Need help with a Metcal MX500 - Page 1
Forum section: electronics / repair

author: kff
author_rank: Contributor
timestamp: January 18, 2015, 07:29:03 am
content: Trying to fix a Metcal MX500. Q4 dies as soon as the unit is switched on, with all three terminals shorting to each other (2-6 ohm resistance). I've double checked all capacitors, transistors, diodes, and now inductors on the board, and everything looks ok. All capacitors that could have any effect on Q4 have been checked out of circuit. With Q4 removed from the board, the voltages on Q4's gate and drain look ok (checked with an oscilloscope), nothing that would exceed the +/-20V gate/source and 200V drain/source ratings of IRF640N. Some questions:1) In what scenario would an N-channel MOSFET fail in such a way that all terminals are shorted to each other? Overvoltage on the gate?2) Is it common for capacitors to test ok at lower voltages (as tested with a multimeter), but fail at higher voltages? What would be a good way to test capacitors that normally operate at ~50V and 13.56Mhz?3) What other components typically fail when Q4 dies?4) Any other tips on how I should proceed?Many thanks!

author: zapta
author_rank: Super Contributor
timestamp: January 18, 2015, 08:04:20 am
content: You do have the schematic and circuit description doc, right?Just checking.

author: kff
author_rank: Contributor
timestamp: January 18, 2015, 08:05:25 am
content: Yes, this one:http://www.mikrocontroller.net/attachment/193474/MX-500P-11.pdf

author: zapta
author_rank: Super Contributor
timestamp: January 18, 2015, 08:22:54 am
content: Quote from: kff on January 18, 2015, 08:05:25 amYes, that one.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: January 19, 2015, 06:55:15 am
content: Overdissipation?U4 can cause grief on this circuit, with 56V on the output if it fails (if my memory serves me...)I also have some recollection of a strange heatsink clamp on this unit (not the metal bar that clamps the aluminium plate but the clamp on the device tabs. One of the transistors has a tin-plate clamp, different to the rest - also are the insulating washers fitted and correct?Regards, BT

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: January 19, 2015, 07:05:41 am
content: Just looked at the circuit -Have you a known good wand connected to the unit?With Q4 out of the circuit you should still get a green LED to indicate that the wand RF coil is present and intact, after cycling the power switch of course.Is all OK on the other side of T1 ?

author: kff
author_rank: Contributor
timestamp: January 19, 2015, 10:46:05 am
content: Thanks BurningTantalum.I do have a working handpiece, and the green light is indeed on even with Q4 removed from the circuit.I am getting a reasonable-looking 13.56mhz sinewave at the gate of Q4. The voltage amplitude is a bit higher than it should be, but that is expected with Q4 removed, as there is no feedback going to U4.Also, I do see 56V on the output of U4. But again this is what I would expect when there is no Q4 (U4 tries to boost output voltage until it sees 1.23V on the feedback pin, and without Q4, the feedback voltage never increases). I suppose I can desolder U4 and try it out on a breadboard with some resistors.

author: kff
author_rank: Contributor
timestamp: January 19, 2015, 10:48:15 am
content: Sorry, forgot to answer, the insulation seems ok everywhere.

author: wraper
author_rank: Unknown
timestamp: January 19, 2015, 10:16:40 pm
content: QuoteYou can bodge one resistor to the schematic and feedback will be there. No need to desolder IC.

author: kff
author_rank: Contributor
timestamp: January 20, 2015, 07:35:00 am
content: Quote from: wraper on January 19, 2015, 10:16:40 pmGood advice. With a 4k resistor across pin 4 of U4 and the positive side of C8, I am seeing 2.4V, which looks right. So U4 appears to be ok.Pretty stumped at this point. Perhaps I can keep the feedback resistor on U4 to limit the output voltage, solder another Q4, and do some probing if it doesn't die right away?Other troubleshooting tips would be much appreciated!

author: Corporate666
author_rank: Unknown
timestamp: January 20, 2015, 09:37:02 am
content: I have an MX-500 with exactly the same problem and manifests exactly the same behavior as yours.Stumped me too but I didn't have a lot of time to mess around with it. Hope you will keep us updated if/when you figure it out.

