Modifications, Hints, and Tips for the Radioddity GD-77 Dual Band DMR Digital Handheld Radio

Introduction

The Radioddity GD-77 is an excellent entry-level dual-band FM/DMR handheld radio offering great value for money. It strikes a balance between affordability and features, making it ideal for ham radio operators who enjoy tinkering and customizing their equipment. This document compiles various modifications and tips for the GD-77, with credit given to the original authors where possible. Modifications are categorized by difficulty: Easy, Moderate, and Advanced.

If you have additional information or modifications to share, please contact the author at vk7zja@gmail.com.

Protective Leatherette Case for GD-77

While no specific protective case is made for the GD-77, cases designed for the Retevis RT3/RT8 and TYT MD-2017 fit well. Cases for the TYT MD-380 are also compatible.

Cross Compatibility of Accessories for the GD-77

Many accessories from other radios are compatible with the GD-77:

• Charger cradle: TYT MD-380 / Retevis RT3 cradles work.
• Programming cable: TYT MD-380 / Retevis RT3 cables work. Baofeng programming cables are not compatible.
• Belt clip: TYT MD-380, Retevis RT3, and Baofeng UV-5R series belt clips fit.
• Multi-bay charger: A multi-bay charging station for the TYT MD-380 also works with the GD-77.
• Antenna: Any decent quality single or dual-band antenna with a male SMA connector will work. The GD-77 has a female SMA connector.
• Protective case: Retevis RT3/RT8, TYT MD-380, and MD-2017 cases are suitable options.

Hint for the Rubber Side Cover

The speaker microphone rubber cover on the GD-77 can be rotated 180 degrees to lie flat against the side of the radio, keeping it out of the way of any plugged-in speaker microphone.

Different Power Supply for the Charging Base

The GD-77 charging base can safely accept any voltage between approximately 10 and 16 volts, as it contains an internal voltage regulator.

Converting Different Versions of Codeplugs

Codeplugs are not forward or backward compatible between different versions of the CPS software and firmware. Colin G4EML has created a codeplug converter to simplify this process. It can be downloaded from: http://www.gb3gf.co.uk/Files/GD77-CSV.zip

Volume Indicator Visual Enhancement (Easy)

To make the volume knob position more visible, fill the groove on the knob with fluorescent paint. This allows for easier identification of the volume setting, especially in low light conditions.

Screen Scratch Protection (Easy)

To protect the GD-77 screen, use a cheap cell phone screen scratch protection film, cut to size. This will help keep the screen free from scratches.

Sitting Up Straighter (Easy)

To prevent the GD-77 from tipping forward when standing on a flat surface, shorten the feet on the battery. File them down until they are almost flush with the battery case.

Headphone Adaptor (Easy)

An adaptor can be made to allow the use of standard headphones with the GD-77. This requires a right-angle stereo 2.5mm audio plug, a stereo 3.5mm audio socket, shielded audio cable, and a 22-ohm resistor. A wiring diagram is provided.

Prevent the GD-77 'USB' Charger from Damaging Other Devices (Easy)

The GD-77 charger outputs 12 volts, while standard USB devices operate on 5 volts. To prevent accidental connection of a USB device to the GD-77 charger, use hot melt glue to permanently fix the charging cable into the charger's USB socket.

Make the Top Panel Orange Button Easier to Press (Early Versions) (Easy)

For early versions of the GD-77, the orange emergency button can be difficult to press. To make it easier, a small piece of thin plastic or a cut cable tie can be glued to the top of the PCB push button to effectively raise its height, requiring less pressure to activate.

Modify a Speaker Microphone to Keep Out DMR Pulsed RF Feedback (Moderate)

To reduce or eliminate RF feedback from DMR's pulsed RF that can distort transmitted audio, a modification involving adding a capacitor across the speaker microphone electret element and an inductor in line with the positive line of the electret element can be performed. Details are available at http://owenduffy.net/blog/?p=9326.

Prevent the GD-77 Charger from Overcharging Batteries (Moderate)

To prolong battery life, the charger base can be modified to ensure the charging voltage does not exceed 8.40 volts. This can be achieved by adding a resistor in parallel with R4 in the charging base. The specific resistor value may need adjustment based on the charger's output voltage.

