re capping

Recapping is most important with high voltage devices like power amps. If they go, they can take out a lot of the amp with them. Especially important if you have expensive tubes, or irreplaceable transistors.  30 years is a good threshold.

Line level devices like your 901 EQ probably won't suffer catastrophic failure if a cap blows, but the electrolytic caps today are much better than their 1980's counterparts. It may be worthwhile, and relatively inexpensive.

Best,

Erik

A recent inquiry into the age of an amp, and my concern for the caps, resulted a response "we've tested the caps and they're all good". Is it possible to evaluate the condition of capacitors (other than working or non working)?

Electrolytic caps kept fully charged will function perfectly well and not degrade!

Nope. There are multiple mechanism for electrolytic capacitors to degrade. We once replaced 7 million of them.

The only way to properly test caps is to pull them all out and test with a quality tester. A cap test on a VOM is probably not adequate.

ESR increases as electrolytics age and increased ESR is never a good thing.

Electrolytic caps are rated for 1000 to 10000 hours @ maximum conditions. If operated at less than maximums, life can be doubled for every 10°C below max °C. Assuming 105°C caps and a 35°C internal temperature, that's between 125k and 1.2 million hours. Only a small percentage of the caps would be expected to have the upper limit. 1977 was more than a million hours ago, so a 40 year old amp is living on borrowed time.

If the ampzilla has never been recapped since 1977, as a technical type, I would never even turn it on without a full inspection and some critical level recapping

Ditto! 1977 is 40+ years and that's nearly 3 times as long as the recommended de-rated 15 years due to seal deterioration.

All capacitors dry out after awhile 15-20 years for sure 
Electrolytic power caps  for sure  . I owned a store and worked modding 
caps , there are several grades of quality  ,coupling caps,of various types like teflon, paper in oil.foils,  as well as  Electrolytic , including the top 4 pole cap types.  You get what you pay for  on average .

for the past 6 years i only use my equipment like 6 hours a week. should i worry about the 'caps'??