Need help with a Metcal MX500

[BurningTantalum]

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

[kff]

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?

[BurningTantalum]

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

[AndersAnd]

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.