Faster Charge for the GD-77 (Moderate)

Li-ion cell charger modules (TP5100) can be used to modify the GD-77 charging base for faster charging. This involves installing a solder bridge to configure the module for two Li-ion cells and removing a shunt resistor to adjust charging current. A 12V, 1.2A power supply is required for this modification.

Reducing RFI from the Charger Base (Moderate)

To reduce RFI noise from the charger base that can interfere with weak VHF signals, modifications include adding 0.1uF ceramic capacitors across the input socket and battery contacts, and replacing the wire to the positive battery contact with one wound through a ferrite bead.

Changing the Electret Microphone Element (Moderate)

The audio quality of transmitted audio can be altered by changing the electret microphone element. While some users have reported improved audio with replacements like the POM-3542P-R, results can vary significantly between individuals and radios.

Dim the LCD Backlight (Moderate to Advanced)

The LCD backlight can be permanently dimmed by adding a 1k ohm surface mount (SMD) resistor in line with the backlight on the flexible ribbon cable.

Introduction to Hex Editing (Moderate)

Hexadecimal (hex) is a number system used by computers. Understanding hex editing allows for direct manipulation of data within files. This section explains the basics of hex, binary, and little-endian formatting, and recommends the HxD hex editor for making modifications.

Out of Band Frequency Entry (Moderate)

The GD-77's frequency coverage can be expanded beyond its stock limits (136-174 MHz and 400-470 MHz) through hex editing of codeplug files or the CPS software. Methods are provided for adjusting frequency ranges, with a caution to avoid editing the 'Basic Information' page in the CPS software after modification.

Cross-Band Frequency Entry & Operation (Moderate)

The GD-77 can be programmed for cross-band operation (transmit on one band, receive on another) using hex editing of channel data. This allows for operation on memory channels, though RF output power is halved, and DMR cross-band operation is not supported.

Is 220 MHz Operation Possible? (Moderate)

While the GD-77's RDA/AT1846S chip is capable of receiving in the 220 MHz band, sensitivity is poor due to the RF preamps being designed for VHF and UHF. Transmitting on 220 MHz is not recommended as it can damage the radio's electronics and cause interference.

GD-77 Tuning & Alignment Information (Advanced)

RF tuning and alignment data, including microphone gain, RF output power, and signal strength meter sensitivity, can be adjusted by editing 'soft' calibration values in the flash memory. More information can be found at: GD-77 Tuning & alignment information.

Improving RF Shielding on the GD-77 (Advanced)

Improving RF shielding involves several steps: applying conductive nickel spray paint to the inside of the plastic shell, ensuring good electrical contact between the PCB and chassis, and using metallic adhesive tape to cover any remaining openings.

Changing Display Fonts on the GD-77 (Advanced)

The display font on the GD-77 can be customized by loading larger or different font types. This involves using a font editor to create a bitmap font file and then writing it to the GD-77's flash memory using specialized software.

GD-77: Some General Technical Information

This section details the major internal components of the GD-77, including the MCU, DMR baseband IC, flash memory, EEPROM, and RF components like the AT1846S and RF power amplifiers.

GD-77 RF Sensitivity & Mute Thresholds

Graphs are provided showing the VHF and UHF RF sensitivity and mute thresholds of the GD-77, illustrating its performance across different frequency bands.

FCC Part 90 Approval Information

Technical information regarding the GD-77's FCC Part 90 approval, including internal photographs and test reports, can be found at: https://fccid.io/2AN62-GD77.

GD-77 External Flash W25Q80BV Memory Map

A memory map of the GD-77's external flash memory is provided, detailing the contents of various address ranges, including codeplugs, DMR ID data, and font information.

GD-77 Codeplug Map

A text file detailing system-level settings in the codeplug .dat file is available, along with a PDF expanding on channel entries. These resources can be found at the provided links.

GD-77 Programming Cable Pinout

The pinout for the GD-77 programming cable is provided, noting that it differs from many other Baofeng programming cables and does not require a driver.

YouTube Video Tour of TYT Manufacturing Facility

A YouTube video showcasing the manufacturing facility of TYT (the manufacturer of the GD-77) is available at: https://www.youtube.com/watch?v=R7xmHnpRCZM.

Known GD-77 Bug List

A list of known firmware bugs in the GD-77 is provided, with Radioddity aware of these issues and planning to address them.

Common GD-77 Faults and Their Fixes

This section addresses common hardware problems with the GD-77, such as soft audio squeal at low volume and blank or white screens, providing potential fixes.