EPT360015C Connex cx3400hp dual mosfet issue.

[Hawkeye351]

First off, thank you all for the helpful technical info you have helped me with on a few other radios. I had to put the other radios you all helped me on up on the shelf for this rush job. God I hate being rushed.

Anyway,
Working on a connex cx3400hp dual mosfet ept360015c now.

Sent to me due to no transmit, carrier or modulation. I verified this and set it aside due to being backed up.

After further inspection, I noticed 2 small solder burs between the output voltage rail from the 2sb827 over to a 5v rail next to it. Obviously I checked the 2sb827 after seeing those burs and it showed as 2 diodes while in circuit, I then removed the burs and rechecked with same results. After I removed it, I noticed a hole and a crack in the face of the 827, so I replaced the 827 with tip36c (st micro brand), the1819 in front of it with a 2sa1012 and replaced all the mosfets (driver and both finals). I then double checked my work and then hooked everything up to test it. It keyed fine, modulated wimpy up to around 18 watts max. I then readjusted all bias' back to 3.70 volts for the driver and 3.50 volts each for the finals by using bare leads of L37, L50 and L35 to check. Got about 22 watts max from just the bias adjustments. I then proceeded to readjust L41, L42, L43 and L44 for max swing, could only get a max of 31 watts max. The mosfets ran cool the entire time I was keyed with modulation, (5 minutes at most) but the new power amp I replaced 827 with (tip36c) got pretty hot (the 2sa102's got pretty warm) so I shut it down and checked a few other things out. The whole time I was keyed, with modulation or not (mic gain up or down) there was this constant steady tone which sounded like a 900hz going with the carrier, modulation would cover it up. Upon checking a few other things, I found that the biasing diodes (strapped across each individual mosfet) were all showing OPEN, even my parts tester said it was a "bad part or no part plugged in".

I am taking a break from it for tonight, but I am curious of a few things:

Would those bias diodes being bad make that tone on a carrier?

Would those bias diodes make the tip36c get hot?

Would those bias diodes affect the wattage that much?

Note:
This radio has had only 3 mods to it that I know of.
A talkback mod consisting of an electrolytic cap, disc cap and diode on the trace side of the board near the tip36c, AMC removed, 2 diodes tied together at the D79 and D65 locations, and maybe another mod from a brown wire soldered to the bare side (top side of board) of R205 (part of a channel mod maybe). Nothing clipped other than the AMC removed. Nothing looks burnt, visually internally it looks brand new, no visual signs of burnt or overheated resistors or caps, no burnt traces, everything else in the radio runs very cool, just the tip36c gets very hot, the 2 2sa1012's get warm, strange tone on carrier, lights do not dim whatsoever when keyed or modulated.

The owner stated he stopped at a truck stop and had them mod it. He stated it ran fine for a week or so and then stopped transmitting, so he dropped it off at a different truck stop and they charged him for new mosfets and tune. He said it worked fine for a week and then smoke came out the back corner where your DC plug is. He said he got frustrated and just threw it in his closet. Now he's heavy into radio and wants me to straighten it out for him. I informed him of the damage and the cost just to get to the starting point of just being able to transmit, then we may run into more money and time, he doesn't care, he wants this radio fixed, I think it had some sentimental value to him.

Anyway, any help would be greatly appreciated as I'm not charging labor on this radio, just the price of the parts to help the guy out because I've had this radio on the shelf for a couple months.

Sorry so lengthy, but you get more detailed advice when you give more detailed information.

Thanks in advance...

 

[Hawkeye351]

By the way, I'm using an MFJ 872 Grandmaster watt meter. Reads AVG/PEAK, not pep.

Was told that the 31 watts I'm seeing on my meter (avg/peak) is actually around 54 watts pep, is that right?

 

[Handy Andy]

This sounds like "burrs" being someone was in there then removed something - this could have been anything from an NPC mod onto a TopGun Modulator onto - a 120VAC extension cord hooked up to a 73 Thunderbird and used their Windshield defrost generator with no governor...

Use a Dummy load - and a good ammeter...

So if you can... or ... did you have a chance to throw an ammeter on the power cord? At the Connector - the TIP36 heating up issue could be something of a red flag for the MOSFET - which isn't the problem, the parts left behind in the radio from the mod are...we don't know the cause of the failure but low power indicates damage to the TX strip - could be from an overvolted attempt and blew the filter caps along the strip to the Driver and Final - onto a High SWR condition from an amp snapping back at it's owner for asking too much from it. (surges RF flashback / backdraft / relay arcs sort of condition).

