Optimizing ICOM AM Operation
Flash! I have discovered a trick for the IC-7300 and 7610 on AM -- no modifications required! There seems to be a "sweet spot" where the positive modulation percentage on AM is improved if you adjust the output power to about 60% of maximum. Use the "Multi" knob to set it. Modulation drops off at lower or higher output power settings (e.g., 100%). With this setting, I am getting 10-12 watts of carrier with modulation peaks of 40-45 watts peak positive.
If you own one of the ICOM multimode transceivers that supports AM mode operation such as the 746, 756, or 7300, you probably already know that they typically won't achieve 100% positive modulation and that the average carrier level droops when you modulate the transmitter. For casual AM operation, most hams are willing to overlook these deficiencies. But if you're an AM perfectionist, they probably "bug" you to some degree. Is there a way to fix them? The answer is "yes", but there will be some effort involved.
First a brief review. Most of the ICOM HF/6 meter transceivers have a 100W Peak Envelope Power (PEP) output rating on SSB, CW, and FM. On AM, older units were adjusted to produce a 40W (maximum) carrier. Newer rigs such as the IC-7300 were designed for 25W, maximum. These rigs all use "low level" modulation, meaning that the AM signal is generated at an intermediate frequency and then amplified to produce the output signal. As a result, the Peak Envelope Power at 100% modulation must be limited to the capabilities of the power amplifier output stage, which is 100W for these rigs. Ideally when operating AM you would see a 25W carrier (average) with modulation peaks to 100W (positive) and nearly zero (negative) at 100% modulation. Instead, what you typically get is either 25 or 40W carrier (maximum) with positive peaks only approaching 50-70W and negative peaks approaching zero.
What is the source of the problem and how can it be fixed? It is due to the operation of the ALC circuit acting on the positive modulation peaks and pulling the power down. At first I thought there was probably no way to correct this, but it turns out that there is enough circuitry in these rigs to modify the ALC operation on AM without affecting any other modes. There is a well known modification that you can do by introducing a small negative voltage to the ALC input and adjusting it until you override the internal ALC with this fixed voltage. It works, but it costs you quite a bit of output power. In my tests it is typically necessary to drop the power to ~12 watts before you can get to 100% modulation. I wanted a solution that achieved the full 25W carrier that you should be able to get out of the rig.
All of these radios use a very similar scheme for the ALC circuit. There are a number of inputs that control it, including forward and reflected power, the power output control on the radio, and so on. There are two transistor switches that are only turned on in the AM mode. One sets the carrier power and the other switches in a capacitor that is supposed to filter out the modulation components so they don't affect the ALC operation. The problem is that they use a value that is too small for this capacitor-- typically 1 uF in most of these radios. This capacitor forms an R-C filter with a build-out resistor on the output of an op amp (they refer to it as a comparator in the circuit description). Typically this capacitor needs to be increased to a value in the range from 22-47 uF to correct the modulation problem.
As an example, I will take the case of the IC-746. In this rig, Q984 controls the carrier power and Q985 switches in C984 (1 uF) which is the averaging filter. These switches are only active in the AM mode. In this case, C984 could be changed to 22 uF. C984 works with R987 (150 ohms) to filter the modulation components when Q985 is "on".
With this mod, in the IC-746 R994 needs to be adjusted so that the maximum carrier output on AM is 25 watts, not 40. Otherwise it would be necessary to remember to turn the front panel output power control down to avoid "flat topping" on modulation peaks. The rig cannot achieve 100% modulation with a 40W output carrier because that would require a PA capable of 160W peak output.
Why did ICOM design these radios to have such low modulation capabilities on AM? I think the answer is that they wanted to accommodate the average ham who these days has little knowledge of AM operation and probably lacks the necessary test equipment to properly adjust their transmitter, such as an oscilloscope and RF sampling network or an accurate PEP reading wattmeter. As supplied, the ICOM ALC circuit completely prevents overmodulation (flat topping) because it is impossible to get even close to 100% modulation.
