Wednesday, February 4, 2015

14194kc DSB transmitter

The most of beautiful band for me is the 20meter. I fell in love with it reason most of the time I can get plenty of the station for listening DX and local. Some are QRP/QRPp/QRO.

Its time, I make the DSB Modulator and be on the air. Lets see how much I fare.
Started the 2Watt linear.

This will be my third prototype. First stage is the 2N2218, Second transistor is IRF510 both working on the 12vdc. This time I will be making the power-supply little up 14 volts. ie LM7812 plus LED.

Plan is to drive the VXO of 14318kc, one DSB 14194kc, 3.68mhz ceramic VFO plus Quadrupler. And the DC receiver integrated with it.......

VU2ESE has designed this linear and it gives about 5watt when carefully made. But ,I call this 2watt as to me it gives me just 2watt without any special care. Happy, this is best for our use.  I could have tried broadband RM96 1watt amp, but that needs more toroids/components
my toroid/ferrite source ( I do recommend VU2PTR for toroids)  (For balun core I bought from tpsonline )

Here we can use the simple plastic pipe for IRF510. And just one balun for its driver. For RFC use the CFL lamps cores. What else you want ! This ckt is best made on the veroboard ( Yes sir !).

A small heat-sink is needed. 2inch by 2inch (5cm by 5 cm square or rectangle aluminum will suffice). But don't over do here. use small piece as heat dissipation will be low.

One thing I remarked that we need 24swg copper wire with just 2-3 twist perinch for biliar winding. When I gave more turns about 6-8 turns during the winding (sliding the wire from cores) I feel insulation was cracking. not joking. I realized the mistake but somehow if this does not work I need to rewind the toroid for IRF640 ( the after burner power amplifier)

05-feb-2015. Finished most of the components. Winding the second set of coil incase the first coil does not work as it has low number of turns. Prepared the heatsink from aluminum. It was in L-shape. Cleaned it. Flattened it with the wood batten and small hammer (250gm). Then with the help of Jig made it into the U-shape. Filed the aluminum surface clean and fat. As the transistor has to made the contact with it. So, it must be flat atleas the contact area. Then put the drill. The heatsink should be outside the pcb not near or on the 2n2218 transistor. Otherwise feed back may arise. I did not had the insulating kit but will use small plastic piece on top bolt to isolate it from Vcc.
Lets hope it works as I am trying an old tech in the days of the DDS Microprocessor controlled VFO........

But my hunch has proven right sofar.....this will also work

All the fun is in making and using it to drive a big amplifier. Yellow are the biasing network for IRF and 5volt dc supply. In blue,The crown heat sink is the driver transistor. In he tblack is the IRF amplifier stage. In corner  black LED to indicate the DC supply...........
One thing I noticed that in the previous two attempt the preset was atleast 2 hole apart from the two 100 resistors... here that is the mistake. So if you are making atleast put the preset 3-4 holes away from the 100 gate resistor. This way during the tuning the screwdriver just don't hamper the coil.

Please use the dedicated LM7805 here. As the work is in progress  at the input and output we need to put one 470 mfd, 25volts each. This way It gives a good stable 5volt. There is one LED placed so it burns the voltage during switched off and reminds me that there is voltage.

I have made a point to add atleast one LED per board so that quickly I can know atleast there is voltage and power supply is not at faults.....

Now the back side of the vero-board.

Here it is important that the IRF510 is soldered in the back side to reduce the lead length. IRF is not inserted into the holes. Just placed over it. If the lead length is big it will sure to oscillate, be it in the copperboard, etched pcb or any damn thing. Make sure the lead length is small. also notice the gimmick 10 ohm resistor on the gate of IRF510. I have also used the 100k+0.1mfd capacitor for negative feedback. This reduces the possibility of the parasitic oscillations. I did not have the insulation kit for transistors, so using the mica washer only. Of course the heat conducting paste is liberally applied. The gates impedance is 100 ohm so it needs more drive from 2n2218. That's okey.

I could not find the round ferrite core used in the CFL lamp. So used its transformer by breaking its half E core into a C and then placing a small slab on it so it becomes sqare bobbin. I also liberally applied the plastic adhesive used in the water pipe / plumbing.

work is in progress will be update from time to time.... whatever the outcome.....

07 Feb 2015.
Working on the Quadrupler and Amplifier. May be today or tomorrow it will be ready. At this time only one band will be operational. But will make it for two bands.

3.68 mh Ceramic VFO + Quadrupler is almost ready. Two Coil needs to be tested/ tuned for 20meter band.  One small amplifier will be added which  will drive a CW as well as DSB modulator plus DC receiver. Lets see......

Actually I wanted to make this for  20 & 40 but in a bit hurry to be on air.... anyway will make dual band also. On searching the net I found somedifferent filter which has the 50 ohm terminations impedance using the capacitor divider method. That's good. So, we don't have to put a tap, that results in physically strong coil. When we put the tap it slacks a little bit.... whatever works is okey for me. So when you re-produce make sure it  works for you.... I don't intend to make another or repeatable thing. Each prototype will be little different, learning from failure and success of the previous modules and new information.

The modules first tank i.e quadrupler required extra 47pf. That means it required (47+68+150)=268pf total with 13 turns 20swg copper wire wound on the 1/4" drill bit. I chose the dril-bit as it is a very precise tool then lots of other available items like sketch pen, plastic tubes etc.

On second thought I think a small 2db pad will solve a harmonics problem. 470 ohm 10 ohm 470ohm preset to control the RF. This was quadrupler get minimum power required without producing excessive harmonics. I will not put the signal directly to tx stage. Instead make one class-A tune amplifier with emitter degenerations. So that harmonics are down further. Will also use more filtering at the  small linear stage.

One friend recently asked why taking this much pain. I said I have in the last year spent atleast 20k in buying old marine receiver ( lasted 6 month), LRR40 receiver ( that piece never worked from day one), VHF DR150 that guy cheated me. So I don't trust. I will not buy anything from the hams. Only VU2PTR & VU2KD was able to give me something which worked as they claimed.

So I am making this much effort to be on air. First will be the CW/DSB tcvr 15watt. Then will move towards the other band.

The receiver was tested with this quadrupler module. It gave me good quality signal in speaker even with 15cm wire as antenna. It has too much gain so there is need of proper shielding and filtering plus 3db resistive pads....


At night time again tested the linear module. There was short developed due to the that green capaciotor. Removed it. There was no connection/ dry connection on Vcc rail....
need some work..... there is some carrier leakage...

This is the first time TA7358Ap module is leakage of RF. On verobaord it was very nice. It might be due to the  copper planes instead of small pin socket...(. it needs some pads / resistor to curb the leaks...)


finally plugged the problems. At the 150 mw pre-driving amplifier on the  DSB modulator put one 10k resistor across the 4:1 transformer. Then things settled down. It still producing more power so will have to put some pad at the IRF510 output. Signal is very clear and clean. Will put one filter after IRF510 amp... Starting to brew the after burner (using IRF840) and the cabinet, VFO. So I will have three things on this TX/ CW full band, DSB on 14318kc xtal and DSB on 14194 kc ......

Half success...One day one day, I will be transmitting ham....

work in progress........

(this ends here, next will be actual on air test and other things.....)


( Till August 2015 not tested it on air. oh the damn 24 Vdc power supply.Will drive a big linear with it. )