u/Clandestine_Caster

The video documents the modification to an Icom IC-M700 Marine Radiotelephone. In stock form, this model has a 6 KHz AM Filter that is used only in the receive mode. In Transmit, the 2.8 KHz SSB Filter is used in all modes. AM is an afterthought on this model and is only added in order to communicate with much older AM radios. When transmitting in AM Mode, the bandwidth is choked down to 2.8 KHz.

We decided to remove the SSB Filter and replace it with a 6 KHz AM Filter. This allows 6 KHz Bandwidth in the AM and SSB Modes, much more suitable for "emergency Broadcasts". After performing the modification the difference in audio fidelity was very apparent. The audio Frequency Response went from 300 - 3000 KHz to 70 - 6000 KHz. No, definitely not Hi-Fi, or even your local AM Radio Station's fidelity, but very acceptable for the noisy shortwave bands.

Here are the initial findings:

• AM Mode: The carrier is running at 80 watts. The Modulator section of this radio was not designed to fully modulate an 80 watt carrier. The carrier needs to be reduced to about 40 watts, which should give close to 100% modulation.
• USB Mode: Excellent sounding SSB with frequency response all within the 6 KHz USB Passband.
• LSB Mode; Here is an interesting, and useful anomaly - this radio only has LSB as an afterthought as all marine comms are in the USB Mode. In LSB the radio is now exhibiting DSB operation. The passband is 6 Khz wide, spread across both passbands, Lower and Upper Sideband.

The physical modification:

• Although the size of the SSB and AM Filters are identical, the 4 pins that connect to the circuit board are placed differently.
• For testing purposes I ran jumpers from the pins to facilitate mounting to the PC Board. In the future I will construct an adapter board for the AM Filter.
• The radio needs an alignment, which will be performed later this summer.

6945 KHz USB at 0300 UTC (8PM PDT)

This information is geared towards experimenters that would like to use a vintage tube, amateur radio transmitter for broadcasting. The most suitable AM Transmitters will use Plate Modulation. Heising Modulation is a close second - the advantage is lower initial cost for a transmitter. This article is not intended for techs and engineers as it leaves out a lot of information that may interest this type.

Plate, Heising, and screen modulation are historical and modern AM techniques used in Amateur Radio transmitters. They differ by where the audio signal is injected: Plate Modulation alters total power at the anode, Heising uses a constant-current choke, and Screen Modulation controls gain at the low-power screen grid.

1. Plate Modulation (High-Level)

How it works: The audio signal is injected in series with the DC voltage supply of the final RF amplifier using a heavy, iron-core Modulation Transformer. The audio voltage alternately adds to and subtracts from the high-voltage DC, varying the plate voltage and output power.

Advantages: Produces pristine, high-fidelity 100% modulation with a highly linear envelope. It is the most efficient and rugged method for vacuum tube transmitters.

Drawbacks: Requires a large, heavy, and expensive modulation transformer capable of handling the full DC input power of the final amplifier.

2. Heising Modulation (Constant-Current)

How it works: A specialized sub-type of plate modulation. Instead of an audio transformer, a high-inductance Choke is placed between the power supply and both the modulator and RF final amplifier. The total current drawn from the power supply remains constant, with the current simply shifting back and forth between the modulator tube and the RF tube.

Advantages: Simpler and lighter since it eliminates the heavy modulation transformer.

Drawbacks: The plate voltage can never reach zero during negative audio swings, meaning it typically tops out around 80% to 90% modulation capability without causing distortion.

3. Screen Grid Modulation (Low-Level)

How it works: The audio signal is applied directly to the screen grid of a tetrode or pentode final amplifier, rather than the plate. The screen grid voltage controls the gain of the tube, altering the RF output as the audio fluctuates.

Advantages: Requires very little audio power. The modulator can be a tiny, inexpensive audio amplifier, completely eliminating the need for bulky iron transformers.

Drawbacks: Inherently non-linear ("S-shaped" curve). It requires careful tuning to avoid distortion and typically limits your maximum carrier output power.

It is illegal to use a transmitter on the HF Bands in the US, without an appropriate license, except under Part 15 of the FCC Regulations.

Clandestine Caster

We often use vintage amateur radio transmitters for AM in the shortwave bands. Unfortunately, there are more transmitters available that don't work well for this purpose, than those that do work well. Many vintage transmitters either don't have a Modulator built-in, or the Modulator used is what is known as "Screen Modulation". This is a poor form of AM Modulation that is weak and is not suitable for broadcasting.

We are interested in either Plate Modulation or Heising Plate Modulation. Both methods are suitable for AM Broadcasting. True Plate Modulation is available on transmitters like the Globe King. Unless we have very deep pockets, the Globe King id usually out of our price range. An affordable transmitter is the Globe Scout Deluxe with Heising Modulation. I own two, my last one was purchased for $70 USD.

Transmitters like the Heathkit DX-35, DX-40, DX-60A, Globe Scout 680, and the Globe HG-303 are not suitable for use as an AM Broadcast Transmitter.

Unless you purchase a vintage transmitter that has been restored, you will likely need to replace all of the electrolytic capacitors.

There are 10 slides in this article.

Vintage Underground Publication