I just completely recapped my 500A, every electrolytic cap in the set needs to be renewed. I can guarantee that they are long since dried out. The build dates on the caps on my unit were 3rd month of 1975 so they're way out of warrantee. The 20uF multisection cap near the front of the unit on the left as you face it will not be available. Use the equivalents or greater and tuck them underneath the chassis. Digi-key is an excellent source of small quantities of those units. There are also tub tubular (paper) bypass caps (small value) that can be replaced by poly-props, make sure the working voltage meets or exceeds the originals. After recapping the sound was amazing, I highly recommend the repair.

Also if you need service notes and schematics you can get the from AGTannenbaum out of PA. Very reasonable and excellent service.

BG