author: kff
author_rank: Contributor
timestamp: January 21, 2015, 09:49:12 am
content: For me, it was a short in T2 -- after desoldering it, the resistance between all 4 terminals is 0.3 ohms. Here is how I figured this out:1) Made a poor man's signal generator by ordering an AD9850 module on Amazon (http://www.amazon.com/Huhushop-Active-Generator-0-40MHz-Equipment/dp/B00IR5I886/, $9.99 shipped) and hooking it up to a TI Tiva launchpad board ($12 shipped). Took only 30 minutes to figure out the protocol and have the board generate a 13.5Mhz square wave with +-2V amplitude.2) Hooked up the generator across C26. The output at the coax connector was only +-0.3V, so there was leakage somewhere. Desoldered C25 to figure out whether the leakage was to the right or left of it -- it was on the transformer side (i.e. without C25, there was no leakage).3) Checked capacitors C21 and C22 and they were fine. At this point it could only be T2.Now, the question is how to replace T2. The schematics specify it as "transformer toroidal 9uH, 10.75 turns each side". Would any other 9uH transformer work here, as long as the number of turns is equal?

author: wraper
author_rank: Unknown
timestamp: January 21, 2015, 11:52:57 am
content: Quote from: kff on January 21, 2015, 09:49:12 amI don't think so. Better to rewind it.

author: bingo600
author_rank: Super Contributor
timestamp: January 21, 2015, 12:26:45 pm
content: Wasnt there some torroid Winding info in the 13.5 MHz metcal psu thread/Bingo

author: N8AUM
author_rank: Regular Contributor
timestamp: January 24, 2015, 01:46:36 am
content: Just a totally wild ass guess: Q4 goes into parasitic oscillation just long enough to wipe itself out ?Good luck !!!!

author: jbryan2
author_rank: Contributor
timestamp: January 24, 2015, 08:53:46 am
content: Quote from: kff on January 18, 2015, 07:29:03 amI would suspect excessive current since it's a 100V FET. Make sure Q4 is not shorted to the heatsink (look at the notes at the end of the parts list). Pull T2 and check the inductances and that the windings are isolated. Work downstream from there.

author: kff
author_rank: Contributor
timestamp: February 01, 2015, 01:27:43 am
content: After rewinding T2, there is still no heat produced even though the green light is on, but at least Q4 doesn't blow anymore. Something else must be broken somewhere.One thing that I noticed is that my unit has IRF640N for Q4 instead of IRF530N. This is a newer model made by Oki, serial number > 120,000. Is it possible that Oki switched to IRF640N recently, or do I have the wrong MOSFET in my circuit? Another difference is that R10 is ~17K instead of 4.7K.

author: wraper
author_rank: Unknown
timestamp: February 01, 2015, 01:37:55 am
content: Probably they had reliability issues and switched to beefier mosfet. Unless someone repaired it before you, unlikely that it is wrong.

author: jbryan2
author_rank: Contributor
timestamp: February 01, 2015, 03:09:44 pm
content: Quote from: kff on February 01, 2015, 01:27:43 amBig difference in the transfer characteristics. Three amps versus 20 amps at Vgs=5V.

author: kff
author_rank: Contributor
timestamp: February 01, 2015, 06:14:16 pm
content: Quote from: jbryan2 on February 01, 2015, 03:09:44 pmThanks for noticing this. IRF640N is probably wrong then, I will go ahead and order some IRF530Ns. The unit was repaired by someone else before me, and given that Q4 was blowing due to a short in T2, it is not surprising they decided to go for a higher rated transistor.

author: kff
author_rank: Contributor
timestamp: February 01, 2015, 06:49:44 pm
content: Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

author: wraper
author_rank: Unknown
timestamp: February 01, 2015, 11:11:13 pm
content: Quote from: kff on February 01, 2015, 06:49:44 pmMounting tab is not gate but drain. Considering working frequency likely because of the capacitance.

author: jbryan2
author_rank: Contributor
timestamp: February 02, 2015, 02:37:27 am
content: Quote from: kff on February 01, 2015, 06:49:44 pmSource is tied directly to ground. If the heatsink is tied to ground you had better isolate the tab/drain.

author: jbryan2
author_rank: Contributor
timestamp: February 02, 2015, 02:44:10 am
content: Quote from: kff on February 01, 2015, 06:49:44 pmOh, and the gate is tied to ground through L10. If you don't isolate the tab/drain then all three pins are - glory be! - tied to ground. It's not going to do much that way.

author: wraper
author_rank: Unknown
timestamp: February 02, 2015, 03:17:20 am
content: Quote from: jbryan2 on February 02, 2015, 02:44:10 amAs I understand he asked about why 2 layers of insulation, not just one.

