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SMK-1 5-Watt Mod
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"SMK-1 5-Watt Mod"
Well, here we go again, hi. I just couldn't keep from trying for more power out of the SMK-1. Here is an easy to do mod, using a cheap IFR-510 Mosfet from Radio Shack, that will give you 5 watts out. It requires a few more parts than the 1 Watt Mod, but gives the rig significantly higher power out. I have a zipped file of the bitmap format (Paint Program) circuit drawing which I will email to anyone who wants it. Study the circuit diagram before starting the mod, so as to get a good idea of what we are inserting into the SMK-1 circuit.
What we are doing is removing C-22, and effectively inserting at that point a PA stage and another section of output filter for added harmonic attenuation, as well as adjusting the output network component values for the higher power out.
Some of the parts are installed on the main board, in ugly style, and others, including the transistor and the added section of output filter, are installed on the side panel of the enclosure. Feel free to vary this installation based on your individual enclosure and connector locations.
You need 14 parts to do the mod. Here they are:
I did the mod "ugly" style with all leaded parts, soldering the necessary leads to the pads on the board. The IFR-510 is mounted vertically on the side panel of the rig's enclosure, about even with C-17, C-18 and R-13.
As I made my enclosure from pieces of PC Board material, and had not yet attached sides or a top, it was easy to build part of the circuit onto the new side panel. You may have to take a different approach, depending on what kind of enclosure you put the rig into.
A Mica insulator type mounting kit is used, as the Drain of the Mosfet must be insulated from ground, and Heat Sink compound is rubbed on both surfaces of the Mica Insulator before mounting the Transistor, as the side panel acts as the heat sink for the Mosfet. Mine does not seem to heat at all in this configuration.
I suggest you read all the following instructions first, while referring to the schematic, to get an understanding of the mod before proceeding with the actual work.
NOTE: If you have already done the 1 Watt Mod, start by removing the two 100 Ohm resistors and the PA transistor you used, leaving the .01 cap attached to the C-22 pad nearest to Q-3, the .1 Cap attached to the other C-22 pad, and the FT37-43 RF choke attached to Pad 4 of T-1.
If you have not already done the 1 Watt Mod, start by removing C-22 from the board (I used two soldering irons, quick and easy). You may want to save it for your junk box, but you will need to mark the small bag or container it goes in, as a .1 cap.
L-5 must be removed from the board, as it will be replaced with the T-37-2toroid with 19 Turns of # 24 magnet wire. Save it for your junk box, in a small container marked 1uh.
If you had already done the 1 Watt Mod and have removed the transistor and the two base resistors, skip the following two steps.
Cut both leads of the .01 cap so as to leave about 1/4", bend a 90 degree angle about 1/8" from the end of one lead, and solder that lead to the C-22 pad closest to Q-3 on the board. Leave the other lead straight for a moment. It should be positioned so as to angle up.
Cut one lead of a .1 Cap to a 1/2" length, bend a 90 degree angle about 1/8" from the end of the lead, and solder that lead to the other C-22 pad. Leave the free lead long for a moment.
Wind 19 turns of # 24 on a T-37-2 Toroid, trim the wire ends to about 3/8" length, scrape about 1/8" of the wire ends clean of insulating material, bend a 90 degree angle in each lead, even with the end of the insulation, tin the leads, and solder the leads to the pads for L-5.
Cut one lead of the 33 Ohm resistor to about 1/4", bend a 90 degree angle about 1/8" from the end,and solder that lead to ground, at the C-24 pad closest to the edge of the board.
Bend the resistor over so that you can solder it with a short lead to the .01 Cap, close to the body of the capacitor. Trim the lead to length, and solder it to the .01 cap.
Cut one lead of the other .1 cap to about 1/4", bend a U shape in that lead, hook it around the lead of the .01 cap just above where the 33 Ohm resistor attaches and solder it there. Trim any excess lead from the .01 cap at this time. Leave the free lead of the .1 cap long for a moment.
Cut one lead of the 2.7K Ohm resistor to about 1/4" length, bend a 90 degree angle in that lead about 1/8" from the end, and solder that lead to ground at the C-21 Pad closest to the edge of the board.
Bend the 2.7K Ohm resistor over so that it can be attached to the free end of the .1 cap coming from the top of the 33 Ohm resistor. Leave the free leads of the 2.7 K resistor and the .1 cap long for the moment, and do not solder them together yet, as they will be attached to the Gate of the Mosfet.
If you had not yet done the 1 Watt Mod, wind 5 turns of # 22 magnet wire on a T37-43 toroid, trim both wire ends to about 3/4 of an inch in length, scrape about 1/8" of insulation from one wire, and about 1/4" of insulation from the other. Bend a 90 degree angle at the end of the insulation on the end that was scraped for 1/8", and solder that end to pad 4 of T-1.
This new RF choke ends up positioned just about right over Q-5, and should be turned so that the flat sides of the toroid are at 90 degrees to the flat sides of T-1, so as to minimize any possible coupling between the two.
As I had not yet made a side panel for my enclosure, and had made the base and front and rear panels from PC Board, it was a simple matter to make a side panel out of PC board material, mount the Mosfet on it using the insulated mounting kit and heat sink compound, and build the rest of the added circuit on the side panel.
