Kraft 7 Channel Series 75 Conversion

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davejones
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Kraft 7 Channel Series 75 Conversion

Post by davejones »

Have this Kraft 7 Channel Series Seventy-Five that I have just started to clean and convert to 2.4
Kraft 7c Ser75.jpg
So far I have the transmitter, a LemonRx transmitter module and a DIY-More Pro-mini Strong board that I have loaded Phil-G's 7ch+s/c encoder code onto. Thanks Phil for making this stuff available.

I couldn't get the code to load using a USBAsp (may have been faulty) so ended up using an Arduino Uno as a programmer which worked fine (I hope).
Dave Jones
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davejones
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Re: Kraft 7 Channel Series 75 Conversion

Post by davejones »

I stripped all parts from the txmtr except for the trainer button as I couldn't work out how to remove it without breaking something.
Cleaned the case which came up really nice. Slightly reamed out the original antenna bezel/mount and fitted the LemonRX antenna plus an earth from the antenna base to the case. Also cleaned and re-fitted the RF Meter.
Kraft7cFront.jpg
Kraft7cAntenna.jpg
Kraft7cMeterRear.jpg
Dave Jones
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davejones
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Re: Kraft 7 Channel Series 75 Conversion

Post by davejones »

Worked out a battery solution.
The original battery box is also a mount for channels 5 and 6 pots/levers. I modified the battery box interior so I can just squeeze in a 6 cell NiMH 1500 battery pack. Just need to install a charging jack either in the bottom or side of the txmtr case. In the second photo, you can see the blue battery connector exiting the top of the battery box.
The transmitter module needs 4.8 to 12 volts and I think 7.2volts should be enough for the pro-mini encoder board.
Kraft7cBattBox.jpg
Kraft7cBattBox2.jpg
Last edited by davejones on 08 Jun 2022, 23:51, edited 1 time in total.
Dave Jones
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davejones
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Re: Kraft 7 Channel Series 75 Conversion

Post by davejones »

I intend to convert the RF meter to a battery level meter but, as I'm an electronics dunce, I have no clue how. I'm sure I read somewhere that it could be done but cannot recall how and can no longer find the info. I'm fairly sure that the Kraft meter was used to show RF output (I could be wrong).
Any help on this part would really be appreciated.
Picture of the meter below.
There is also a picture showing the rear of the meter 2 posts up.
Kraft7cMeterFront.jpg
Dave, I have resized your pictures, I hope you didn't mind.//Tobe
Dave Jones
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Re: Kraft 7 Channel Series 75 Conversion

Post by Ginger »

Very nice!
Heres the meter thread: viewtopic.php?f=62&t=132
G
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stuart mackay
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Re: Kraft 7 Channel Series 75 Conversion

Post by stuart mackay »

Case and sticks look in great condition.

Yes use the reverse Zener and variable pot (50k) to tune the needle position and you shouldnt be too far out on the meter scale.

Phil did one of his now infamous videos on this a couple of years back:

https://youtu.be/B-vqEBa9PnY

Keep the pics & updates coming
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davejones
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Re: Kraft 7 Channel Series 75 Conversion

Post by davejones »

Thanks for the links chaps. That's where I saw it.

Found some Kraft service manuals here: https://www.vintagercfiles.com/ Use the Files button down on left side.

According to the parts list in the Kraft service manuals, the meter is 0-1mA which, I think and for the purpose of the math, is a 0.001A meter.

Using a 6cell Nimh battery should give a voltage range of about 9v when first fully charged down to about 6v when flat.

So according to ohms law and the math in Phil's video (thanks again Phil) I will use a 6.2v Zener Diode which would make V=2.8(voltage) and I=0.001(meter).
2.8 divided by 0.001= 2,800 So is that 28k Ohms or 2.8M Ohms? 2.8M Ohms, I think, maybe, not sure.
Ignore my woriking out in red above and see Phil's post below

Anyway, I'm off to Jaycar (electronics shop) to buy some bits and do some testing. With a bit of luck I've worked it out correctly and won't destroy the meter.

Tobe - Apologies for the rather large picture sizes. I have now read the 'Image size and PM thread' which I should have done before posting. :oops:
Last edited by davejones on 09 Jun 2022, 04:17, edited 1 time in total.
Dave Jones
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Re: Kraft 7 Channel Series 75 Conversion

Post by davejones »

Phil_G wrote: 09 Jun 2022, 01:11 Hi Dave
2.8 volts divided by .001 is 2800, ie 2.8 kilohms.
Heres a cheat Dave. In maths you can trade milli-whatever for kilo-whatever.
So if we work in milliamps the .001 becomes 1 and we can expect the result to be in k.
2.8 over 1 is 2.8 and thats 2.8k since we traded kilohms for milliamps. Easier to do in your head than to explain in words :D
So, you need a 2.8k ohm resistor - with 2.8 megohms it will hardly move.
The nearest preferred value is 2.7k which I would expect to be near enough, red, violet, red then whatever the tolerance band.
Cheers
Phil
Thanks for the explanation Phil - it makes sense now. I am a slight maths gumby - especially when it comes to where to put the zero's.
You were absolutely right, the meter needle didn't budge with the resistors I used.
For testing, I used a small variable power supply that I normally use when anodising model engine parts. In the end, just using the 6.2v Zener Diode (1n4735A) without any resistors gave me just about the right movement. The needle just starts moving at 5.9v and reaches the stop at 9.4v.
Just for interests sake, without the diode or resistors, the needle starts moving at 0.1v and reaches the stop at 3.5v.
Pic attached of Diode mounted to meter and also shows polarity of meter which is not marked, or obvious from the original wires (both were black).
Kraft7cDiodeMeter.jpg
Dave Jones
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davejones
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Re: Kraft 7 Channel Series 75 Conversion

Post by davejones »

Mounted the 7c encoder on the end of the battery box using M3 nylon nuts, bolts and washers.
Fitted the charge port into the bottom of the case, in between two of the rubber feet. This is the only extra hole I plan to make in the case.
Wired up the two aux channel pots.
Cleaned the main switch. They don't make them like that any more - it's just about bullet proof.
Wired up all the power stuff - switch, battery, charge port, meter and encoder.
Switched on, the meter moved, the encoder board leds came on and no smoke came out from anywhere so happy with that. :D

3d printed a blanking plug that fits into the bottom of the battery box to fill the space left by the original trainer/charge port.
Kraft7cEncoderMount.jpg
Kraft7cCaseBottom.jpg
Dave Jones
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davejones
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Re: Kraft 7 Channel Series 75 Conversion

Post by davejones »

More progress.
Cleaned and tested the gimbals and their pots.
Wired the pots and channel seven toggle to the encoder. Note to self - use thinner servo cables next time as it gets difficult to find room to route everything so the trim levers etc don't impact on the wiring.

Next up is the Lemonrx DIY transmitter module. The plan is to cut some FR4 fiberglass sheet and mount this sheet where the original pcb was mounted with the two screws that are visible, above the pivots, on the back of each gimbal. The transmitter module will be mounted to this in such a way that the indicator leds will (hopefuly) be visible shining through the meter. Also hoping to be able to make some holes in the fiberglass sheet and use them to cable-tie some of the wiring up out of the way.
The original trainer switch (top left in the picture) will become the bind / low power range check button.

Pic of progress so far:
Kraft7cGimbalsWired.jpg
Dave Jones
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