Metcal MX-500P blown U4 (LM2576HVT) - Page 1
Forum section: electronics / repair

author: Metcal4Ever
author_rank: Newbie
timestamp: August 01, 2025, 01:27:00 pm
content: Hello, I have a Metcal Soldering Station model MX-500P that died while being only connected to mains but not turned on. Main failure is U4 which shows a crack in its body at the level of pins 1 and 2 (Vin and Output respectively). I noticed the failure when I tried to use the iron and it did not heat up. In the primary failure other components also broke:- Q1 (ZTX749) with leads melted and burning mark on the PCB.- Q3 (YN01D9N5 N-Channel MOSFET) by shorting source to drain.- Main Filter Capacitor C6 by bulging.- U4 (LM2576HVT) Blown.All other components in the PCB were tested meticulously both passively and under load, and all tested good. Iron was also tested in another unit and worked properly.The faulty components were replaced noticing that the original main filter cap is rated for 50 volts and the circuit when functioning normally handles 53 volts at this point (bad design?). Also noticed that U4 Vin is 53 and its rated for 60 volts max input. I found some articles mentioning that LM2576HVT can fail in some situations when Vin is greater than 48 volts.I proceeded to repair by replacing all faulty components but the unit keeps blowing LM2576HVT as soon as the iron is set to heat or sometimes even only by connecting the unit to mains. U4 blows in few seconds by making a whining noise. At this point the unit has broken three U4 replacements and the in these failed repairs only U4 gets damaged.I have tested all components in the board until frustration and now I am thinking that maybe the problem is the component itself (U4) that is not the exact original and maybe in this particular circuit fails even with Vin in range.I wonder if anybody has solved this same issue and consult also to the experts in the forum in regard to some ideas about maybe modifying the circuit to reduce the Vin voltage to the LM2576HVT as I don't think it requires 53 volts to work properly (maybe 40 volts would suffice) and since U4 blows even without the wand being active then its most likely because of over-voltage considering that at this point the IC is not even working and no current is supposed to be pulled from it. I also tested current draw at output pin anyways just to be sure and no current is pulled when the iron is not active and yet U4 blows sometimes even when the unit is connected to mains with main power switch off. Also no high frequency (13.56 Mhz) is detected at LM2576HVT pins with the oscilloscope.Finally I carefully read all posts in this and other forums about this particular failure but in none it has been solved. Metcal itself said its not a common failure (I disagree - this problem not only affect the MX-500P but also the Metcal 5000 series) and that they can not help. I would appreciate any help and thanks to all.P.D. I know a working used unit can be purchased in ebay by around 200 dollars. But this is mainly about the repair exercise.

author: tszaboo
author_rank: Super Contributor
timestamp: August 01, 2025, 01:33:48 pm
content: There is a schematic posted here, might help:https://www.eevblog.com/forum/repair/metcal-mx-500p-coldIt's a good station, hope you can repair it.

author: benj38
author_rank: Frequent Contributor
timestamp: August 03, 2025, 10:42:54 am
content: I still have an MX-500P with the exact same problem that I never managed to repair. Everything checks out fine (I even replaced the inductors), but then the LM2576HVT blows. After the third LM2576HVT blowing I gave it up, and it is sitting in my scraps bin ever since.Sure, if I were to wage an all out war on this thing I would have found the problem, but the list of other projects is too long.If you chose to fight this to the bitter end, and find the solution, please make sure to report your findings here.Good Luck!