[A/S] univolt..radio question

[AirstrmCamper@xxxxxxx]

Hi
I still have the original Univolt on my 79 Airstream. It also has the 
original Motorola Radio and 8 track. I added a portable cd player and an 
audio component switch so that I could chose between the 8 track and the cd 
player and just input the signal into the Motorola.
It works real well. there are two things I have noticed with this unit. One, 
when the trailer battery gets low the radio seems to get a lot of distotion 
in its sound. And when the trailer is pluged in to ac there is a very loud 
hum from the speakers no matter what the vloumn is at , even if the volumn is 
all the way down or whether it is on tape, am, fm, fm stereo. It only does 
this when it is pluged in. On battery it is fine untill the battery gets low. 
How much power does this old Motorola draw?Would I be better off just 
replacing it with a car unit? I had wanted to keep it and the 8 track as they 
are a conversation piece and I like the "original " look. Any suggestions?
Jon