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: February 05, 2015, 03:10:48 am
content: KFF - if you are still around.I have several MX500s but all are a long way from me at present. I have a unit here that had a dead U4 and a dead Q4. I replaced both and the unit worked again but obviously not correctly as I could not set the output of U4 correctly.The dead Q4 was an IRF640 so I replaced it with another as I had one in stock. I have now ordered, as you have, some IRF530Ns.I some memory of Q4 having an insulation on the tab that was not standard - some extra washer and a tinplate 'clip', but it was obviously missed off this unit when the original Q4 was replaced.Could I trouble you for a brief description of it ? Is it simply an extra thick insulating pad ? I pic would be above and beyond the call of duty if you haven't reassembled the unit yet !Thanks in advance,Burning Tantalum

author: kff
author_rank: Contributor
timestamp: February 05, 2015, 09:04:41 am
content: Don't have a pic at the moment, but basically you need:- A spacer to insulate the MOSFET tab from the screw (and hence the heatsink). This is a small white plastic part that slides inside of the hole in the heatsink- A piece of tape to insulate a small grounded metal strip from the heatsink (see below).- A small strip of metal that is grounded. Why it is needed, I am not sure (the heatsink is also grounded)- Another piece of tape to insulate the tab of the MOSFET from the grounded metal stripBasically, you don't want the transistor tab to touch the grounded metal strip, the heatsink, or the screw. The grounded strip should not be touching the heatsink.I think the double insulation is needed to reduce the capacitance seen at the gate of the MOSFET (it needs to switch quickly).I wonder if IRF640 is indeed the right transistor for Q4 in later versions of MX500. There are quite a few reports of people seeing them in Metcals on Google -- could they all be replacements? It turns out I also have IRF510 for Q3 in my non-working unit. Is there any chance that Metcal ran out of their stock of VN0109N5s and starting substituting Q3 and Q4?

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: February 07, 2015, 05:17:53 am
content: Thanks for that , KFF.It jogged my memory and now I recall the tinplate 'tab' with clearance hole for the plastic bush, fitted with an insulator either side of it.I have just removed the PCB from the case and can now see the pad and hole on the PCB to accept a pin on the metal tab. The pad is indeed connected to the ground plane/track. I cannot for the life of me see how this is any different to a single insulator mounted to the grounded aluminium heat sink, but I'm not too hot on RF and this IS 13.5MHz.The only observation is that the aluminium heatsink is grounded only via the tab and ground lead of the 18V reg, and possibly the tab and ground lead of U4, so from a capacitance point of view the metal tab serves a purpose either to isolate Q4 from noise from U4 or vice versa.I guess I will fabricate a metal tab when the IRF530 arrives. I have since found an IRF614 in my MOSFET drawer - this seems to have a closer transfer characteristic to the IRF530 than the IRF640, but as replacing this device is a bit fiddly I will await the correct device type.I seem to recall a discussion about a replacement for the original and obsolete Q3 but cannot locate the thread.Many thanks, BT

author: AndersAnd
author_rank: Frequent Contributor
timestamp: February 09, 2015, 08:55:37 am
content: Quote from: kff on February 05, 2015, 09:04:41 amAs I mentioned in my post quoted below I bought two defect Metcal MX500 power supplies on eBay almost a year ago. Both came with Q3 = IRF510 and Q4 = IRF640N.Q4 were shorted on both devices, and I replaced them with new IRF640N's and they have both been working since.However they haven't seen too much use since as they are only for private use. So I can't tell about the longevity with these transistors for all day indsutrial use. But so far so good and both IRF510 and IRF640N are easy to get hold off, unlike the obsolete Supertex VN0109N5.Quote from: AndersAnd on April 28, 2014, 03:27:12 pm

