Author Topic: Need help with a Metcal MX500  (Read 10414 times)

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Offline kff

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Need help with a Metcal MX500
« on: January 18, 2015, 07:29:03 am »
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!


« Last Edit: January 18, 2015, 08:06:08 am by kff »
 

Offline zapta

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Re: Need help with a Metcal MX500
« Reply #1 on: January 18, 2015, 08:04:20 am »
You do have the schematic and circuit description doc, right? 

Just checking.
Drain the swamp.
 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #2 on: January 18, 2015, 08:05:25 am »
 

Offline zapta

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Drain the swamp.
 

Offline BurningTantalum

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Re: Need help with a Metcal MX500
« Reply #4 on: January 19, 2015, 06:55:15 am »
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
 

Offline BurningTantalum

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Re: Need help with a Metcal MX500
« Reply #5 on: January 19, 2015, 07:05:41 am »
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 ?
 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #6 on: January 19, 2015, 10:46:05 am »
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.

 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #7 on: January 19, 2015, 10:48:15 am »
Sorry, forgot to answer, the insulation seems ok everywhere.
 

Offline wraper

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Re: Need help with a Metcal MX500
« Reply #8 on: January 19, 2015, 10:16:40 pm »
Quote
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.
You can bodge one resistor to the schematic and feedback will be there. No need to desolder IC.
 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #9 on: January 20, 2015, 07:35:00 am »
You can bodge one resistor to the schematic and feedback will be there. No need to desolder IC.

Good 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!
 

Offline Corporate666

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Re: Need help with a Metcal MX500
« Reply #10 on: January 20, 2015, 09:37:02 am »
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. 
It's not always the most popular person who gets the job done.
 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #11 on: January 21, 2015, 09:49:12 am »
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?
 

Offline wraper

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Re: Need help with a Metcal MX500
« Reply #12 on: January 21, 2015, 11:52:57 am »
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?
I don't think so. Better to rewind it.
 

Online bingo600

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Re: Need help with a Metcal MX500
« Reply #13 on: January 21, 2015, 12:26:45 pm »
Wasnt there some torroid Winding info in the 13.5 MHz metcal psu thread

/Bingo
 

Offline N8AUM

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Re: Need help with a Metcal MX500
« Reply #14 on: January 24, 2015, 01:46:36 am »
Just a totally wild ass guess: Q4 goes into parasitic oscillation just long enough to wipe itself out ?

Good luck !!!!
 

Offline jbryan2

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Re: Need help with a Metcal MX500
« Reply #15 on: January 24, 2015, 08:53:46 am »
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!

I 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.


 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #16 on: February 01, 2015, 01:27:43 am »
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.
 

Offline wraper

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Re: Need help with a Metcal MX500
« Reply #17 on: February 01, 2015, 01:37:55 am »
Probably they had reliability issues and switched to beefier mosfet. Unless someone repaired it before you, unlikely that it is wrong.
 

Offline jbryan2

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Re: Need help with a Metcal MX500
« Reply #18 on: February 01, 2015, 03:09:44 pm »
One thing that I noticed is that my unit has IRF640N for Q4 instead of IRF530N.

Big difference in the transfer characteristics.  Three amps versus 20 amps at Vgs=5V.
 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #19 on: February 01, 2015, 06:14:16 pm »
Big difference in the transfer characteristics.  Three amps versus 20 amps at Vgs=5V.

Thanks 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.
 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #20 on: February 01, 2015, 06:49:44 pm »
Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?
 

Offline wraper

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Re: Need help with a Metcal MX500
« Reply #21 on: February 01, 2015, 11:11:13 pm »
Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?
Mounting tab is not gate but drain. Considering working frequency likely because of the capacitance.
 

Offline jbryan2

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Re: Need help with a Metcal MX500
« Reply #22 on: February 02, 2015, 02:37:27 am »
Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

Source is tied directly to ground. If the heatsink is tied to ground you had better isolate the tab/drain.
 

Offline jbryan2

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Re: Need help with a Metcal MX500
« Reply #23 on: February 02, 2015, 02:44:10 am »
Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

Oh, 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.
 

Offline wraper

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Re: Need help with a Metcal MX500
« Reply #24 on: February 02, 2015, 03:17:20 am »
Also, just curious, why does Q4 need to be double isolated from the heatsink? Is it to reduce capacitance between the gate and ground?

