[Phlogiston]

OK, I got un-lazy and looked at the schematic. The 7A8 does all of those jobs - as a converter. So look at everything prior to and including the 14A7.

Your 455 signal is being rectified by the 2nd detector just as it would be in normal operation. There is also the remote chance that it could be the 2nd IF trans. The closer the failure is to the detector the less volume you will get with your injected signal. You can use this to trace the failure - if you have the right probes and knowledge- so be careful.