• So, if you can - disconnect all Driver And Finals - then do a preliminary check - just TX the thing no TX hi-power parts hooked up - check for Draw - if you got an amp or more - indicates shorting condition - then you might need to replace several caps along the strip starting at Driver - hook it up - check for Draw (amps) - try to find your SHORT!

​

Did you have any issues trying to adjust BIAS?

If you haven't messed with MOSFET's - best to TEST Gate voltages WHEN THE GATES ARE DISCONNECTED - unsoldered - from the board - should be less than 4V - when these are OVERVOLTED the ZENERS used at the DIODE from Gate to Source - may have blown OPEN. That condition would show HIGH voltage above 4V at trimmers setting 3/4 up - SHORTED condition would be no voltage to GATE at all. Fix any thing that doesn't let you set these trimmers to MID point and see about 3.5 to 3.8V DC and is adjustable WITH GATES unsoldered not connected - to PREVENT mis-steps.

Some Connex radios DO NOT adjust bias - they run straight - but I'll let you examine this post then get back with us as needed...

So, to TEST VOTLAGE at gates, it's done with no gates connected - just verify the trimmers are not sending more than 4V into the GATE.

• When I say that I mean to check the BIAS section of the board to send the correct amounts of power to let the Gate trigger on RF - and Not stay left on once you're done TX-ing the equipment. A trickle voltage is there to let the Gate see RF arriving and turn on and FOLLOW the RF signal in a more linear fashion.
• I'm referring to the GATE PAD - test across to Source (Emitter Pad if this helps to understand this) - voltage should be adjustable - and not go above 4 volts.
  • Why this 4 Volts? MOSFET require a voltage to turn on, doesn't need any current behind it - so the VARIABLE has to control voltage as a STAND ALONE means - so the Zeners Protect the Gate - but cannot PREVENT the Gate from an action called Clamping. The Voltage arriving needs to remain steady so the Gate can work thru it's Linear Region correctly and not act like a switch.

When you set BIAS - you set in mA Setting (Amperage) and check Source (Emitter - Ground) - Drain (Collector is Center Leg on Bipolar - HOT from AM Regulator) (from Gate idle) mA draw - through our Test Point to AM Regulator. Slowly bring up the trimmer to Midpoint - as the MOSFET starts to conduct that MOSFET will latch on - you need it to be at the JUST ABOUT THERE moment. This is the "linear" region of the part.

Once BIAS seems ok, remember to keep the Trimmers to a MIN setting - lowest voltage adjust - then bring them up - one at a time starting at Driver - then finally to last final - all trimmers need to be kept at minimum until set...

Considering everything - they use a 5.1V 1W Zener type - might/should be changed to fresh to eliminate a future problem later. IF this radio arrived with blown finals - then spikes and bad VSWR may have killed a lot of parts from the power dissipation it had to take on until it failed - usually a cascaded type of failure...

I use 7.5V Zeners - others just replace with a 1N4148 REGULAR diode - which I don't recommend. Regular Didoes will work but no protection from High SWR conditions leaking back into this area.

​

The thing has got a TP jumper outline on the board, this outline will help you get a grip on the Bias and voltages present on the TX BIAS FEED line (those TP 7, 8 and 9 silkscreen outlines)

You will have to remove the jumpers that arrive from the AM Regulator - two types, one long jumper from one TP to another - then the other is a SHORT jumper just connecting two holed in the same outline of that Test Point.

You keep the SHORT one in place - You remove the LONG ones - like the one with the white weave braid shown above - that also goes to another jumper filling in that same hole - heading off to TP 9 - that TP9 is from your AM Regulator.

See Green Arrows? Use your NEGATIVE LEAD jumper here - RED ARROW is Positive - Start at Driver

Best to unsolder the legs of the ones you are NOT adjusting - start with Driver - get that to NO MORE than 50 mA You can then leave this soldered in - the TP jumper then is removed - so you're safe - move to the Meters jumper test lead to be on the Finals TP

Then at the Final - start at One, that parts proper trimmer Fix it's mA to 50 ~ 70 mA MAX - unsolder then solder / fix and adjust to SAME reading on other of the pair.

Remember - resolder all legs.

Then verify your work and re-establish the TP wiring.

