A product from the NorCal QRP Club ...
Put Your SMK-1
on 80 & 160 Meters

by Wayne McFee, NB6M

About the AmQRP Club
Frequently Asked Questions
Kits
Projects
QRP Forums
Contesting
References
Links                                                     


Having had enough fun with my VFO equipped and 5 Watt Mod equipped SMK-1s on 40 meters to want to try something different, I thought, with 80 and 160 Meters now open during the winter, why not put an SMK-1 on one of the lower bands and see what activity I could scare up.

By staying with crystal control, the SMK-1 can quickly and easily be put on a different band by making appropriate changes to the receiver's tuned input circuit, changing a few capacitor values in the oscillator feedback circuits and changing the values in the transmitter's output network. Crystals for either 80 or 160 Meter operation are readily available from Radio Shack, either online or by calling 1-800-THE SHACK. The rest of the parts needed to make the change may well rest, awaiting use, in your parts bins.

In addition, the circuit can very easily be modified to provide for a separate receiver RF input so that the low noise advantage of a small, tuned loop antenna with an appropriate RF preamp could be realized, especially on 160 Meters.

Sure, the amount of tunable frequency range will be even less on 80 or 160 than it is on 40, but the SMK-1 is basically a rockbound, single frequency transmitter with a slightly wider bandwidth receiver anyway, so the limitation is really not all that constricting. Think of it as having a rockbound transmitter and a rockbound receiver with RIT. My 80 Meter SMK-1's receiver has just over 1.5 Khz of frequency range.

As observed with the 40 Meter SMK-1, the receiver's Local Oscillator drops out of oscillation past a certain point of VR2's range. This doesn't really hurt anything, but is something to be aware of, especially when you first turn the rig on after modification. Start with the pot all the way to the left, counterclockwise, and it will be oscillating just fine.

Also, as the SMK-1 is known for its characteristic small amount of transmitter chirp, especially on the higher end of the transmitter's tuning range, stability is improved, resulting in no chirp, by disabling the transmitter's tuning circuit. Since there was very little transmitter tuning range anyway, and there will be even less on 80 or 160, this is no loss and your signal will sound much better. The transmitter's tuning circuit can be disabled easily by removing C16. Or, if you are just getting around to building your SMK-1 and want to put it on either 80 or 160 Meters, don't install C16, D6, D7, R7 and VR3.

In order to gain some flexibility with the transmit frequency in the SMK80, adding a SPST switch and a 68 pf NP0 capacitor, in series, between the C16 (removed) pad closest to the front of the rig (junction of X2 and C16) and ground will provide about 600 Hertz of shift, so that one could at least move the transmitter a little further away from nearby QRM when appropriate. This amount of shift is still within the tuning range of the receiver.

Color burst crystals for 3.579 Mhz are commonly available, but with PSK-31 and other digital activity now in that part of the 80 Meter band, there seems to be little or no CW operation there. Radio Shack part number 900-5085, a crystal for 3.686 Mhz, at $1.18, would make a much better choice, as that frequency falls right into the old Novice portion of 80 Meters. More CW activity has recently been heard in that frequency range than in the low end of the band.

Crystals for 160 meters, 1.843 Mhz, Radio Shack part number 900-5089, are available for $2.00 each.

80 Meter Mods

Here are the changes I made in my SMK-1 to put it on 80 Meters:

Change X1 and X2 to 3.686 Mhz crystals
Add 130 pf NP0 across TC1
Change C1 to 160 pf
Change C2 to 940 pf
Add 240 pf NP0 across L2
Change C4 to 390 pf NP0 (probably should be 470 pf or so, but I used the 390 that was the old C24)
Change C5 to 150 pf NP0
Change C18 to 270
Change C24 to 820 pf, Silver Mica or NP0
Change C25 to 160 pf, Silver Mica or NP0
Change C26 to 1000 pf, Silver Mica or NP0
Change L5 to 2 uh, 22 T # 24 on T37-2

Note that wherever a capacitance value is increased, rather than having to remove parts, another capacitor of appropriate value can be soldered across the existing part. In this way, only L5 would actually need to be physically removed.

If that approach is taken, in addition to changing the crystals, do the following for 80 Meter operation:

Add 130 pf NP0 across TC1
Add 82 pf NP0 across C1
Add 470 pf NP0 across C2
Add 240 pf NP0 across L2
Add 270 pf NP0 across C4
Add 82 pf NP0 across C5
Add 100 pf across C18
Add 390 pf Silver Mica or NP0 across C24
Add 82 pf Silver Mica or NP0 across C25
Add 470 pf Silver Mica or NP0 across C26
Remove L5 and replace with 22 T #24 on a T37-2 toroid

The 130 pf across TC1 was added in order to resonate the TC1, L1 combo to 80 Meters.

C1 and C2 changes, while perhaps not absolutely necessary, were made so as to maintain the same impedances seen by the signal coming from VR1 on 3.5 Mhz as was seen on 7 Mhz. Probably, since the impedance of a parallel tuned circuit is very high at resonance, only the impedance ratio between C1 and C2 is important, and added capacitance wasn't needed. You are welcome to experiment. 240 pf was added across L2 to resonate the TC2, L2, C1 and C2 combo to 80 meters.

