Earlier in 2024, I bought a large lot of very interesting vintage electronics – including test gear, computers, and audio equipment – which belonged to a local electronics engineer, who had sadly passed away quite recently, and whose family wanted it to go to a good home.
Among this was a very cool 1950s Radio City Products (RCP) Model 314 vacuum tube tester, which was in good condition for its age, but obviously needed a clean up and a tune up.
The most obvious issue was the mains cable – the original rubber cable had become brittle and perished, and the insulation had started to crack and fall off, which is not particularly safe for mains voltages as live and neutral can short together or you can get a nasty shock.
To replace it, I opened the unit up (removing all of the flat-head screws surrounding the faceplate, and lifting it out), desoldered the original mains cable where it connects to the electronics (taking care to note the location of live versus neutral, as the original cables were not coloured – if you’re stuck, this is also available on the schematics), and fitted a new mains cable with 3A-fused UK moulded plug – I also fitted an inline power switch to make using the unit easier.
The electronics in the 314 is quite simple – there’s a large number of switches to connect various tube sockets into circuit depending on the type of tube that is being tested, as well as to set the heater voltage. There’s also a large wire-wound potentiometer to set the line voltage. All of these could be quite corroded, so I cleaned them all with contact cleaner.
There were only two electronic components that I replaced preventatively: a 0.03uF 400V wax/paper capacitor at C1 which I replaced with a modern 33nF 630V polypropylene axial capacitor, as the original was electrically leaky; a 1kOhm “flexohm” cloth resistor at R2 which I replaced with a modern 1kOhm 0.5W 1% film resistor as the originals are apparently quite unreliable and frequently fail open-circuit.
The two cables for the anode caps were also quite badly perished and the insulation was flaking off, which is not good for high-voltage cabling. They are two different lengths and have different end connectors – one has a large clip which solders to the cable and pulls out of its shell, the other has a small clip which the cable solders into directly.
To replace these cables, I removed them from the chassis, taking note of where they originally soldered into – I then cut two lengths of 1kV-rated black test lead cable to size, fitted the anode caps, then refitted the new cables to the chassis.
The pilot lamp holder was corroded and snapped off, and the bulb itself was badly corroded too – I replaced these with a new E10 holder and a new 6.3V 0.5A E10 bulb.
I brought the unit up on a variac and dim-bulb tester in case the 6X5GT rectifier tube was shorted, but it seemed to work OK – its heater lit up and its DC output looked good. The pilot lamp also lit up OK.
The tester had probably been stored in a damp environment, as it had rust on its latches, hinges, and screws, and corrosion on the aluminium tube sockets and surrounds.
I used white vinegar and a wire brush to clean up the brightwork; I also used a Dremel on a low speed with a polishing tool to clean up the tube sockets and surrounds; some of the stainless steel sockets had discoloured, and I used Brasso wadding to clean these.
With the clean-up done, I reassembled the tester to try it out for the first time.
The first vacuum tube that I tested following the repair was a 6BF6 (triode and dual diode), using the user manual and supplementary tube chart – both it and the tester seemed to work OK.
The tube tester cleaned up really well, and I’m very pleased with it!
