I don't think the DC receiver matters for qrp as much as for high band noise or
crowded band conditions. It should have a 500Hz filter--but it is an audio
filter, not crystal.
If you can live with the bump in noise then the only thing I would be concerned
with is the PTO. I contacted TenTec a couple months ago and they didn't have
PTO rebuild kits. Should look into the finals and see if they are available.
I forget if the C22 has AGC. I believe the C21 did not. Oh, and the C21 had a
funky sidetone, iirc it was fixed in freq and rather low at that--and a
sawtooth too. Not sure what bits carried over. But the C21 had a built in power
supply, while the C22 does not.
TenTec mailing list