It has been a long time since I have posted here but I wanted to share this for anyone who may have a "dead" AM21 off-air tuner.
My AM21 stopped working and the HR24 display showed both off-air tuners as "removed". Tried resetting, different USB cable, etc. but no joy.
I decided to crack open the AM21 and noticed that two of the 10uF 250V capacitors were bulging a bit.
Replaced both caps and it works again. $2.50 in parts and a bit of soldering - not bad.
Sorry I did not note the cap numbers. Be sure to get low profile replacement caps as the box is so thin.