When entering the are of old tube receiver repairment, I found that a signal tracer device is crucial. With this device, you can trace a signal in the stages of the radio device and identify the stage that is not working anymore, so it is a great tool to troubleshoot radio receivers.
Signal tracers are not produced anymore, I think they passed when receivers and amplifiers were designed with ICs, where you have multiple or all stages of an electronic circuit in a single chip. Then a signal tracer is if no help anymore. So, usually a used a signal tracer usually must be found.
ebay is your friend here and after some days checking, I got the Grundig SV41. Grundig is a famous name in germany, but today chinese-owned as far as I remember. The signal tracer is from the 70ies I guess.
It can be operated from battery or external power supply at 9V=. My device showed dead and leaked batteries in the battery pack section. The external power supply connector was broken, maybe during delivery. I replaced the external power supply and cleaned the battery pack.
The device offers a 1 Khz signal generator with attenuation -10 .. -60 dB, an amplifier, a speaker and a panel meter. It has connectors for generator output and signal input.