Metcal MX-500P-11 power comes on but no heat. - Page 1
Forum section: electronics / repair

author: labjr
author_rank: Frequent Contributor
timestamp: September 12, 2022, 02:49:27 pm
content: The surplus seller got me. I bought two surplus Metcal MX-500P-11 power units last year. However, I didn't have a handpiece to test them until yesterday. The power comes on and green LED comes on when the channel with the handpiece is selected but the tip doesn't heat.Both units have the same problem. I have two handpieces and they work fine in the third MX-500P-11 unit I picked up yesterday.Anyone know if there a particular failure with this model that causes these symptoms?Thanks

author: Runco990
author_rank: Frequent Contributor
timestamp: September 12, 2022, 07:35:27 pm
content: You didn't mention this in your PM to me. I'd check the capacitors and then the FET. You have already proven your hand pieces and tips are OK.

author: georges80
author_rank: Frequent Contributor
timestamp: September 12, 2022, 07:45:36 pm
content: I recently 'blew' up my mx500 power supply. Bunch of fault finding (schematics are out there) and I found that one of the small inductors right at the output (where the RF signal gets to the coax connector) had blown. I was (stupidly) soldering and had power active on the thing I was soldering (+12V at a few amps). There is a DC path through that inductor. The inductor failed and no output, though I did get an orange light in that case). Easy repair (with a second soldering station...) was to replace the little inductor.You can probe each stage of the RF path all the way along and easily find where the RF 'disappears'. It's actually quite a simple circuit/path.cheers,george.

author: labjr
author_rank: Frequent Contributor
timestamp: September 12, 2022, 07:56:18 pm
content: Quote from: Runco990 on September 12, 2022, 07:35:27 pmYes the handpieces work fine with a third MX-500P-11 I just got. I'll continue to post my progress in this thread.

author: quadtech
author_rank: Regular Contributor
timestamp: September 13, 2022, 08:07:37 am
content: I had these possible failure points in my notes , probably gathered from an old post here -- C4 C25 330n 63v (100) capacitors - also check R2, R3- breakdown of the Zener diode D2 1N4736A (6.8V)- "fried" resistor R2 180 Ohm ; R3 - 470 ohms- Fried D2 Zener - 6.8V 1W (1N4736A)- check Q1 (ZTX749) transistor (possible replacement with 2SA966, KSA473YTU, TIP32C or BD140 - check pinouts before installing !!!)- Check Q2 (possible replacement - BD139)- If Q1 failed, check R1 (100 ohms) also - may be faulty- Q4 may have failed- check C24, R27 & R28- check 100nf C0 (C5) and C24, as well as 330nf C21 and C25

author: georges80
author_rank: Frequent Contributor
timestamp: September 13, 2022, 08:08:46 pm
content: http://colinoflynn.com/wp-content/uploads/2012/05/MX-500P-11.pdfFault finding & schematic.cheers,george.

author: labjr
author_rank: Frequent Contributor
timestamp: September 13, 2022, 10:12:41 pm
content: Thanks for your reply,So far Q1 (ZTX749) tests bad. R1 (100 ohm) is smoked. C24 (.1uf 63v Wima) is melted.In this unit Q3 is an IRF510 and Q4 is an IRF640N. I can never seem to remember how to test MOSFETS. They both seemed to have a short but tested good using the little LCR-T4 tester. The problem is I practically ruined them by pulling them out. Might as well just replace them now.Can someone tell me what L1 is supposed to measure for DC resistance. I disturbed it while trying to remove D1. Not sure If I damaged it or not. Measures 62K in circuit. Then again it may have fried when Q1 went bad. I don't know.**EDIT I broke the lead of L1 off the board. I can touch it with a meter probe and it measures around 2ohms. I'll have to resolder it carefully.Source-Drain pads on PCB still tests 11 ohms with removed Q3. Will check C4. Do all these Wima caps randomly go bad?Thanks

author: labjr
author_rank: Frequent Contributor
timestamp: September 14, 2022, 04:18:38 am
content: Update:First Unit: Successfully resoldered L1. Can't go any further without new parts.Started troubleshooting the 2nd unit. C24 was also melted. I pulled Q1 and it seems to test OK. Can't find any other problems. Replaced C24 with one borrrowed from the first unit but still won't heat. RF generator won't start up. I don't think C24 melted on it's own but not sure what causes this issue? This is getting frustrating.I used a recently purchased third MX-500P-11 to remove and replace parts. It's more like doing precision dental work using a long bent tip. Works better than anything I've ever used.

