Thanks Bruce. Complex (read - more than rewiring a contactor) electronics are really not my thing. I guess I have nothing to lose so I'll open it up and take a look!
I am pretty sure that these have classic "linear" power supplies that work as follows. Power comes in at line voltage/frequency, and passes through a power switch and transformer. It's very unlikely that anything is wrong there. But with a multi-meter you can check that the transformer has 230V AC across the primary side. If needed, spend a few minutes reading about transformers on the internet.
The secondary side of the transformer has several windings. These provide lower voltages, for example 6 or 12 volts AC. Again, it's unlikely that anything is wrong here. That lower AC voltage is then sent through a device called a bridge rectifier, made from four diodes, that converts it to a DC voltage that is about 40% higher than the AC one. Each of the windings will have its own rectifier. One or more of these diodes may be burned out. If so, you may see/smell damage to it.
Again, you can read on the internet about how a "full wave rectifier" or "bridge rectifier" works. It's not hard to follow. It's also easy to test the four diodes (often contained in a package with four pins) with a multimeter.
The DC voltage from the diodes is then "smoothed" by some electrolytic capacitors. These can dry out after decades and fail. Your eyes and nose can sometimes sense this, sometimes not.
Finally, the DC voltage from the capacitors is often reduced and tightly controlled by voltage regulators, that are often three-pin semiconductor devices with a ground pin, and input pin, and an output pin.
In total, there are probably three or four different voltages produced in this way. Often they are labeled on the circuit boards as VCC or V+.
If you have a cheap AC/DC multimeter and some patience, you can identify and test the different power supply output voltages. If something s wrong, none of the main components (apart from the transformer, which is probably just fine) costs more than pocket change, so can be swapped.
Do you have a local ham radio club? Or an electronics club at a local school or university? Either of those should have people who can easily do this or show you how.
Does anyone have any thoughts on replacing the bellows with the metal ones? Worthwhile upgrade or not?
Personally I prefer the fabric ones because they take up less of the Z travel. Unless they are torn or otherwise defective, I don't think it's worth replacing them until they fail. At that time, if you don't mind losing the vertical travel, then go with the metal-armoured ones.
Cheers,
Bruce
EDIT: I just looked on-line and found a service manual for the TNC 113.
Service Manual TNC 113
If you look at Section 4.5 it shows the input and output voltages of the power supply. Suggest you test these with a multimeter. Be careful not to touch line voltage wires, 230VAC can hurt you or kill you.