Doepfer

I'm really confused on this one.

I have an a121 multimode filter that doesn't filter external sources.
At first I thought the filter was not functioning at all. The
resonance will self oscillate and allow hints of what was coming in
from the external source.  Then, I patched it in a conventional synth
path.

This works:

When I patch it in vco -> a121 -> vca, it modifies the sound as
intended (hp/lp/bp/notch).

These don't:

When I patch in external source (drum machine/iphone) -> a121, I
barely get anything.  The same if I patch ext source -> vca -> a121 or
ext source -> vca -> a121 -> vca.
When I have more complex sources like vco -> vca -> bbd (188-2) ->
a121, the result is the same as the external source.

Conclusion:

It only seems to filter when I send in simple wave forms from a vco.

Notes:

My other filters (a120/a106-5), allow me to patch in external sources
(iphone/drummachine) and work as expected.

Is there something unique about the a121 and complex wave forms that
I'm missing?  Is my a121 toast?