If you don't have a 746, all of their other multimode transceivers that support AM operation have very similar circuits. You will have to look for these components in the schematic and locate them in the rig. They will obviously have different part designations and in some cases, slightly different values. I will say that the 7300 is one of the more enigmatic variants; it's not entirely clear why some of the extra components are present or what they are intended to do.
If you own one of the ICOM multimode transceivers that supports AM mode operation such as the 746, 756, or 7300, you probably already know that they typically won't achieve 100% positive modulation and that the average carrier level droops when you modulate the transmitter. For casual AM operation, most hams are willing to overlook these deficiencies. But if you're an AM perfectionist, they probably "bug" you to some degree. Is there a way to fix them? The answer is "yes", but there will be some effort involved.
First a brief review. Most of the ICOM HF/6 meter transceivers have a 100W Peak Envelope Power (PEP) output rating on SSB, CW, and FM. On AM, older units were adjusted to produce a 40W (maximum) carrier. Newer rigs such as the IC-7300 were designed for 25W, maximum. These rigs all use "low level" modulation, meaning that the AM signal is generated at an intermediate frequency and then amplified to produce the output signal. As a result, the Peak Envelope Power at 100% modulation must be limited to the capabilities of the power amplifier output stage, which is 100W for these rigs. Ideally when operating AM you would see a 25W carrier (average) with modulation peaks to 100W (positive) and nearly zero (negative) at 100% modulation. Instead, what you typically get is either 25 or 40W carrier (maximum) with positive peaks only approaching 50-70W and negative peaks approaching zero.
What is the source of the problem and how can it be fixed? It is due to the operation of the ALC circuit acting on the positive modulation peaks and pulling the power down. At first I thought there was probably no way to correct this, but it turns out that there is enough circuitry in these rigs to modify the ALC operation on AM without affecting any other modes. There is a well known modification that you can do by introducing a small negative voltage to the ALC input and adjusting it until you override the internal ALC with this fixed voltage. It works, but it costs you quite a bit of output power. In my tests it is typically necessary to drop the power to ~12 watts before you can get to 100% modulation. I wanted a solution that achieved the full 25W carrier that you should be able to get out of the rig.
All of these radios use a very similar scheme for the ALC circuit. There are a number of inputs that control it, including forward and reflected power, the power output control on the radio, and so on. There are two transistor switches that are only turned on in the AM mode. One sets the carrier power and the other switches in a capacitor that is supposed to filter out the modulation components so they don't affect the ALC operation. The problem is that they use a value that is too small for this capacitor-- typically 1 uF in most of these radios. This capacitor forms an R-C filter with a build-out resistor on the output of an op amp (they refer to it as a comparator in the circuit description). Typically this capacitor needs to be increased to a value in the range from 22-47 uF to correct the modulation problem.
As an example, I will take the case of the IC-746. In this rig, Q984 controls the carrier power and Q985 switches in C984 (1 uF) which is the averaging filter. These switches are only active in the AM mode. In this case, C984 could be changed to 22 uF. C984 works with R987 (150 ohms) to filter the modulation components when Q985 is "on".
With this mod, in the IC-746 R994 needs to be adjusted so that the maximum carrier output on AM is 25 watts, not 40. Otherwise it would be necessary to remember to turn the front panel output power control down to avoid "flat topping" on modulation peaks. The rig cannot achieve 100% modulation with a 40W output carrier because that would require a PA capable of 160W peak output.
Why did ICOM design these radios to have such low modulation capabilities on AM? I think the answer is that they wanted to accommodate the average ham who these days has little knowledge of AM operation and probably lacks the necessary test equipment to properly adjust their transmitter, such as an oscilloscope and RF sampling network or an accurate PEP reading wattmeter. As supplied, the ICOM ALC circuit completely prevents overmodulation (flat topping) because it is impossible to get even close to 100% modulation.
If you don't have a 746, all of their other multimode transceivers that support AM operation have very similar circuits. You will have to look for these components in the schematic and locate them in the rig. They will obviously have different part designations and in some cases, slightly different values. I will say that the 7300 is one of the more enigmatic variants; it's not entirely clear why some of the extra components are present or what they are intended to do.