I guess you mean TU-919. and that the composite signal goes in fine ( as monitored at the H detect jack for multi-path) but does not come out of LPF01.
Not going to be easy to find as it will probably only be available from a donor unit.
I have not taken one apart, but it maybe possible to repair it, if it can be opened up and components can be accessed. It is a L/C &/or notch filter of sorts to attenuate 114KHz etc.
LPF01, could also be removed/bypassed with slightly less adjacent channel rejection capabilities. Most tuners never used one.
Post in tuners forum, Mike might know better than I.