HAMMERHEAD VERSION D PROBLEM

[daverave]

i just powered my hammerhead for the first time tested and ive run into problem with the coil drive...when i connect the coil i cannot control the tx pulse current and the circuit is then drawing 1.5 amps and mosfet starts to smoke...i scoped the output of the 555 timer ic and adjusted the tx pulse and it is adjustable but will not decrease mosfet gate drive !!!!

[daverave]

i have replaced the mosfet and the 555 timer but still the coil drive is drawing nearly 1.5amps and the preset for the tx pulse is working but not controlling the coil drive....ive checked my solder joints and parts but cannot find the problem...i need some expert advice pls.

[mickstv]

Have you got a CRO, if so have a look at the gate pin of the TX Mosfet and post a CRO shot, so we can see what's happening.

[daverave]

Photos taken from gate and tab of mosfet

[mickstv]

Looking at the first picture it shows that the gate of the mosfet is being turned on for approx 700us then of for approx 70us, this is wrong. Check to make sure resistors and caps are in correct locations around the 555.

Check the picture below, sorry for the bad drawing but you'll get the idea.

[daverave]

the problem was caused by selecting the active tx drive which was inverting the positive pulse so i changed it to passive coil drive and now the positive pulse is present but yet another problem the tx pulse is only 1 volt in amplitude and the mosfet is not turning on unless the 2nd mosfet i installed has become damaged so not sure about this problem ????

[mickstv]

Most likely the second Mosfet has been damaged, below is a picture of what to expect when the circuit is working properly (without the coil connected).

[daverave]

im still having no luck with this even though i changed the mosfet the gate voltage is positive but only about 1 volt tx pulse...i wonder if maybe i need a pull up resistor ???? with my surf pi circuits i had no problems !!! but this hammerhead just problems from the start even though i made it to a good standard.

[daverave]

the only way i could get the tx pulse working correctly was to remove Q2 and connect the collector output from Q1 direct to the gate of mosfet Q3 and use a pull up resistor from the base of Q3 to ground and now its working perfectly so why should i have had to modified this to make it work ???

[mickstv]

Don't know it should work maybe check and make sure Q2 isn't damaged.

[daverave]

i changed Q2 but still the same...it was inverting the positive pulse to negative...im confused why this happen ???

[mickstv]

Ok on the output of the 555 you should have a pulse that is 12v (common) and goes to -5volts for each TX pulse. Q1 inverts this pulse to produce an output that goes from battery 0v to battery 12v during the TX pulse. Q2 is a buffer.

So check the 555 and make sure you have a short pulse going to -5v.

Which side of Q2 do you have the mosfet connected to (collector or emitter)

[daverave]

i first connected the mosfet to the collector side of Q2 to give me active coil drive which inverted the tx pulse to negative...i then tried the emitter side of Q2 and i got a approx 2 volt positive pulse but mosfet which not switch on !!! i really find this hammer head confusing....also why is it i can hear the coil frequency coming through the head phones ???

[daverave]

i have the tx working now without Q2 with 12v pulse to mosfet....everything now works apart from the audio !!!! yet again seems to be a problem with transistor this time Q6 which is not driving pin 7 of the vco oscillator...im wondering now if i must have bought a bad batch of these 2N type transistors which are wrongly marked !!!! are their any known problems with this hammerhead like im getting ????

[porkluvr]

i first connected the mosfet to the collector side of Q2 to give me active coil drive which inverted the tx pulse to negative...i then tried the emitter side of Q2 and i got a approx 2 volt positive pulse but mosfet which not switch on !!! i really find this hammer head confusing....also why is it i can hear the coil frequency coming through the head phones ???

I believe you need to replace R7 with a jumper if you want to use Q2 emmitor to drive the FET.