Then using the TP jumper any open the bare leads - make sure you are not sending more than 8V into the TP section from the AM Regulator - test from jumper to BOARD ground Same Ground as NEGATIVE on the power leads at the rear connector - you can adjust this AM POWER after you get things to settle down.

Issues with low power from this point on usually mean the Disc caps may have suffered a spike and got toasted - the output combiner and the ones towards the output matching network are usually suspected...

You say you have a "tone" or unnatural sound - could be damages to the Mic amp or a bad repair job onto soldering not completely soldered in correctly. A squeal from a bad cap or poor soldering or even loose connectors from and to the front panel.

Shame on this mess for these are great radios - just not a lot people know how to treat them, let alone fix them.

 

[Hawkeye351]

Thank you Andy, you always come with the best detailed advice. I will check these out tomorrow.

As far as adjusting the bias', they all seemed to adjust fine, around halfway on all 3, no erratic movement or anything.

When I saw those burs I immediately took a picture and sent it to him. I asked him who did that talkback mod and he told me but I can't say openly. Those burs were just across that mod so I assume it came from whoever did that mod.

As for the tone on the carrier, I did get that out, it was a cold joint on a resistor beside the input of the driver. Now, there's a faint hum in the carrier, but modulation covers it up, not a loud hum but a faint hum.

I'm using an MFJ 872 avg/peak meter. On my meter this radio will only max at 31 watts. The specifications for this radio with dual mosfets claims it will do 45 to 55+ PEP. According to my understanding, an avg/peak meter will not show that 45 to 55+ watts pep. Is this true? And if so then should I calculate the 31 watts on my avg/peak meter to find the pep watts? According to my meter, does it sound like the radio is doing what it should for 2 mosfets?

 

[Hawkeye351]

Well, just hooked my big window MFJ 868 (rms/PEAK) meter up and it shows 1/4 watt carrier with rf power all the way down swinging to about 35 watts on peak, and 8 watts with the rf power all the way up swinging to about 47 watts peak.

My MFJ 872 avg/peak meter shows 1/4 watt carrier swinging to about 16 watts peak with rf power all the way down, and a 8 watt carrier swinging to around 31 watts peak with rf power all the way up.

Both meters were bought new from mfj by me and never been into.

So, are both meters correct in how they are designed to work (avg/pk vs rms/pk) and showing about right, or should I look into using one over the other?

 

[Eldorado828]

I believe the 868 has a true peak reading circuit but it doesn't distinguish harmonics if it isn't clean. My thoughts are the 868 would be a more trustworthy source over the 872 that acts more like a dosy.

 

[Hawkeye351]

Thank you for such a prompt response. I was just doing some reading on the difference in watt meters and realizing some things.

Now I just got to figure out the heat issue in this radio by doing what Andy posted above tomorrow.

Gonna stick with the 868. I've never seen over 35 watts on any radio on that 872 in peak mode. Different design I guess.

I love this site, most knowledgeable and helpful people I've ran across when getting stumped on a radio issue.

 

[Handy Andy]

The "heat" issue can be a simple "gotta' clean this up" - if your rear panel is Aluminum - then you might have the Oxides of March - marching on. Aluminum is great for transferring heat away and making a meal...

But I digress - just steel wool where the TIP36 mounts - and use goo and the insul-pad - should lessen the Hot-Spotting effect a little less. Aluminum Oxide is a great insulator - but does little to transfer the heat out of the part.

But work in stages - keeping a Dummy load on the Antenna - so you don't blow it up...

Keep the TP jumpers in place but don't let the Final and Driver connect their DRAIN (Collector in Bipolar days - if this helps) until you test each stage to make sure they are not causing the problem.

Remove - just unsolder the legs off the board - Disconnect the Driver and Final - then test in TX mode to check for Draw...If you are seeing more than an amp - something in the AM Regulator onto the Driver and Final SECTION that is still connected is causing this fault - you may even feel the heat from the Regulator at this time - some heating is normal.

• The interesting thing about this radio versus others is the "have to do it this way" moments. It's because of the variable AM power and other features - require the radio to have a LINEAR power supply and have a means to put Audio onto that power line at the same time and send it off to the TX section. Then POOF and almost like magic - you have Radio...

The AM Regulator is a Amplifier and Regulation Device
Regulates and Amplifies (Imposes) Audio and DC levels together​

It's why I showed Caps earlier - they bear the brunt of heavy modulation and have to sit there and take the abuse. IF they get pushed too hard, they expel gas - a stench of sulfuric acid - meaning they got hot and will probably need replacement soon - if not - the next heavy push can dry them out and it can do one of two things - cause a DEAD SHORT (your heavy current draw) or Explode - Funfetti - and either way - you'll have a h*ll of a mess to clean up.

