[mikethedruid]

Looked closely at the face plate of the unit and it is a model 601B. I looked in my Rider CD for volume 1, and found Model 603B which appears to be about the same circuit wise, but mine differs a good bit from the pictures of the 603B interior I found in the links above.  Here's what I found inside mine:
[Image: https://66.media.tumblr.com/3c6c6ec060c9...o2_500.jpg]

[Image: https://66.media.tumblr.com/4fe1bb521bc8...o3_500.jpg]

[Image: https://66.media.tumblr.com/3a61380cbcbb...o4_500.jpg]

I'm figuring how to remove the whole guts. Looks like 3 large machine screws from the bottom, and the small machine screws that hold in the front panel.  I may actually have lucked out. There is a schematic for it inside the unit behind the rectifier tubes. There was a picture of it taken by the previous owner before he made the substitute rectifier tubes with modern diodes inside. I certainly DO have to replace the line cord which is beginning to fray. It is the ancient two cloth covered wires twisted together type! Fortunately, an antique lamp supply place I have dealt with before carries it.  https://www.antiquelampsupply.com/light-brown-twisted-lamp-cord.html