Patched HiFi AM firmware for Kenwood TS-950SDX
Bill Paul N1GPT firstname.lastname@example.org
It's long been known that you can get the Kenwood TS-950SDX HF to produce amazing ESSB audio using a few simple power-on menu tricks. However what's also long been known is that these tricks don't work on AM. That is, if you try to transmit on AM with the HiFi TX filter configuration, you won't get a carrier.
It turns out however that there's a way around this limitation. The problem is due to a bug in the way the gate array chip in the DSP unit handles the commands sent to it by the main CPU. The CPU sends the correct command for HiFi AM mode, but the DSP unit doesn't handle it right. You can fake the DSP unit out though, by sending two command words: one to start the radio transmitting in normal mode, and another to change the filter selection for HiFi mode (without releasing the transmitter in between). This can be done by modifying the filter selection from the front panel while transmitting, but you have to do it every time you transmit, which makes operation clumsy.
Since I like to operate on AM, and since I have some experience with embedded software development and debugging, I finally decided to do something about this, so I've produced a patched version of the TS-950SDX firmware that implements the DSP fake-out trick internally. When the user configures the radio for HiFi mode (menu 21 LPF set to "off"), the firmware will actually send two command words: a normal AM/3100Hz command word to get the transmitter going, followed immediately by a second one to get the filtering set correctly for HiFi AM. This is completely transparent to the user: the radio just works as expected when you turn the LPF off in AM mode.
The patch is based on the most recent firmware available for the TS-950SDX, version JQGA. This means it includes all the fixes Kenwood made during the radio's production run.
You can download the firmware by clicking the link at the bottom of this page. This file includes the original JQGA firmware, the patched version of the firmware, a detailed description of the modification, disassemblies of both the original and modified version of the firmware images, datasheet and user's manual for the NEC uPD7810 CPU, the Kenwood TS-950SDX service manual and a disassembler tool that handles uPD7810 code. Installation instructions are included that explain how to replace the firmware chip in the radio.
Note that updating the firmware requires replacing the firmware EPROM on the digital unit. You can make your own chip if you have an EPROM programmer and a 27c512 part. I am not providing the chips themselves, just the firmware image.
Patched HiFi AM Kenwood TS-950SDX firmware image