Hello! This is my first post on the forum, so apologies in advance if I buck any conventions. Last week I bought a used and ostensibly working MX-500P from eBay, along with a brand-new cartridge and -AV hand-piece from Amazon. I plugged it in and it heated up, but I almost immediately realized I forgot tweezers so I turned it off, set it down, and about 40 s or so I came back and found the tip glowing red so I unplugged it. A minute or so later, I plugged it back in, heard a pop and then something rattling around the case.
Here's what I've found so far:
- It still had an OKI International warranty sticker, so it hasn't been opened before.
- My unit is a MX-500P (not MX-500P-11 or MX-500P-21). As far as I can tell, this just means that C53 is soldered on the back of the PCB, just like C7 on the schematic.
- The top and bottom of C6 (50V 2200uf) were expanded (the rubber on the latter was ruptured) and a lead had come free of its solder joint. I imagine the expansion was pre-existing and the shipping pulled the lead free.
- U4 (LM2576HVT-ADJ voltage regulator) had exploded - part of the front of its package was what I heard rattling around in the case.
- Nothing else showed any signs of damage.
I replaced C6 and U4 and now the unit is stable and the LEDs are behaved but the iron does not heat up. Following the repair notes, I checked the FB pin on U4 and it's about 0.3 V when it should be 1.3 V. On a whim, I checked the two generator MOSFETs and, if I'm testing it correctly, it seems the gate and source Q4 (IRF530 N-channel MOSFET) are shorted (I removed it first, since I believe the gate should always appear shorted to ground due to L10 right next to it).
In typing this, I also just realized that I made a mistake in reconnecting C53, which should have been between pins 2 and 4 (OUT and FB), to pins 2 and 5 (OUT and OFF).
So, I have a few questions for how I proceed:
1. The original International Rectifier IRF530 (
https://www.infineon.com/dgdl/irf530npbf.pdf?fileId=5546d462533600a4015355e386b1199a) is discontinued. Is Vishay's
https://www.mouser.com/datasheet/2/427/irf530-1768291.pdf a valid substitute?
2. I presume Q4 was damaged by U4. Is there anything else I should look at when I order a replacement for Q4?
3. Does it seem like my mistake with C53 should cause any significant damage? Nothing seems wrong that I can see - perhaps I can just rewire it and hope for the best?