When considering a repair:
How much would you actually use it? Would it be worth $50 to $150 to repair it if you never used it?
You will need to buy and know how to use a multimeter, to check resistances and battery voltages for troubleshooting if you have trouble.
If you or a friend attempts a repair, you will need to spend money to make a battery, before you have any idea if the mine detector will run, and shipping cost for parts can add up. Do you really have the tools and experience to solder parts in tight locations?
Condition checklist:
What is the condition of the insides of the amplifier box? Are the three tubes present? Is there any obvious damage, broken wires, or burns?
Are the cables in good condition, with no more than the smallest amount of cracking?
Are the cable connectors and battery connections free of corrosion? The battery connections can be cleaned; cable connections are difficult to clean.
Do knobs turn freely? The battery compensator can be replaced, but not the control box controls.
Do the switches seem to switch smoothly? The amplifier box switch can be replaced; the control box switch would be a nightmare to replace.
A few pre-repair checks:
Disconnect the speaker/headset. With a small piece of wire and a third hand, connect a 1.5 volt battery across the speaker or headset connector. You should hear a scratchy noise in the speaker/headset, if not, try a 9V battery, but only for a short time.
Some resistance readings:
M-356 speaker/resonator: 1K ohms
Choke (component 2), as measured across the variable capacitor 15 (the white component): 2.5K ohms.
Resistances at the battery connections of the amplifier, with everything hooked up, and switches in the on position, and NO BATTERIES INSTALLED: at BA-33 connectors: greater than 1M ohm, at A-1 connectors: 4.5 ohms, at A-2 connectors: 5.0 ohms. Your readings may be off 25%, and probably not a concern.