There are four different AK model 20's. The earliest (4640) is known among collectors as the big box. 20 The AK 20 deluxe (4920) (sometimes known as the 24) is the same as the big box 20, except for a fancy cabinet. Obviously these radios are larger than the others. They can be distinguished from the other two by the fact that they use binding posts for the battery connections. The next model 20 (7570) is smaller and uses a battery cable. It has brass tube shells to hold the tubes in. The last model 20 (7960) looks the same externally as the 7570, but lacks the brass tube shells and must use UX tubes.
Here is a list of AK literature and service data available by download. The documents are in Word 6 format. For some reason Word 97 will probably not open these. Wordpad (part of Windows) will work to print and view them. The schematics are in tiff format. I can supply these in jpeg format if needed:
AK Owners manual Vol. 2 (9C, 10C, 12C, 19, 20, 21, 24, 20C (7570)) : akvol2.zip
Small insert for the above to cover the AK 20C (7960) : 7960.zip
Schematic, part values, and continuity test diagram for the 4640 & 4920 : bigbox20.zip
Schematics, part values, and continuity test diagram for the 7570 & 7960 : compct20.zip
This is part of an article about using and restoring the AK 4640. I wrote this for the Antique Radio Gazette many years ago. It is not very well written, but may be helpful for a beginning collector.: 4640art.zip
Any questions, comments or problems? Please email me at radiola@erols.com