[Kenwood] TR-2400 ground wire to battery

[Dave Edwards]

I was hoping someone on the list could help with a simple problem. 
I have a TR-2400, new to me that would barely work with a good battery. 
A good battery or 10 volts....same result. 
It would show the low battery indication, not transmit, and very low and distorted audio. 
Plugging in the charger, would correct things....but would exhibit that 60 cycle hum that one would 
expect from running off a charger. 
In fooling around, I found that if I hook a alligator clip wire from the negative terminal of the battery to the ground of the radio that the problem is fixed. In fact, the battery positive to ground of the radio only shows about 8 volts...when across the battery is a bit over 10.
So, I have a bad black battery wire going into the radio. 
Does anyone have a schematic that indicates where the black battery wire goes? 
I ask because it seems to go between the transmit and receive boards, and I am not sure I want to do major surgery to check that wire. It may be easier to just route a new one to a conveniant ground. 
Thanks for any tips!!
....Dave