Modifying the Icom IC-R100 for SSB!

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Modifying the Icom IC-R100 for SSB!

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Submitted by: Jason, VK7ZJA.



icr100.jpg



Icom released the IC-R100 communications receiver late in 1989, and at the time it was one of the most advanced receivers available, with 0.1 to 1856 MHz coverage coupled with AM, FM and wide FM reception modes, along with attenuator and preamplifier, multiple tuning steps, automatic frequency control & automatic noise limiter features, all in a case not much larger than a paperback novel. Conspicuous by it's absence is SSB reception. Described here is a do-it-yourself circuit to add in SSB reception to the R100. Be warned, though, this is no 'snip a diode' or 'cut the blue wire' mod, it will require a certain degree of skill to build & install.



Some Theory:

A single side band (SSB) signal is essentially an AM signal 'cut in half' and the carrier removed before being transmitted. SSB is a very efficient way of transmitting voice, using only half the bandwidth of AM and is four times more power efficient in that to get an intelligible signal through, you only need a quarter of the power on SSB as you would using AM. The disadvantage is that SSB requires much more complex circuitry to generate and receive. However, a somewhat crude yet effective way of allowing almost any AM receiver to be able to receive SSB signals reasonably well is with the use of a beat frequency oscillator (BFO) circuit, which adds back in the carrier that was originally removed from the transmitted SSB signal. Described here is a relatively simple BFO circuit that you can install to the R100 and be able to receive SSB signals quite well.


How To Use:

Once installed, the BFO is very easy to use. Tune in to an SSB signal in AM mode, press the 5 key to turn on ANL, and the BFO circuit is activated. At this point, the squelch is forced open and the squelch control becomes your SSB fine tune 'clarifier' while ever the BFO is on. To turn off the BFO, simply push the 5 key again, and the squelch control resumes its original function. In this way, the add in circuit is a 'no holes' installation, meaning there are no external switches to add, preserving the original look of your R100. This also means that the ANL and BFO features are turned on at the same time, and you can't independently activate the two. It is possible to separately switch the BFO using an external switch if you wanted to, though.


What You Will Need:
  • Vero or experimenters PCB board to build the circuit on
  • 680 ohm resistor
  • 1k ohm resistor
  • 22k ohm resistor
  • 33k ohm resistor
  • 130k ohm resistor (but see below text, may need 150k ohm, 120k ohm or 100k ohm depending on your circumstances)
  • 10M ohm resistor
  • 1pF disc ceramic capacitor
  • 2x 47pF disc ceramic capacitors
  • 0.01uF capacitor, monolithic / greencap / film type
  • 4066 quad bilateral switch IC, preferably the CMOS CD4066 version, but others will do
  • BC548 NPN small signal transistor, or equivalent like 2N2222, just be sure to observe correct pinout for the type you choose
  • 455 kHz ceramic resonator such as ZTB455E or CSB455J
  • small hook up wire of different colours
  • heatshrink, both small to cover a single resistor & large to cover the built circuit
  • fine solder
  • fine tipped, small soldering iron to get into tight corners without burning surrounding wires or components
  • good soldering skills!
  • for those of us with ageing eyes, a magnifying headband is a great help

The 130k ohm resistor needs a little explaining here. This resistor determines the level of BFO injection into the IC-R100 circuits, and depending on your installation, may need some change. If you don't have a very efficient antenna for HF reception and you only receive quite weak signals most of the time, you may find better results using a 150k ohm resistor instead. On the other hand, if you have a good antenna and regularly receive strong signals e.g. lighting up all segments on your signal strength meter, then a 120k ohm or perhaps even a 100k ohm resistor may be the better choice. The stronger the average signal, the lower the resistor value should be, but don't go below 82k ohm.

The general rule is: If you find that strong SSB signals sound very 'chuffy' or still sounding like SSB normally would on AM, then you should go down a value or two of this resistor. If you find weak signals are sounding like they are swamped by a carrier (this is the BFO signal being injected) then go up a value for this resistor.

You'll also find that judicious use of the R100's built-in attenuator will not only reduce general HF band noise, but also under strong signal conditions, reduce distortion of received SSB signals due to the fast AGC action of the R100's circuitry.

Apart from miscellaneous bits like hookup wire & heatshrink, this should total about $10-15 in parts.


How The Circuit Works:

Icom division in France produced a BFO circuit module called the BLU100, which is now unobtainable and incredibly rare to find on the second hand market. This circuit borrows quite a few ideas from that circuit and how it is implemented, with one major difference (more on this later).

When the user pushes the '5' key to turn on the ANL feature (and this BFO circuit) five volts appears at connector J9 pin 1 on the R100 main PCB. This voltage is tapped and is used to turn on all four gates of the CD4066 quad bilateral switch. A bilateral switch is like an electronic relay, able to pass AC & DC signals when on, and open circuit when off. The first gate (CD4066 pins 1, 2 & 13) is used to force the squelch circuit to be always open when the BFO is engaged.

