[FIXED] Repair of Metcal RFG30 Power Supply - requesting assistance

[Johndi]

Hey All,

So I have a Metcal RFG30 power supply that I've purchased because it was knackered.  After opening it up, I took some pictures (after staring in marked wonder at the simplicity and straightforwardness of the circuit boards) of the circuit boards and a gratuitous one of the transformer.  The unit was stated as being unable to power on, and from the first picture I can see that the R8, R9, and R10 resistors are burned out.

I've read the thread https://www.eevblog.com/forum/projects/broken-metcal-rfg-30-soldering-base-unit-fixed/ and I also have located the thread https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply/.

As such, I have the Metcal MX-500 schematic, and what's listed as values for those resistors are R8 = 3K3 and R9, R10 = 4K7 5% Carbon Film resistor. 

Now, I can blindly replace the parts but I was wondering if someone could help me identify what may have caused the damage shown in the first picture?  I don't happen to have a scope handy, nor to I have a handle for testing (looking for a deal on ebay) so I'm restricted to my cheap multimeter for time being.

Besides the resistors, what should I consider replacing while I'm at it?  For any general jellybean stuff I'm going to grab from Tayda Electronics as I think that everyone has raided all the resistors from Radio Shack, and I can't remember if they carried 5% ones anyway.

Thanks for any assistance and thanks to all that have posted before.  Thanks to the DIY thread, I know all the options for finding a new handpiece if it comes down to it.  If I have to purchase new, I'll probably go with Hakko's handpiece, although there are a bunch of used tips that are still in great condition available for Metcal's MX-RM3E.

[poot36]

I would start by checking all of the semiconductors for shorts and also the capacitors as well.  It looks like the board with the burnt resistors receives power from the first board, I would start by disconnecting that board and testing the voltages out of the first board are correct.

[Richard Head]

Also, use a DC bench PSU with the current limit set low to supply the DC input rather than the mains transformer. That way if there is still a problem you won't get too much smoke!

[Pat Pending]

You may have the wrong schematic for the MX500. This is an earlier model spread across 2 pcbs, later models had just 1 pcb.

C12 couples RF into the gate pin while the parallel combination of R8, R9 & R10 (all 100R) pull the gate to ground.
The source is grounded and the drain sees the full PSU voltage. 
Most likely the power fet has developed a internal short between gate and drain.

[Johndi]

Thank you for the responses.  Unfortunately I don't have a benchtop DC source yet, but I do plan to build a simple linear one.

Pat Pending: thank you for the clarification.  You're right, I do have the wrong schematic as I have the one for the single board unit.  I can't find one for the RFG-30 and I was blindly hoping that the later ones weren't changed too much.   I think the FET you're speaking of is the TO-3 package UFN130, Q3 I think?  The only other FET I see is the one labeled VN01065N5 that's a TO-220 package. 

You don't happen to know the value that C12 is supposed to be do you?

Edit: Disregard my question about the caps, I see the ones on mine are CK06BX103K so according to Mouser and Digikey they are 10,000pF 200V ±10% MLCC.

Should I go ahead and replace the power supply capacitors while I'm at it?

[Pat Pending]

UFN130 yes that's the one, you may find that gate to drain measures as a short.

Q2 in my case is a Siliconix VN67AD.

Judging by the 87 date codes replacing the five electrolytics can't hurt
but before doing so, I'd try find that elusive Metcal handpiece first.

[Johndi]

Thank you for sharing.  From the cross reference sheets I found, it looks like I just need to go tripping through the list until I find a part number still in available in TO-3 (NOS most likely) and purchase a couple of them.

Looks like someone experienced the same problem here: http://sci.tech-archive.net/Archive/sci.electronics.design/2009-03/msg01172.html and they seem to have replaced theirs with an IRF140 (original was IRF130), and the IRF140 looks to be available from US sellers on eBay in TO-3 package.

[Pat Pending]

Ouch! that replacement is pricey.
I'd be tempted to Dremel the top off the TO-3 package and solder in a IRF530.

[Johndi]

Yeah.....Tayda Electronics has the IRF540 for $0.56.  Meanwhile the cheapest I've found the IRF130 for is $7 from Taiwan.  Slightly cheaper from China but I'm not confident that I wouldn't get a fake.  Not to say that I wouldn't get a fake from Taiwan but I feel the likelihood is lower.

[Johndi]

I was able to acquire an MX-RM6E cord and STTC-126 tip.  I think I'm going to pursue an MX-RM3E cord as I don't like the longer reach of the 6E.....but the price was right and it was NOS.

So I replaced the three toasted resistors with 100 Ohm 1/2W new ones, as well as replaced the UFN130 with a JANTX2N6756.  Powered it on last night and found that the transformer gets immediately gets extremely hot. Measuring the output in circuit where it feeds the bridge wave rectifier diodes, I read around 1.0VAC.  I desoldered the leads from the board and then measured and measured about 17VAC.

Checking the diodes in circuit and they don't seem to be bad, but I'll probably have to remove them for best check.  I think I might have a short somewhere perhaps, just have to track it down.......

[Johndi]

So as it turns out, I had improperly reassembled the unit, and the case of the TO3 mosfet was grounding straight to earth!    Swapped around the plastic and metal standoff and spacers/inserts and then tested and all was well.  Tip heated damn near instantly and it was ready for service!

Now all I need to do is get it its own fuse and replace the incandescent bulb in the front and she's good to go.  That, and get myself an MX-RM3E handpiece.