If your side panel is not removable, or your top panel does not remove easily to provide access to the side panel, you may have to use an alternate approach.
I mounted the IFR-510 vertically, with the pins down, so as to provide short leads to connect with appropriate parts. If your top panel is removable but the side panel is not, you can mount the transistor with the pins up, or sideways, whichever way will give you enough room and access to mount the remaining parts and solder the necessary connections.
The IFR-510 is positioned on the side panel so that it will be about even with C-17, C-18 and R-13. The top of the tab of the transistor is about 1/32" from the top edge of the side panel, just enough to provide clearance from the top panel so it does not short out.
Once the transistor is mounted on the side panel, using the insulating mount kit and heat sink compound, bend the Source lead down to the surface of the side panel, close to the transistor, and solder it to the PC board copper foil. Bend the Drain and Gate leads so they stick out at a 90 degree angle from the surface of the side panel.
With short leads, solder the 470 pf Silver Mica capacitor between the Drain lead and the copper foil surface of the side panel, with the grounded end of the capacitor towards what will be the front of the rig.
On the other side of the transistor, with short leads, solder one lead of the 10 Ohm resistor to the copper foil surface of the side panel, solder the anode of the 12 Volt 1 Watt Zener diode to the free lead of the 10 Ohm resistor, and trim those leads.
The cathode lead of the 12 Volt Zener diode attaches to the Gate lead of the transistor. Trim any excess Zener diode lead, and solder it to the Gate lead of the Mosfet, close enough to the body of the transistor so that there is room to attach the leads from the 2.7 K Ohm resistor and .1 Cap.
Cut one lead of the 390 pf SM Cap to about 1/4", and solder that lead to the copper foil surface of the side panel, even with the tab of the transistor and about a half inch away from the tab, on the side of the transistor towards what will be the rear of the enclosure. Leave the other lead long for a moment.
Now position the side panel on the enclosure and tack solder it to the PC board front and rear panels, so as to provide a good ground connection. In my SMK, I soldered the Ground wire which is next to the antenna wire on the board, with a short lead, to the side panel copper foil also.
Position the free leads of the 2.7K resistor and the .1 cap against the Gate lead of the transistor, cut them to length, and solder them to the Gate lead. Solder the free lead from the T37-43 RF choke to the Drain lead of the transistor.
Wind 16 turns of # 24 magnet wire on the other T37-2 toroid core. Cut the wire leads to about 3/8" and scrape about 1/8" of insulation away from the wire ends. Tin both leads.
Solder one lead of the T37-2 coil to the Drain lead of the transistor. The other lead of the coil attaches to the free lead of the 390 pf capacitor and the free lead of the .1 capacitor which goes to the C-22 pad closest to the edge of the board.
Position the three leads, cut the leads to length as necessary, and solder the three leads together.
Check visually, and with an Ohm Meter, if necessary, to be sure you have none of the "ugly" style components or leads touching the main board or other parts.
Check with the Ohm Meter from the Drain tab of the Mosfet to ground to be sure it is insulated from ground.
Check with the Ohm Meter from the copper foil surface of the side panel to ground on the circuit board to be sure the foil surface is well grounded. And that is it. Hook up power and try it out into a wattmeter and dummy load. I get a solid 5 Watts out of mine, and you should get approximately that much as well.
With this circuit the Mosfet draws no current on receive, as checked with a Milliammeter hooked in series between the drain of the Mosfet and the RF choke.
Also, if you have another type of QRP transmitter that you want to increase the power out of, and the driver (or PA) circuitry provides 150-250 or so milliwatts, this circuit could be added, with appropriate output network value changes if it is to be used on a band other than 40 meters.
Enjoy.
Wayne NB6M
SMK 5 Watt Mod - Addendum
For those who want to use the 5 Watt Mosfet PA in another transceiver, remember that the Mod makes use of the fact that there is a section of output filter network already in the SMK-1, which consists of a 390 pf cap, a 1 uh coil (replaced in the mod) and a 470 pf cap. Additionally, there is an 82 pf cap across the coil, which is a trap for harmonics. Another 82 pf cap could be added across the 16 turn T37-2, if desired.
In order to use the amp in another 40 meter rig, you would need to ensure that the entire output filter matches the effective result of the Mod, which means that you need a 470 cap, a 16 turn T37-2, an 820 pf cap (780 in the SMK with the mod), a 19 turn T37-2, and another 470 pf cap, in that order, as a five element output filter network.
Because of the fact that there was already an existing section of output filter in the SMK-1, It was easy to add a section and change the value (and size) of the coil in the circuit in order to effectively have the values indicated. The .1 cap between the two sections is nothing more than a coupling capacitor, and the two 390 pf caps are effectively paralleled, making 780 pf, close enough. If you use a single 820 pf cap, the .1 can go between the Drain of the Mosfet and the filter.
So you can either modify an existing output filter network by changing values and adding a section as was done in the SMK-1, or replace it with the entire five element filter listed above. For use on other bands, simply scaling the values should work.
In addition, be sure that you either have adequate supply voltage filtering for the 5 W PA stage, where you insert the amp into the circuit, or add a 100 uf electrolytic and a .1 cap at the top of the FT37-43 RF choke.
72
Wayne NB6M
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Page last updated: April
15, 2004