Metcal MX-500P-11 power comes on but no heat.

[labjr]

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   

[Runco990]

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.

[georges80]

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.

[labjr]

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.

Yes the handpieces work fine with a third MX-500P-11 I just got. I'll continue to post my progress in this thread.

[quadtech]

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

[georges80]

http://colinoflynn.com/wp-content/uploads/2012/05/MX-500P-11.pdf

Fault finding & schematic.

cheers,
george.

[labjr]

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

[labjr]

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.

[labjr]

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. 

[labjr]

Update:

After waiting 6 months for ZTX and IRF parts, I've successfully repaired both units. Thanks for all your help.

[quadtech]

Great! What were the failed components in each unit ?

[labjr]

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.

[labjr]

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.