>> I have the board out it doesn't look damaged although a few of the solder joints have gone black (like bad joints) <<
Examine closely to see if that isn't the black marker pen of the factory inspector first.
If it isn't, then don't gloss over the (very) obvious.
When you see *anything* out of the ordinary in a circuit that you are troubleshooting, correct it. That might be all you need do. Clean out those suspect solder joints and resolder and test.
If that doesn't do it, the method of troubleshoot is simple enough, all you need is a good DMM set to the 20VDC scale, then monitor the 6V line while operating, the regulator on the board is there to keep that voltage absolutely constant. Any fluctuation at 6V output pin 13 on the board is the clue.
It is a rather simple regulator circuit using Integrated Circuit Q4 as the pass element -- *Everybody* new always suspects that IC! -- I have seen very few of them actually go bad, although it does happen. Suspect this last would be the smart troubleshooter's path.
Download this particular Service Manual if you have not already done so.
Fox Tango site has it: http://foxtango.org/FT-101SVCMan/Foxtango%20101%20Service.htm
Don't overlook this particular manual -- almost everything you will need to know, right down to even soldering techniques on the PCBs, is in there!
And don't be too overly concerned with model suffix difference here, either. "E" - "EE" - "EX" or whatever, the basics are the basics on these and don't change that very much. The power supply is a basic.
Some things that can cause this problem other than the black solder joints (perhaps that black is marker pen, too, examine closely -- inspectors often used marker pen to proof that they had inspected):
*VR4 (sometimes id as "VR-3) located near that IC is an adjustment trimmer resistor on the board and can get dirty, I've also seen some give up the ghost. This controls both receive and transmit voltage and is the adjustment internally for that. Often a quick spritz of control cleaner plus turning it end to end to clean, then realign for the critical 6V reading is all there is to it.
*If, once you have cleaned up all solder joints on Q4 IC and associated components, cleaned, checked and/or replaced VR adjustment trimmer and still cannot acheive a locked 6VDC situation, suspect the 1000mFd and 100mFd electrolytic capacitors associated with the circuit before decrying that you need that new IC.
*Attach the DMM + lead securely to pin 13 (without an extender card this can be done by tack soldering an insulated small gauge wire at aan appropriate point to tap the regulator output to pin 13 thus bring the testpoint out where you can safely attach and alligator clip) and securely attach the negative lead of the DMM to chassis Common (GND). Use a "spudger" -- and insulated tool such as a bamboo or plastic chopstick -- to tap lightly around the IC regulator circuit, looking for changes in that voltage with mechanical vibration.
*The board end connections and its mating socket can get corroded or dirty over time, too, clean the board contacts with a common pencil eraser and reinsert a few times to wipe the female side clean.