So if it is not drawing lot of current - hold the TX on for a minute and double check the TIP36 itself - if it's got issues like a SUBSONIC feedback squeal - this signals that you will have to research ...

Where? Well look back there for fresh work from the OTHER guys...the Electrolytic caps in that AM Regulator can be fiddled with in values and they might replace them.

A popular one they "change" is C190 ... THAT one is a 100uF - but if someone added / swapped in or substituted in a LARGER value CAP - like a 220uF or 330uF - NO NO NO! - Replace! Put it back to the 100uF it needs to be there - you're killing the AM Regulator amp using too much capacitance to try and get audio thru it.

• C190 along with the 15Ω resistor R241 - form a GAIN Cell - slows down the Regulator action by allowing a lag in charge and discharge of a capacitor thru R241 - the Cap holds the "results" from the output of the regulator thru the 560Ω (R240) and gives the feedback section something to draw from and use as a level gain. This slowing effect " opens up " the DC bias range for Audio to get imposed into this " space "

​

So if there has been changes, you'll have to reverse them out and pretty much bring it back to stock...

You'll have to figure out which of the places this draw is occurring.

So best to keep the TEST simple and work in blocks - easier and simpler to find - kinda' tedious but worth the effort once you've found the problem - it's a lot easier to solve.

This can be caused by only one part hogging all the current - so if the Driver doesn't seem to really cause the "heating up" problems - that is fine - the test passes there - onto the Final stage which has two of these things...

Do one "check" at a time and watch your ammeter - make sure both Finals are drawing proper current levels AT IDLE.

No Audio - Turn that Mic Gain down...then check - if one pulls - say 0.75 Amps - it's gotta be checked - adjust the BIAS to it first - see if it's too much Gate voltage to cause the "latch" - The Draw you see on the meter - it's part is pulling the DC present at the AM Regulator thru it - so this will cause the AM Regulator to heat up - the higher the draw - the hotter it gets - faster...

 

[Hawkeye351]

Well, the diode across the driver is open and all 3 disc caps across each mosfet is shot.

Hopefully that's the whole problem.

 

[Handy Andy]

Wow, this is a signal that you had a catastrophic fail - one part took out two parts.

You are fortunate that the chain didn't continue to break apart - ones the RF power was killed off - the rest of the system was preserved - hopefully in good enough condition to continue.

You might have to work not just at the driver - but downstream - towards the Network too - the output caps from the Tank circuit can also be damaged.

 

[Hawkeye351]

Andy, I appreciate how you go into detail on how to do the tests. And thank you so much for always chiming in.

Now I'm new to testing mosfets, so bare with me here please.

So you are saying to check for what's stressing my regulator when I transmit I should do the following:

1. Unsolder and lift all the legs on all 3 mosfets.
2. Key the radio (mic gain off)
3. Check the gate voltage on the gate pad on the board and adjust it if need be.
4. Move on to next mosfet and do the same as step 3.
5. If my regulator is still getting hot while doing it this way, then check the electrolytic and disc caps in the rf section (even the Mylar caps at the tank circuit section)?

Is that correct?
What gate voltage would you recommend?

I do remember when I put these mosfets in that all 3 legs of the driver mosfet touched the chassis of the radio at the same time when in was installing it. I read up that the tone I heard could have been a sign that one of the mosfets was in the constant (ON) state, which can occur when all 3 legs touch together at any time and I would have to use my finger to short all 3 legs out to take it out of the "ON" state.. If this was the case, then would that mess up the mosfets?

Please bare with me Andy. I'm used to the old ways of doing things. Just a little guidance, I listen good when explained to me step by step.

I've actually got 3 other radios I had to put on the side for this one. The owner loves this radio for some reason.
I've got a 2000 gtl I'm currently in the process of completely recapping, restoring. A friend's galaxy saturn (ept360014b) which I think you were helping me with, lights dim and freq counter goes nuts when you modulate and has low power. A friend's durland superstar 3900 (ept360015c) which you were also helping me out on, low power. I also have an emperor ts5010 that needs fixing when I get time, another galaxy saturn to fix, 2 teaberry stalkers (9 and 15) to restore when I get caught up.