author: stretchyman
author_rank: Regular Contributor
timestamp: September 19, 2024, 03:29:09 pm
content: An old topic but a good one as many of these irons are available for under £/$200 with bits.If you want a decent soldering iron there really is no contest, these are superb.Just fixed two and read all the advice on here which helped considerably.The most common fault it would seems is Q1 going bad. I can remember replacing this (ZTX749) on a station years ago and it fixed the problem (green light but no heat to bit).However the two I've just fixed were very similar but Q3 (IRF510 is fine) and R1 needed replacement too but still no output.Measuring around with a 'scope the RF waveform was present at Q3 drain but not at Q4 gate.C24 checked and was ok so suspected C4 (this is mentioned quite a lot here) and yep was very low in value measuring around 500pF so dumping all the RF.I'm thinking C4 going bad will be the main cause of failure here so best to change it (I fitted a 470n 100V as it was all I had).BTW there's mention of an extra tab on Q4 and some confusuion as to it's use.Well people it's a cheap high voltage low value capacitor for Q4 as this circuit is class E not C.If anyone has any links or info on how to corretly calibrate these units I'd be very greatful.Happy SoLdering (Sodering is something else !)J.

author: reddish75
author_rank: Regular Contributor
timestamp: March 13, 2025, 09:21:04 pm
content: Could anyone advise the wattage of resistors R1 R2 and R3 on the metcal MX500?

author: thm_w
author_rank: Super Contributor
timestamp: March 13, 2025, 10:25:12 pm
content: Quote from: reddish75 on March 13, 2025, 09:21:04 pmR2 says 2W in the schematic above. R1 and R3 I would assume are 1/4W.But you can just measure the size of the resistor and that will give you its near enough wattage.

author: reddish75
author_rank: Regular Contributor
timestamp: March 14, 2025, 08:48:17 am
content: Great thanks,Im collecting some spares for mine just in case so...Has anyone used Bd140 and Bd139 forQ1 and Q2, do these need heatsinks?Has anyone found a replacement power switch?

author: benj38
author_rank: Frequent Contributor
timestamp: March 14, 2025, 03:49:28 pm
content: Quote from: reddish75 on March 14, 2025, 08:48:17 amAre you referring to the modern versions of the MX-500 with an LCD display and a round power button?If you are, then this is a replacement power switch for the MX5200 and MX5000, which I believe have the same switch as these modern MX-500:https://www.mouser.com/ProductDetail/ZF/RRA32H3BBRLN?qs=4EnmSMTL3HMobUXvdJZgpg%3D%3D

author: reddish75
author_rank: Regular Contributor
timestamp: March 14, 2025, 09:17:42 pm
content: Referring to the original MX500

author: benj38
author_rank: Frequent Contributor
timestamp: March 15, 2025, 08:18:02 pm
content: Quote from: reddish75 on March 14, 2025, 09:17:42 pmThese are as boring and easy to find as a 10K 1/4W carbon film resistorFor example:For 110V USA market units:https://www.amazon.com/Position-Rocker-Toggle%EF%BC%88Quality-Assurance-Years%EF%BC%89KCD3-101/dp/B07MV56LKS/?th=1For 240V units on the other side of the pond:https://www.amazon.co.uk/eROOSY-Position-Motorcycles-Household-Appliances/dp/B098JDZS5J/

author: stretchyman
author_rank: Regular Contributor
timestamp: March 22, 2025, 09:11:59 am
content: I'd mentioned this in another post as there seems to be some confusion over the 'extra' metal pad on the output fet.Its a capacitor from drain to ground as this is needed as the design is class E.

author: reddish75
author_rank: Regular Contributor
timestamp: March 26, 2025, 10:23:06 am
content: Well, I've received my MX500 and...... the it doesn't heat the iron, ( iron is OK) looking through the service document posted all is OK until we get to U4 this measures on pin 4 261mvdc and pin 5 bounces between 2-15mvdc, obviously something not right here! Where to look next?Also Q4 is measuring short between s and g with the fet out of circuit?

author: reddish75
author_rank: Regular Contributor
timestamp: March 26, 2025, 11:20:27 am
content: And after further testing pin 2 on U4 when an iron is connected goes up to 56vdc instead of i think 18vdc

author: quadtech
author_rank: Regular Contributor
timestamp: March 27, 2025, 06:09:00 pm
content: Per post #4, 56V on U4 could mean it is bad.See post #9 earlier in this thread to test U4.Some potential failure parts to check and possible replacements -R2 - 180R 2WR3 - 470 ohmsR27/28 - 100R 1WC0/24 - 100nF,63VC21/22/25/4/1/7 - 330nf,63VD2 - 6.8V 1W Zener (1N4736A)Q1 - TIP32C (or BD140)Q2 - BD139Q3 - IRF510NQ4 - IRF640N