Oh, 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.
As I understand he asked about why 2 layers of insulation, not just one.
 

Offline BurningTantalum

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Re: Need help with a Metcal MX500
« Reply #25 on: February 05, 2015, 03:10:48 am »
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
 

Offline kff

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Re: Need help with a Metcal MX500
« Reply #26 on: February 05, 2015, 09:04:41 am »
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 strip

Basically, 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?
 

Offline BurningTantalum

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Re: Need help with a Metcal MX500
« Reply #27 on: February 07, 2015, 05:17:53 am »
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

 

Offline AndersAnd

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Re: Need help with a Metcal MX500
« Reply #28 on: February 09, 2015, 08:55:37 am »
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?
As 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.

P.S. Why you use IRF510?
It has a 0.5ohm internal resistance, may be something similar to IRFb4019 with 0.008ohm be more efficient?

P.P.S. Sorry, IRF630/IRF640/IRF740 i mean. Still, it is a UHF modulator ;)

Basically for two simple reasons:

- RF circuits are not really my field of expertise, so for the initial design i limited myself to the types that i had already used once in circuits operating at the same or higher frequencies, so i would know they should work in that application.

- Because i wanted to reduce the overall complexity of the circuit, i chose to use a readily available FET driver chip instead of the much more complex driver circuit used in the original Metcal supply. However, that chip is very tiny, and to avoid overheating i chose a FET with a really low gate charge and capacitance.

(Edit: Of course i might have been too paranoid with that. I just chose to stay on the safe side so i can get something to work correctly)

Of course that is not to say that only the IRF510 would work. But since i lack proper equipment to really evaluate other FET's, it would be up to others to see what else can be used there. The IRFB4019 that you mentioned looks interresting. It has even faster switching times, while still having a relatively low total gate charge. Might be worthwile to experiment with that one.

Greetings,

Chris
I've just repaired two MX500 power supplies (won at an eBay auction), with the help of the schematic and documentation here: https://www.mikrocontroller.net/attachment/193474/MX-500P-11.pdf
Mirror: http://scopetechniques.com/Metcal/MX-500P-11.pdf

The documentation says:
Q3 = VN0109N5 http://scopetechniques.com/Metcal/465-1342-0-VN0109.pdf / http://scopetechniques.com/Metcal/VN0109N5.pdf
Q4 = IRF530(N) http://www.irf.com/product-info/datasheets/data/irf530npbf.pdf

But in the two defect MX500's I bought they were different:
Q3 = IRF510 http://www.irf.com/product-info/datasheets/data/irf510pbf.pdf
Q4 = IRF640N http://www.irf.com/product-info/datasheets/data/irf640npbf.pdf
Both branded International Rectifier.

The amber/orange led lit up all the time in my two defect devices while the green led was always off.
I found out Q4 was shorted in both devices and after replacing it with a new IRF640N both devices work again.
It looks like Q3 (IRF510) had been replaced before in at least one of the devices, so not sure if they came with Q3 = IRF510 and Q4 = IRF640N from the factory. But it seems to work, at least so far.
Has anyone else seen an MX500 with Q3 = IRF510 and Q4 = IRF640N?
Looks like VN0109N5 from Supertex is obsolete, so maybe Metcal started using IRF510 instead?
Now when you search Supertex.com VN0109 seems to only be available in wafer / dies and TO-92 housing called VN0109N3, but no TO-220 option.
And for Q4, IRF640N is a 200 V MOSFET, while IRF530(N) from the schematic is only rated at 100 V, so maybe they have replaced it because there was problems with too high voltage spikes, toasting the IRF530's? Doesn't look like IRF530N is obsolete.
Haven't done any measurements after repairing them, so not sure about the voltages Q4 handles? But since the IRF640N was toast in both of them, maybe that's not the issue?


Much to my surprise I also received a Metcal solder stand with the defect power supplies, even though this wasn't advertised. A very nice surprise as Metcal solder stands are actually very expensive and I didn't have any. Later I received a new and very nice MX-H1-AV handpiece as a birthday gift.
This came from RS Components where MX-H1-AV seems to be quite cheap compared to many others for some reason, even though RS usually isn't know for being cheap. So if anyone needs a new MX-H1-AV alloy handpiece for your DIY, Metcal or Thermaltronics/Easy Braid power supply, try to check out the price at you national RS Components site.
« Last Edit: February 09, 2015, 09:06:15 am by AndersAnd »
 


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