With these changes, TC1 and TC2 seem to peak the signal/noise level quite well on 80 Meters, but some experimentation may still be in order as to the exact value of capacitance to add across TC1 and L2 in order to get the "double peak" in the tuning range of the two trimmers, which should be there in order to indicate true resonance of the circuit.

In this case, the approximate values needed were determined by the simple expedient of first wiring an air variable capacitor across TC1, tuning for maximum signal, removing the air variable and measuring the resultant value of capacitance. An appropriate value fixed capacitor was then soldered across TC1, and the same procedure was used to determine the approximate value needed across L2.

Since C4 and C5 determine the feedback necessary to the oscillator in SA612, their values were changed so that their individual impedances would be the same on 3.5 Mhz as they were on 7 Mhz, in order to be sure that a sufficient amount of feedback for reliable oscillation was provided. C18, in the transmitter's oscillator circuit, was changed for the same reason. Again, you are welcome to experiment.

The remaining changes, to the transmitter's output network, were made in order to scale the values from 40 to 80 Meters.

Although the one quarter to one third of a watt normally realized from an SMK-1 should be enough to have many enjoyable QSOs on 80 meters, more than that may well be needed on 160. To increase the transmitter power output of the SMK-1, there are at least a couple of ways to go. One very simple modification which raises the transmitter output to around a half watt on 40 Meters, and results in three quarters of a watt of output on my 80 Meter SMK-1, is simply to lower the value of R-14 in the PA circuit.

Carl, K5HK, simply bridged R14 in his SMK-1 with a wire, directly grounding the emitter of his PA transistor. As he reported no obvious problems after several months of operation with R14 bridged, I gave it a try as well. This very simple mod raised the output of my VFO equipped 40 Meter SMK-1 from one quarter to one half a watt. Not earthshaking, but a significant improvement for very little effort. After several months of operation now with that mod in place, no overheating or other adverse effects on the PA transistor have been observed.

I tried running the emitter of the PA transistor in my 80 Meter SMK-1 directly to ground as well, and realized just over .8 Watt. Due to the higher gain of the transistor on 3.5 Mhz, and wanting to forestall any possible problems of overheating or instability, I ended up bridging R14 with a 2.2 Ohm Resistor, which resulted in .76 Watt output into a 50 Ohm dummy load, with the little rig operating from an eight AA battery pack. No overheating or other problems with the PA transistor have been observed, and on-the-air signal reports are good. If you do this mod and your PA shows either any signs of instability or heating, try changing the 2.2 Ohm resistor to a higher value, perhaps 4.7 or even 10 Ohms. You should still be able to realize a higher power output than the stock SMK-1, with no overheating or other problems with the PA stage.

If you care to operate at the QRP Gallon level, a 5 Watt Mod as performed on the 40 Meter SMK-1 could be added to the rig. Suggested output network values are listed below:


                      C1               L1               C2                L2             C3

80 Meters      820 pf         21 T #24       1600 pf        25 T # 24      910 pf
                                         T50-2                             T50-2

160 Meters    1600 pf         30 T #24   Two 1600 pf    35 T # 24      1800 pf
                                        T-50-2       in parallel         T50-2
                                           or                                    or
                                       28 T # 24                        32 T # 24 
                                         T68-2                              T68-2


160 Meter Mods

To put the SMK-1 on 160 Meters, the following changes are suggested:


Change X1 and X2 to 1.843 Mhz crystals
Add 590 pf NP0 across TC1
Change C1 to 330 pf
Change C2 to 1800 pf
Add 1430 pf NP0 across L2
Change C4 to 820 pf NP0
Change C5 to 330 pf NP0
Change C18 to 390 pf NP0
Change C24 to 1600 pf, Silver Mica or NP0
Change C25 to 330 pf, Silver Mica or NP0
Change C26 to 1800 pf, Silver Mica or NP0
Change L5 to 4.4 uh, 30 T # 24 on T37-2

As noted above, except for the removal and replacement of L5, capacitors of appropriate value can be soldered across existing parts to make the necessary value changes.

And, as indicated for the 80 Meter version, some amount of experimentation may be needed in order to bring the receiver's tuned input circuits within the range of the two trimmer capacitors.

How you effect these changes will depend on how well stocked your parts bins are. You may need to remove some parts and solder in the correct value, and you may be able to add the necessary value across other parts by soldering on additional capacitors. Surface mount parts are very easily removed by the use of two low wattage soldering irons at once. A handy damp cloth is recommended for wiping the removed part off of whichever iron it sticks to when it comes off the board.

If you, too, have had enough fun with your SMK-1 on 40 Meters to want a change, warm up your soldering irons and make the simple changes needed to put the little rig on 80 or 160. I hope to see you on the air.

Enjoy.

Wayne NB6M

Copyright 2001

 
Material and concepts presented on The American QRP Club (TM) website is Copyright 2003 by The American QRP Club, Inc.
These pages are designed and maintained by George Heron, N2APB
Page last updated:  April 15, 2004