Red fault light comes on immediately, nothing connected. Alert beeper sounds. Was working well with FT-2800 2-meter rig on medium power setting.

You mean FT-225RD not DR ?

The RD is a base station all mode 2 meter rig. Older model.
Set the Channel switch to VFO.
Use the Band selector to set the vfo to 144, 145, 146, or 147 mhz.
Then turn the vfo knob to select the frequency.

sounds like you need to test your power source. Plug another item into your wall socket like a table lamp and see if it works.
If it does you need to check your power lead.- you will need a test meter or make a basic test kit; A torch bulb, a meter (3 ft length) OF insulated wire, and a dry battery joined so that when you touch two wire ends together, the bulb lights - or take the cable to an electrical shop!.
1st check the fuse in it
2nd check each wire in the cable; test kit to red at each end, then black at each end or whatever colours there are to see if bulb lights.
If all pass, you may have a faulty PC switch or fuse.

this is a very nice radio (it was one of my first) , it works on cb and 10 meter ranges and has many high end features for a radio as this

if it wont light up , ck the inline power fuse that is inline to the red lead that connects to radio and the black lead needs to be well grounded to car frame , also this radio runs on 12 volts dc (std car battery) and NOT house mains power
also I know this radio did not come with a microphone included as they wanted user to customize there rig to there own tastes , a microphone being properly connected may be required on this model (I know it does on other models) to get radio to power up correctly ,

also never connect radio to power without a proper 10 meter balanced antenna connected properly to input on rig this will cause instant damage to radio

Hi Mag,
I have this steps that might help you with beeping sound on your Scooter. Hope it helps.

UNIT LIGHTS UP/HORN WORKS BUT UNIT WILL NOT MOVE
Possible Cause:

• This often occurs when the scooter is in free wheel mode and is not in drive mode. Re-Engage the free wheel lever which is located down near the rear wheels (normally a yellow or red lever).
• If you hear a beeping noise, the beeping you hear is a fault beep indicating the scooter main control box has detected a problem. Contact your dealer who can diagnose the beep code.
• Check batteries with load tester - If either battery drops more than 1 volt under a load, the battery(s) are bad. Replace "both" batteries.

I also find these additional steps on how to know beep alarm indicators:

This main control box has a self-diagnostics feature & cannot be tested with a volt meter. When a
problem is present the main control box will diagnose the fault, prohibit the scooter from being driven
and sound an alert beep. Each alert beep indicates a different issue which helps to determining the
problem by narrowing the fault into one of a few main areas of the scooter. Reference this information
below for the alert beep indications.

Very Rapid beep- Batteries are low. Charge & test batteries.
Medium Fast beep- Motor/brake wiring fault. Free wheel lever disengaged.
Solid beep- The charger is plugged into the scooter (some models may vary).
Very Slow beep- The throttle pot was not at center when you turned the unit on. Throttle pot off
center or wiring (yellow & orange wires) fault.

On electronically controlled units, a light out issue may alert you to a failure situation,(minor).. Replace the bulb and see if that was the fault

I’m happy to assist further over the phone at https://www.6ya.com/expert/david_dc2f9e0a44104720

You have a module problem:

Evaluate your batteries. 12v Single or 24 together.
Flash codes:
1. Stall Timeout, Joystic out of neutral
2. Battery Voltage too over 32VDC hi or too low under 17VDC
3. Left motor Fault
4. Right Motor Fault
5. Left Brake Motor fault
6. Right Brak Motor fault
7. Shark Control Module Faul
8. Power Module Faul
9.Shark Module Faul

1. Stall Timeout, Joystic out of neutral
2. Battery Voltage too over 32VDC hi or too low under 17VDC
3. Left motor Fault
4. Right Motor Fault
5. Left Brake Motor fault
6. Right Brak Motor fault
7. Shark Control Module Faul****this will be your problem***
8. Power Module Faul
9.Shark Module Faul

Contact me Direct 909-590-1388 x115 CTM HOMECARE OFFICE

At this stage we have found 2 ways that might work but not guaranteed to work with all machines.

Method #1

1. POWER off printer
2. Hold RESUME button then press and hold POWER, the beeper will sound once
3. Hold POWER and release RESUME
4. Press RESUME twice
5. Release POWER button
6. When the indicator light is steady press RESUME three (3) times. Indicator light should be orange
7. Press POWER to set data

Sounds to me like your finals in the rig have blown stopping you using the CB mode, Either that or you have a loose connection on the power output inside the radio.

Solder bad connects in power circuits....Check all power supply
voltages with meter, consult service manual........T.