I used to be somewhat of a shade tree tech, no fancy equipment or such, just books and schematics. I worked on a few friends equipment, even though there were 2 professional techs around here. But just recently the last cb tech around here passed away, now the closest one is around 50 to 75 miles away. So, my friends ask me to get back into it to save them some money. I'm currently in the process of getting the equipment I need, oscilloscope, spectrum analyzer, function/signal generator, probes, attenuaters, samplers, etc...

Thank you Andy for the awesome advice and detailed outlines. Please hang in there with me. I may have questions that others may laugh at, but I have the basic knowledge, i just need some guidance.

 

[Handy Andy]

You're actually doing great...

To Know Someone is to Love them and to Hate them...

I think the same could be said about MOSFET's

Although I kinda' "botched" the description - Yes, firstly disconnect ALL the MOSFET's so you can see the "Voltages" and make sure the voltages to the Gates of each one can be adjusted.

So you know, MOSFET's Gates DO NOT CONSUME a lot of power - so they are high - impedance devices - so once you set a gate voltage - that is fine - but the CURRENT they consume from Drain to Source (Collector to Emitter - if that helps) AFFECTS this Voltage while it is in operation. So you just can't "set" Gate voltages and think your ok.

No, you make sure they can be adjusted then set their trigger by looking at the mA draw - once they start to draw current - this changes their Gates' Behavior - so the trimmer for that Gate compensates for this "event".

You also need to verify that the AM Regulator is not thinking your drawing more power - if it's heating up - you have to fix that - with the Finals and Driver out of circuit - there is no draw. So gotta' fix that first.

There are two types of Voltages you'll need to check;

The AM Regulator - has to be adjustable so you can set 5.6 Volts AM POWER - if dones right - will have it's trimmer set to about Mid-point.​

• With the MOSFET's disconnected - check at this time to see if Lo - to No - load causes this heating up...
• IF it does, you have a problem - or several, with shorts or bad caps - gotta fix this before you can even proceed.

There is only one output to the AM Regulator - that is towards the Driver and Finals - if there is a short causing this draw - it will be on the route from the OUTPUT of the AM Regulator to the Driver and Final

Make sure you can adjust AM POWER to 5.6 volts and 4 ~ 7.2 Volts range - but set it at 5.6 Volts - should be at Mid-point.

Place Radio in SSB MODE can you see (At Least) 8 volts? Across AM Regulator output to ground.

MOSFET's - need to check and verify the Gate voltages can be set - from as low as 2.5V to as high (no more than) 4.5V - any out of range needs to be fixed. Do this in TX mode for AM - NO AUDIO.
​

• - the Gates for all 3 MOSFET's are SECOND one - you set them to about 2.5V (Flat 2.5) make them equal for all three. There is a small catch...
• MOSFET's are Voltage sensitive - so the above check is to make sure none of them are going to be turned on when you power these up and everything is connected.

It is best to work in SECTIONS - focus on one; Driver or Finals first - to adjust mA Current Draw.

So CONNECT APPROPRIATE Power device to their proper pads.

If you TRUST your work, then install an AMMETER in line with the POWER cord.

Install the DVM at the TEST POINT from the AM Regulator place your Positive lead there

Negative Lead of the DVM to the DRIVER (OR FINAL - your choice) TEST POINT

Place Radio in SSB MODE - TX into DUMMY LOAD

Note mA reading on DVM - adjust THAT POWER DEVICES TRIM up slowly until the mA Reading is 55mA.
Unkey TX - so far so good.

If you have more than one POWER DEVICE on your TEST POINT (like Dual finals) - then remember at the Test point you will set the next one to DOUBLE the mA Draw of the first - so that both READ one number 2X that of your First mA reading for that STAGE OF POWER DEVICES.

This is so that your "matched pair" work as a set using the INTITAL mA you SEE - you DOUBLE THAT on that same TEST POINT for BOTH of them - so 55mA - set both together to read for 110mA. DON'T RESET BOTH - LEAVE YOUR INITIAL TRIMMER SETTING FOR YOUR FIRST ONE ALONE - THAT IS YOUR BASE LINE.

IF you goofed up - thats ok, just reset your trimmers to lowest stop - then redo starting at the beginning again for that Test point - one at a time - then double on the next - if you find that the trimmers seem to interact - they are not supposed to - investigate because if you can't keep the voltages separate while your trim mA to their GATES, you will have a bigger fail later.

