Panasonic TR-5041P B/W 5" Portable - help

[Electronic M]

On the eBay one there is a balun built in to the black part....on this Amazon one the black part looks to be just a handle and wires straight to the terminals from the 1/8". .https://www.amazon.com/CERRXIAN-Term...030621&sr=8-20

Your first 2 links are 75 ohm to 300 ohm baluns (which are bidirectional usually ). Those baluns do 2 things: they match impedances between different impedance transmission lines, and they convert between BALanced and UNbalanced (hence the name) transmission line...

If the connector on the set isn't making contact with adapter then either the connectior on the set is non-standard, has dirty contacts, or damaged. You could spray contact cleaner, replace it, or if you plan to keep the plug plugged in to the set for years on end you could try placing a bit of aluminum foil between the plug and socket to shim it and make electrical contact...it ain't the most elegant solution but if it works good enough for you that is all that should matter.