Steve Macdonald G4AQB describes a topband converter that can be used with receivers (typically, ex-military and similar) that don’t cover that part of the spectrum.

 

When I first got my licence back in 1971, topband (160m) was very popular for AM contacts. At the time there were lots of ex-ministry and WWII receivers available, including the famous R1155 receiver. One limitation with some of these receivers was the lack of topband coverage, the R1155 included. I remember tweaking the coils of an old R109A receiver to make it cover 1.8 to 2.0MHz and used it as my first receiver.

I have always wanted an R1155 receiver and to be able to spend a bit of time renovating and modifying it for my own use. Recently I acquired an early version of the R1155 and began to work on it. I now have it working fine. Only a couple of later versions of the R1155 (the L and N models) had topband. Most versions covered 3.0 to 18MHz along with the medium wave (MW) and long wave (LW) bands. I found various old circuits and articles for converting the receiver to topband, ranging from valve converters to major tweaking of the trimmers in the RF unit, which I did not want to attempt!

After a bit of research, I came up with a simple solution that uses just a handful of components and is based around an NE602A integrated circuit. This is a versatile 8-pin device and is frequently used in superhet and direct conversion receivers.

The NE602A is a double balanced mixer with a built-in oscillator. Although it can be used in various configurations, it is used in its simplest form for the Topband Converter. The circuit is shown as Fig. 1. It just requires a tuned input inductor covering 1.8 to 2.00MHz, a suitable crystal and a power supply.

 

Choice of Tunable IF

Because the R1155 receiver covers the MW band of 600 to 1500kHz, I decided to use this band for the converter output frequency. This part of the spectrum has a wide bandspread on the R1155 tuning dial. The mixer will change an input frequency of 1.8 to 2.00MHz between pins 1 and 2 to an output frequency of 800 to 1000kHz on pin 5. To achieve this, a 1MHz crystal is required in the crystal oscillator tuned circuit on pins 6 and 7 with the capacitors C3 and C4 set to appropriate values. These values are C3=100pf and C4=1000pf for use with a 1Mhz crystal. The NE602N mixer will produce an output of 800kHz (1.8MHz – 1.0MHz), although it also produces an output of 2.8MHz (1.8MHz + 1000kHz), the image frequency. This has not been a problem because the front-end tuning inductor is selective enough to eliminate the image frequency. At first. I was going to add an extra inductor stage to the front end but found it to be unnecessary.

The input inductor can be any TOKO type coil tuned to approximately 1.9MHz. Although TOKO type KANK3333R 10mm coils are hard to find these days, equivalents are available. I got mine from Spectrum Communications.

 

Construction

I built the converter unit on a small printed circuit board, Fig. 2, but because there are few components, it can easily be built using a ‘dead bug’ type of construction. In Fig. 2 you can see that I also included holes for an additional front-end inductor and tuning capacitors. The unit requires a supply voltage of approximately 6V. After testing first with a bench power supply, I used a rectified 6.3V heater voltage taken from pin 2 of the Tuning Indicator valve in the R1155 receiver. This consists of a single 1N4005 diode smoothed with a 3300µf 25V capacitor, giving an output voltage of 6.3V DC.

I added a 4-pole 3-way rotary switch to the front panel of the R1155 so that the converter can be switched in and out as well as having a mute facility for earthing the receiver antenna input if required. Unused tags on the rotary switch were used to mount the printed circuit board inside the R1155 receiver, Fig. 3.

 

Testing

I tested the unit on the bench first of all without the switch in place. I connected an antenna to the input pin and a lead from the output pin to the antenna input of the R1155. Using 6V from a bench power supply, I set the wavechange switch on the R1155 to 1500 to 600kHz and tuned to 900kHz on the dial (1.9MHz when converted). On switch-on you should hear noise coming through with an antenna connected. Using a suitable adjusting tool, peak the inductor for maximum noise. Depending on the inductor used, the capacitor C1 may need a different value in order for the inductor to peak half-way up the core. I found that 220pF was the best value to use.

If there is no noise at switch on, set the dial on the receiver to 1000kHz (1MHz) and listen. You should hear the crystal oscillator working. If you can’t hear it, check the oscillator circuit C3, C4 and X1, ensuring that you have the value of the capacitors correct because they can be critical.

I found that the converter works very well. The NE602N is very sensitive and stable giving an output that suits the R1155 receiver front-end. A couple of additions could be considered, such as a tuned output inductor and an additional tuned circuit on the input, but I was quite happy with the operation of the basic circuit.

Although the converter was used in an R1155 receiver, it could be used in any older receiver that does not cover topband but has Medium Wave. This could be built as an external unit in a small box. The converted output frequency can also be changed in order to convert to a different frequency by simply changing the crystal frequency and values of C3 and C4. The values can be calculated from the following formulae: C3 = 100 √ F(MHz), C4 = 1000 ÷ F(MHz)

The photograph, Fig. 4, shows the modified R115 – the 160m switch position is on the switch to the right of the front panel.

 

PW

 

Parts List

C1                       220pF Disc Ceramic

C2                       100nF Disc Ceramic

C3                       100pF Disc Ceramic

C4                       1000pF Disc Ceramic

T1                        TOKO Type KANK3333R or equiv. Inductance of approx. 45µH

XTAL                  1MHz

IC1                      NE602N

Switch                4-Pole, 3-Way Rotary Switch (if required)