Telrex Laboratories


Myron Pogue W6VAX

SB-33 and MP-10

So you are planning on buying one of the new transceivers! After complete evaluation of the available models (see "73" magazine for April), I chose the Sideband Engineers model SB-33, Everyone's reasons are usually quite personal for their choice of a transceiver —mine included:

1- I didn't want to give up 15 meters.

2. T liked the lower priced units as a personal investment,

3+ I liked the small package with a built-in ac power supply and speaker—good portable rig.

4. I liked the idea of mostly transistors for reliability, less heat, low level handling of rf and inherent stability.

5. I wanted selectivity—2.1 kc mechanical filter was just right.

6. Easy conversion to mobile by an ac inverter appealed to me (here is where my story really begins),

So I traded for an SB-33 Transceiver!

At the home station with a beam antenna for 15 and 20 and a trap inverted V antenna for 40 and 75, the transceiver met my fondest expectations. I even worked out barefooted on 20 meters on weekends reasonably well.

There was one small awakening, however, 1 tried several microphones which were not recommended, only to receive reports of questionable or poor audio quality. Ceramic mikes won t work even acceptably on the SB-33. Most dynamic mikes represent a sufficient mismatch so as to provide less than the desired audio quality. Finally, : purchased the recommended Turner model SR 90-D with an impedance of 25,000 ohms and. Eureka!

have also tried various antennas on the rig. The receiver is very responsive with a resonant antenna for the band being used. The latitude of impedance matching of the transmitter seems limited but is adequate for resonant antennas with an impedance in the 50 ohm to 75 ohm range.With appropriate antennas, it is a great transceiver.

So ar, the special Inverter provided by the manufacturer had not been available in my area. Being impatient to trv the rig as a mobile, 1 ordered the Heulhkit Model MP-10. The ratings of the Heathkit Marine Inverter are more than ample. It is rated at 175 watts continuous power output with 12 volt input, while the SB-33 requires only 165 watts for rated PEP of 135 watts input The SB-33 works on the MP-10 Inverter, but two things were readily apparent, First, there was a terrific 120 cycle hum in the receiver. Hie Heath Instruction book states this will happen and provides a solution: put 1 or 2 mmfd 200 volt capacitors (non polarized) across the ac output of the inverter. Two 1-mmfd capacitors reduce the hum to an acceptable level.

Next, the power input and output of the SB-33 using the MP-10 was only about ii of that from the self-contained ac power supply when connected to the power lines. This comes about because the voltage tripler in the high voltage supply looks at the ¡>rak value of the power line voltage, or 166 volts rather than the 117 RMS. What to do? I finally decided to take the transformer of the MP-10 apart. After removing the laminated core, it was apparent that I could add one layer of wire and connect it in series with the output winding to provide the required higher voltage. Experimentation proved that one layer (about 75 turns1 of no, 24 enamel wire provided the desired voltage and would fit the space in the window of the transformer after first removing the outer covering and using brown paper sprayed with Krvlon for insulation between layers.

The original output of the Inverter is connected to pins 1 and 4 of the SB-33 socket on the rear of the transceiver. Hie new winding on the MP-10 is connected in phase to ad voltage by connecting from pin 4 to pin 5, This, of course, is actually done inside the MP-10, and I replaced one of the ac receptacles with an octal socket and wired to the same pin numbers as on the 11 pin socket on the SB-33. Now I can operate with full power as a mobile using my modified Heath MP-10 Inverter. Im not sure I would go to this trouble again if the special Inverter were available. However, the Heathkit Inverter costs about half as much as the special Inverter and the modification is quite easy to make. The result is gratifying and on the air I can tell about rewinding a transformer to make my transceiver mobile. , . , W6VAX

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