The second gate (CD4066 pins 3, 4 & 5) is used to add in a 0.01uF to the detected AM audio path via R82 on the R100 main PCB. This 'rolls off' the audio response, a bit like a tone control, to reduce audible hiss when receiving SSB signals.

The third gate (CD4066 pins 8, 9 & 6) is used to supply the oscillator circuit with 5 volts via the 22k ohm resistor and turn on the BFO when it is engaged.

And lastly, the fourth gate (CD4066 pins 10, 11 & 12) is cleverly used to inject a voltage to the squelch pot. That voltage is then used via the 33k ohm resistor to take manual control of the AFC (automatic frequency control) feature that is normally only used in FM, and engage it for fine tuning or 'clarifier' control while receiving SSB signals.

The oscillator circuit, powered with 5 volts via the 22k ohm resistor, consists of a common NPN small signal transistor whose oscillation rate is accurately fixed by the 455 kHz ceramic resonator on it's base. This signal is then tapped off via the 1pF capacitor and 130k ohm resistor for feeding in to the IC-R100 circuitry, mixing in with the SSB signal at the final Intermediate Frequency (IF) stage in order to 're-insert' the carrier signal, and then be detected as if it were a standard AM signal.

The big difference between the BLU100 circuit and this circuit is where the BFO signal is injected to the R100 circuitry. In theory, the most logical place to inject a BFO signal is at the last point in the final IF stage, right before the AM signal is detected, in the Icom ICR100's case, that's the detector diodes D10 & D11, which is exactly where the BLU100 injects it's signal. In practice, at least for the IC-R100, injecting at this location requires quite a high level BFO signal, and any circuit connection at this point very significantly detunes the L10 tunable IF transformer, and loads the IF stage down. This results in lower sensitivity to weak signals.

This circuit instead injects the BFO signal slightly further up the IF stage, at the base of IF amp Q7 (and R68) where the injection level can be lower, so no fancy high impedance buffer amplifiers are needed like the BLU100 uses, and no detuning of the IF circuit occurs. This makes the BFO circuit presented here both simpler & smaller, and in my opinion, will give better performance than the BLU100.

When the BFO is engaged, it injects a very stable 455 KHz in to mix in with the raw SSB signal, which effectively re-inserts a carrier, and this can then be detected and received by standard AM circuitry - the result is quite legible & understandable audio from SSB signals.

r100 ssb circuit.jpg
The Circuit - Click To Enlarge

Each of the four small boxes with switch symbols inside represents one of four gates of a 4066 quad bilateral switch IC, with corresponding pin numbers as indicated. Don't forget the ground / 0v connection on pin 7, and the +5 volt connection for pin 14, which comes from wire link W5 on the R100 main PCB. A suitable ground / 0v connection can be made to the top of crystal case X3 on the R100 main PCB. The 33k ohm resistor doesn't need to be installed on your vero / experimenters board, it
can be installed directly from R19 to R156 on the R100 main PCB. Insulate it with some small heatshrink. When building the circuit on your PCB, try to keep it as compact as possible, there's not a lot of room inside the IC-R100. Here's what my nearly finished board looks like:

r100 ssb pcb.jpg


Installing The Circuit To The IC-R100:

Use the following pictures as a guide to installing each connection of the circuit to the ICR100 main PCB. You do need to make each connection exactly as shown on the correct side of each component on the main PCB, otherwise, it won't work.

When installing connecting wires to pins on the plugs for connectors J9 & J12, it is best to remove the pin from the plug by using a needle or extremely fine flat blade screwdriver to lift the plastic tab that catches the metal barb on the pin, which will release the pin and allow you to pull it free of the plastic plug. Use minimal solder when making the wire connection to the pin, otherwise it would then be difficult to refit the pin to the plug.

Keep connecting wires reasonably short so as to pick up minimal noise.

The BFO output from your circuit to the injection point at R68 on the IC-R100 main PCB needs to have the insulating paint carefully scraped off the leg of R68.

This connection can be made with thin shielded audio cable, using the metal case of nearby L11 as your ground point. This isn't absolutely necessary, however, and you can use just a single unshielded connecting wire to R68.

Pictures showing the general layout of the attachment points, and close ups of each attachment point in detail are below:


R100 main PCB attachment points overview-min.jpg
Overview of connection points. Click to enlarge.



R156 & W5 connections.jpg



R119 connection point.jpg



R19 & J12 pin 1 connection points.jpg



33kohm and X3 crystal case earth.jpg
33k ohm resistor and X3 crystal case ground point. Click to enlarge.