author: labjr
author_rank: Frequent Contributor
timestamp: September 15, 2022, 07:11:22 pm
content: I had pulled Q1 from Unit#2 and temporarily resoldered it to the back of the PCB after testing it. Started working. Burnt my fingers.

author: labjr
author_rank: Frequent Contributor
timestamp: March 15, 2023, 01:31:14 pm
content: Update:After waiting 6 months for ZTX and IRF parts, I've successfully repaired both units. Thanks for all your help.

author: quadtech
author_rank: Regular Contributor
timestamp: March 16, 2023, 05:30:10 am
content: Great! What were the failed components in each unit ?

author: labjr
author_rank: Frequent Contributor
timestamp: March 21, 2023, 02:46:22 am
content: Basically, the usual components that fail.Unit #1 Q1(ZTX749) R1(100Ω) C24(0.1µF)Unit #2 C24(0.1µF)In one of them I replaced Q3 (IRF510) and Q4 (IRF640N) because I mangled them while trying to remove for testing. I probably could've reused them but I wanted a clean loooking repair. In Unit #2, I replaced Q1 for the same reason.

author: labjr
author_rank: Frequent Contributor
timestamp: December 27, 2023, 05:50:17 pm
content: I bought another MX-500P-11 on ebay. Seller said it was tested but has a blown buck converter IC (LM2576HVT) that's cracked in half. I'm wondering if anyone has seen this issue? Someone started another thread about the issue but looks like it was never resolved.

author: dufflespank
author_rank: Newbie
timestamp: July 28, 2025, 03:15:25 pm
content: My MX-500P-11 failed, and upon opening I also discovered the blown out LM2576HVT. Upon closer inspection, I noticed that one of the Wima film caps (C4) blew up, and probing across it (in-circuit) shows a short.

author: labjr
author_rank: Frequent Contributor
timestamp: July 29, 2025, 03:31:22 pm
content: I would say change the damaged parts and see if it fails again. I tried fixing one with a blown buck converter and it failed again. So not sure what caused the failure. However, I don't remember it having a shorted Wima cap. That may be the cause of your failure.I contacted Metcal tech support and they told me it's not a common failure. So they weren't much help.

author: dufflespank
author_rank: Newbie
timestamp: August 07, 2025, 06:02:44 am
content: OK so I replaced the buck converter and the wima cap.1. This appeared to have fixed the problem in that the cartridge is now heating up and working as normal.2. When I pulled the blown up Wima cap (C4) from the board and probed it out of circuit, it does not show a dead short, but rather several megaohms.3. I noticed a new problem. The station dissipates 30 W of power (measured with a kill-a-watt) when plugged into the wall, even if the switch is off and the green light is off. The unit gets quite hot after 20-30 minutes, kind of scary.Now I'm wondering if something else is wrong which is causing the unit to overheat, which eventually causes the buck converter to fail. I'm planning to probe pin 5 of the buck converter which is the ON/OFF control to make sure that it is off when the green light is off.

author: labjr
author_rank: Frequent Contributor
timestamp: August 07, 2025, 04:17:11 pm
content: That isn't working right. I measure about 3W when it's shut off. When I turn it on, it shoots up to about 70W then settles down at about 25w when the tip reaches temperature. With no iron connected, it stays at about 5W when turned on.So something else going on there. Check the parts I changed in my post above.Does it continue to heat the tip when it's shut off? Does it draw power with no iron connected?

author: dufflespank
author_rank: Newbie
timestamp: August 19, 2025, 06:16:01 am
content: OK fixed. Q1 (ZTX749) was broken, it was stuck on regardless of the base drive from Q2. I installed a new ZTX749 and now I get 2.4 W on the kill-a-watt when the green light is off and a normally functioning metcal station when the green light is on.

author: labjr
author_rank: Frequent Contributor
timestamp: August 19, 2025, 02:36:35 pm
content: Good job. Did Q1 measure good? Was it shorted? I would have checked Q1 in the unit I had? However, I didn't remove it to measure.

author: dufflespank
author_rank: Newbie
timestamp: August 20, 2025, 02:49:47 pm
content: I removed the failed Q1 from the board, and the DMM is showing a low resistance (consistent with dead short) from emitter to collector.

author: dufflespank
author_rank: Newbie
timestamp: August 20, 2025, 03:00:37 pm
content: Looking at the datasheet for ZTX749, absolute max V_CE is 25 V. Considering that the Q1 is switching 25-ish V to the RF generator, this would seem like very little margin against the rating. But looking at the schematic more closely, I think the 6.8 V zener D2 is clamping the collector voltage at 6.8 V when Q1 is off which should keep the V_CE comfortably below 25 V.