Just started MPP project

[Chet]

Chet, maybe I just have no clue as to how an osilloscope works! Here is an interesting shot.

Its hard to read your display, but it looks like very low level voltages which may just be noise from the chopper in the 5 volt power supply. It looks like U4 is still not oscillating.

Try resoldering the connections and try another 555 chip if you have one.

Regards,
Chet

[Chet]

Back to the drawing board! As good as it gets (attached). Regular pulse. I have the 740 and R3 removed, but that should not be in play.

Hi multieagle

Did you check to see if you have 0 ohms between pin 2 and pin 6 of U4? And check for 0 ohms between pin 4 and pin 8. There may be a cold solder joint on one or more of the pins. It would be worth a try to sweat a little more solder into all of the U4 connections.

The wave shape you have now will have the 740 MOSFET turned on 50% of the time. It will still run hot. When the pulse is right it will only be on 50 usec every 950 usec.

Regards,
Chet

[multieagle]

Chet, maybe I just have no clue as to how an osilloscope works! Here is an interesting shot.

[multieagle]

Hi multieeagle

It may be that your oscilloscope isn't triggered properly but the display should be like the one you previously posted in post 14 of this thread.
The signal should go positive for 50 usec then negative for 950 usec then positive again for 50 usec.

Regards,
Chet

Back to the drawing board! As good as it gets (attached). Regular pulse. I have the 740 and R3 removed, but that should not be in play.

[Chet]

Removed the MOSFET and R1. Am again getting a pulse. See image. Will pick up the 470 tomorrow and do a temporary install, along with a new 740. Fingers crossed.

Hi multieeagle

It may be that your oscilloscope isn't triggered properly but the display should be like the one you previously posted in post 14 of this thread.
The signal should go positive for 50 usec then negative for 950 usec then positive again for 50 usec.

Regards,
Chet

[multieagle]

Removed the MOSFET and R1. Am again getting a pulse. See image. Will pick up the 470 tomorrow and do a temporary install, along with a new 740. Fingers crossed.

[Chet]

I need to replace R1 (470 ohm 2W). Locally, can only find 470 ohm 5W. Any problem using that? Otherwise there is a 470 ohm 1/2W.

Hi multieagle

The 5W is no problem unless it is a wire wound resistor. A wire wound resistor may add inductance or capacitance to the circuit. Other wise the leads on the 470 may be too large to go through the holes. You might splice some smaller leads onto the fatter 5w resistor leads. 470 ohms will probably be a temporary value. You will probably change the value after final assembly to get the best performance with a particular coil.

Regards,
Chet

[6666]

I need to replace R1 (470 ohm 2W). Locally, can only find 470 ohm 5W. Any problem using that? Otherwise there is a 470 ohm 1/2W.

You can series link more than one value to get your 470 ohm :IE 200 +200 + 70
and it takes the stress off just one R.

[multieagle]

I need to replace R1 (470 ohm 2W). Locally, can only find 470 ohm 5W. Any problem using that? Otherwise there is a 470 ohm 1/2W.

[Chet]

Chet, to save you some trouble, I replaced the 555 and almost immediately the Mosfet began to overheat. Replaced that with another, same result. I have rechecked all solder points and everything looks okay. Went over it with a magnifier and there were no bridges. Therefore, it is probably associated, IRF740, R2 or R3. Will first replace the 740 and go from there. Thanks for your help.

Hi multieagle

Remove the 740 then check for a pulse from U4. If you don't have a pulse do not install another 740 until you have a pulse.

Regards,
Chet

[multieagle]

Chet, to save you some trouble, I replaced the 555 and almost immediately the Mosfet began to overheat. Replaced that with another, same result. I have rechecked all solder points and everything looks okay. Went over it with a magnifier and there were no bridges. Therefore, it is probably associated, IRF740, R2 or R3. Will first replace the 740 and go from there. Thanks for your help.

[multieagle]

Excellent idea!

[6666]

as there was a lot of heat applied to the Mosfet

When I am makeing such projects as a metal detector I mount mosfets, damping resistors or others components that may need replacing on PCB pins,
does not look pretty but only takes a few seconds and the job is done, does not require a lot of heat and does not damage the PCB.

[multieagle]

If you don't have a signal at TP1, then the TX oscillator is not running. If U4 pin3 is stuck high, the MOSFET will be hard on, and connecting the coil will definitely make it get hot. You need to fix the TX oscillator problem before proceeding any further.

Problem is, it was working (see post #14) and I continued on to Step 4 & 5. I was missing a part at step 4, and instead of waiting until I received and installed it, I went ahead to step 5. I picked up the needed replacement and installed it. Something happened at step 4 & 5 that caused step 3 to fail. Does that make sense?

I had to remove IRF740 and replace it. I scewed up the middle leg recepticle, the insert broke free and also the top soldering point. The bottom soldering point (ring) remained in place. I installed the replacement and am getting 12v both outer legs, and 0v centre leg. I'm thinking it may relate to that?????? I should maybe replace R3, as there was a lot of heat applied to the Mosfet at that point.

Pin values
U4
1: +12 # 5: 0
2: +9.7 #6: +9.7
3: +10 #7: +9.7
4: 0 #8: +11.8

R3 +4V

TP! +11.8v

[multieagle]

Ha, ha . . . that damn TP1-10. Yes connected correctly.

Thanks for your continued help, online and via email. Hightly appreciated.

Gord