coppercone2:
well interesting it turned on 10/10 times after it was left alone for a while. I guess the solder needed a bit of time to set (people have a misconception that it works as soon as its solid) ??? I have no other explanations. I am gonna give it a try some time in the future when it has a chance to acclimate to its new parts ??? the electrons got used to it ? ??? the only thing I did different when it started to work, is I flicked the mains switch rather hard, instead of letting the spring do the work, I kinda slammed it. I guess it pushed the electron into correct spot ??? |
| DisasterNow:
But what output signal do you have on this welder? Just a dc voltage? And what value should it be? |
| coppercone2:
The output was getting near the usual 80V before it shut down, but I did not try it on load with an arc, because I thought it might make damage worse, until I prevent it from shutting down randomly and killing the display However I am happy to report that after changing the opamps, comparator, logic gates on the boost board, and cleaning the small amount of dust on the inverter control board (even though its insulated with manufacturers varnish) it does seem to spark an arc and weld. I am waiting for a vial of silicone lacquer from MG chemicals to re coat the PCB that I worked on, then I will reassemble it. I will leave the LEM current sensor and relay in parts storage if their ever needed. I had to scrape off all the old varnish with mg chemicals varnish remover so I can work on the PCB Based on that picture schematic someone drew of the board, I started to think that the NAND gate is acting like a oscillator for the -12V rail, and that it was damaged.... maybe thats why the -12V rail would drop to like -7 and dance around. I don't know HOW that switching power supply works, I don't think I want to know, I think I would use a control chip instead of whatever the hell they tried to do there. Also, I have no idea why that 20V IGBT driver power supply diode fried, its not related to anything. But I did replace it with a 5W diode, when it was a 1W diode before. The other two diodes I modified for 5W, but I left one of the smaller ones in place (3x 20V isolated rails). I feel like that diode ends up breaking if anything goes wrong on the two cards. I think one of em controls a switch that makes the input voltage for the flyback transformer, and when one of the rails get messed up, it makes too high a voltage and blows the diode (maybe goes to very high duty cycle). Or that its just not specced properly, and needs a very robust diode to survive on 1W, because other resistors on the PCB for this welder are known to brown.... again I would have put them on standoffs away from the PCB and used a higher value resistor! |
| DisasterNow:
It is better to cover with lacquer, after all these are machines that are used in environments where there are many iron splinters. When I opened mine, there was a lot of metal dust on the board, without lacquer I think it would break down quite often. A test would be to try at least a couple of electrodes continuously at high amperage. I would avoid 3.25mm electrodes, in my opinion on these machines they only serve to stress the power part too much. |
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