Now you are in a spot that if your not careful you can "latch" your work - shut power off and reset your test point jumpers - so this time start working on your next stage to and set their mA reading to 55mA - latching can occur if you did not recheck your Test points and make sure you're on the right test point - and then you are ok to set that POWER Devices' mA draw.

Now you can solder on the power devices of your next stage - see how this works? Your keeping a "Breakpoint"
in the TX chain so that if anything "bad" were to happen - you don't destroy the strip.

However, the chances for failure are now higher - you have a power section ready to engage the next one - so NO AUDIO - SSB MODE - Dummy Load on the Antenna Jack - put it in TX and continue...

Power back on, Turn on radio - look to your power cord - your POWER CORD Current Meter should be showing less than 3 amps - anything higher - you have problems. If less that 3 amps - your stable and can continue - else there is a problem and it may be back as the LAST stage you just got done setting. It can happen...

Looking at your DVM - set that Stages' POWER DEVICE to 55mA - check your POWER CORD Current draw???

Over 3 AMPs? Ouch power off and let's review.

Still under 3 Amps? Good Job - if you have other stages to do - or things to finish up now is the time to do it.

I'll stop here and let you GO OVER the above steps - if there is something I've missed - let me know...

 

[Hawkeye351]

Ok Andy,

1. I unsoldered all the legs on all the mosfets.
2. Tested each individual mosfet with a dmm.
Results:
Negative to source, positive to drain shows around 500 to 600 on diode mode.
Negative to drain, positive to source shows nothing in diode mode.
All other pin configurations show nothing on diode mode or ohms mode.

Circuit board results:

Rf power all the way up, Mic gain completely off, unkeyed gate voltage on driver gate tab on board is 0.03v, 1st final gate voltage tab is 0.02v, last final gate voltage tab is 0.02v.

Keyed gate voltage tab on board is 4.19v, 1st final gate voltage tab is 3.67v, last final gate voltage tab is 3.09v.

No draw on my power supplys ammeter at all. I keyed it for 10 minutes solid, regulator did not get hot at all, not even warm. The other 2 regulators and IC on that same side were still ice cold. Only thing that got warm was the pre-driver but not hot.

 

[Hawkeye351]

Correction:
I had the negative lead of the dmm to the drain and positive to the source and had 500 to 600 on dmm in diode mode.

Reversed the dmm leads, negative on source and positive to the drain I had nothing.

I did not test it in the "ON" state though.

 

[Handy Andy]

Ok, you didn't need to test the MOSFETs - I left out the word "Pads" or the foil traces to those leads.

I neglected that - so I apologize to you for that error.

Wanted you to check - and you did - but the GATE voltages have to be variable and so that means the voltage will rise and fall as you adjust the Gate Trimmer - that is what I wanted to see or know from you that you fixed the Gate Bias circuit and how have X.xx volts to X.xx volts adjustable.

Then just turn the trimmers down so their Gate voltage are at the lowest setting - about 2.5V and adjusts or can get turned up to about 4.5V maximum.

So we're on the same page...

You did say...

Keyed gate voltage tab on board is 4.19v, 1st final gate voltage tab is 3.67v, last final gate voltage tab is 3.09v.

(oh nevermind, you did - thanks!)

All of them are high - but below 8 volts - if these trimmers are set at Mid-point, it's not bad - but yes, still high.

This is good news, just need to know if the trimmers are adjustable and what voltages are you getting at lowest setting and highest setting on the trimmers? IF one opens up as in drops off, you may have found a bad spot and the trimmer will have to be cleaned, rechecked and see if it can be made to recover or else we'll have to replace it.

What I'm looking for is the ability to "finely set" the trimmer to get the Gate voltages right at each part so we can make them turn on correctly at the right time.

So they (The Trimmers) need to go below 3 volts (at the Gate pad measured across to ground) and not much more than 4 volts at maximum.

Then the MOSFET's can get soldered in - one at a time - Final or Driver - would recommend you start at the Final since we do have readings that are higher than I'd like - meaning they can force the MOSFET on and you wouldn't know it until it popped that it latched on and stayed on. That is why I want you to turn down the Gate voltages to ensure the MOSFET's won't turn on when they shouldn't before you're ready to test them in circuit.

As a reminder we don't set voltages to turn on the GATES - we use the trimmers to set the CURRENT at the Drains (Collectors in Bipolar days) - by adjusting the Gate Trimmer voltage USING OBSERVING - when the Trigger level is reached - the Drains start to conduct we see it as a rise in the mA reading - each one will be a slightly different voltage trigger than the others.