[DMcMahon]

Good input from Glyn above, be careful not to try that in Record mode, the fairly large HF record bias signal going to the record head may possibly bite a little.

Another possible thing to try Neil is the following (Note I am not sure if your TK 121 works the same as my TK 141/146s (have not had time to compares the circuits).

Setup the recorder for recording using a microphone or music source connected into the DIN input. Press the record button and select Pause (the record button should then stay locked down).

By either playing music into the DIN input or speaking into the microphone adjust/increase the Record level slider and you should see the record signal on the VU meter.

Then by increasing the volume control slider you should hear the record signal through the loudspeaker, i.e. you are monitoring the source. If this works, at least it proves that the later stage (s) of the Playback amp are functional. This is the bit I am unsure about if the TK 121 has this Source Monitoring functionality, the same as the TK 141 /TK 146, if you have the User manual for the 121 it will reference monitoring, either explaining the same as what I have just explained or otherwise it will say something like monitor via earphone/headphone connected to the headphone DIN socket.

Then you can switch from Pause to Play and it will record, you can then playback the new recording to see if any sound played back, if the source monitoring through the speaker works during Paused record it will also work during actual recording.

Note - If doing microphone record tests, then if the microphone gets too close to loudspeaker area and volume is set too high then you will almost certainly get loud feedback howl.