J9 pin 1 connection point.jpg



R68 connection point 1.jpg
R68 connection point.



R68 connection point 2.jpg
Another view of R68 connection point.



R82 connection point.jpg



pin removed.jpg
Pin 1 removed from J9 plug for soldering.



pin soldered.jpg
Pin soldered.
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BFO heatshrink.jpg
Circuit encased in heatshrink & ready to go.




Testing The Circuit:

Once you have made all connections and are sure everything is ready to go, temporarily insulate the BFO circuit with some unshrunk heatshrink tubing or electrical tape. Power up the IC-R100 with a 1 amp fused power cord; this will protect the radio in case something shorts out. Go into VFO (not memory) mode, set the R100 to AM mode with 1 kHz frequency steps, and push the '5' button to engage ANL. With a bit of luck, hopefully the BFO circuit will spring to life as well. You should be able to hear a slight change to the audio output. Connect an antenna, and tune across an AM signal and you should hear a heterodyne like tone mixed in with the AM signal - that is your BFO working! Adjust the squelch control and the tone should smoothly change in sympathy - that confirms your fine tune 'clarifier' is working too.

The acid test is to tune in to an SSB signal, and with some fine tuning using the squelch control, you should be able to quite clearly resolve the SSB signal. Finally, turn off the BFO by again pushing the '5' key and ensure the SSB signal sounds like it normally would in AM mode, and that the squelch control resumes it's normal operation as a squelch.

Tune to a variety of SSB signals, at different times of day, across different frequency bands, and have a close listen. Determine if you should make any changes to the 130k ohm resistor on your BFO circuit; remember the general rule is if you find that strong SSB signals sound very 'chuffy' or still sounding like SSB normally would on AM, then you should go down a value or two of this resistor. If you find weak signals are sounding like they are swamped by a carrier (this is the BFO signal being injected) then go up a value for this resistor.

Once you are happy with the BFO circuit's operation, place a suitable size bit of heatshrink tubing around it, shrink it in place, and the job is done. Alternatively, you could loosely wrap the circuit in electrical tape to protect it.

How Well Does It Work?

This BFO circuit is able to resolve SSB signals quite well considering how simple the circuit is. It is able to equally receive LSB and USB signals, just tune it in for best audible reception with the fine tune (squelch) control. In either case, it seems the best frequency to tune on the IC-R100 is 1 kHz higher than the suppressed carrier frequency (for both LSB & USB) and then fine tune appropriately. For example if you want to listen to 8867 kHz USB, set the R100 for 8868 kHz. Likewise, to listen to 3600 kHz LSB, set the R100 for 3601 kHz.

Fine tune 'clarifier' range is about 1.5 kHz and BFO stability / drift is 80 Hz in the first 10 minutes, over the next hour around 100 Hz, and drift after that is negligible. As mentioned before, judicious use of the IC-R100's built in attenuator on the HF bands will both reduce band noise and reduce distortion on strong SSB signals.


You can see the BFO in action here: https://youtu.be/d0SVB88dmzw

Further Improvements?

The IC-R100 isn't a serious HF receiver, and does have quite a wide AM IF filter for good fidelity reception of broadcast AM stations. Being a narrow bandwidth mode, this wide bandwidth filtering isn't the best for SSB reception - the receiver is less selective than a good SSB receiver. It could be made better by replacing the stock 6 kHz wide AM filter with something that would be a nice compromise of around 4 kHz, like a KBF-455R-4AS or CFWS455G or CFU455G2, or if you do a lot of SSB listening and don't mind sacrificing some AM fidelity, a more narrow filter like the CFU455HT or CFW455HT will make it just that little bit better for SSB reception, but I didn't bother doing that myself.
Certainly a 'proper' SSB circuit using a balanced ring demodulator with better audio shaping would do much better, but that is starting to go overboard and into the realm of diminishing return for the effort put in. Those sorts of changes would need to be made on the IC-R100 PCB itself, but really it would be best if those were designed into the radio in the first place.

In order to minimise frequent adjustments of the fine tune 'clarifier' (squelch) control from frequency to frequency and LSB to USB, it would be good to put the R100 through an alignment, especially section 7-4 Receiver Adjustment steps 1-6 of the official Icom ICR100 service manual. This aligns the 2nd Local Oscillator for accuracy of the last kHz of each tuned signal. It's not essential, but is a nice thing to do if you can, and makes using the BFO a more pleasant experience.

Acknowledgements:

I'd like to thank Adam for taking voltage measurements on his Icom IC-R100 for comparison to similar measurements made on my IC-R100, which helped in the overall
development and simplification of this circuit.

Disclaimer:

While all the information presented here has been carefully checked and verified, I take no responsibility for any omissions, inaccuracies or misinterpretations of this information. You perform this modification entirely at your own risk.

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