⚠ Important Production Caveat

Barry Kirkwood W0IY and the Team XCR group, who have inspected and modified more 516F-2 units than perhaps anyone in the community, report significant production inconsistency in the factory wiring. Some 516F-2 units have been found wired almost entirely with white wire, and others use the same colour for completely unrelated circuits. ^[4] The colour code table above is a guide — not a guarantee. When colours are degraded, the ohmmeter method described below is the only reliable identification technique.

Step 1 — Isolate the Transformer

Disconnect all leads from the circuit. Remove all connections from tube sockets, terminal strips, and any other components. Every wire must be free-floating for accurate readings. Remove the bell covers if you have not already done so. Photograph everything from multiple angles before disturbing any wires — even if colours appear unreadable, strong side-lighting or UV illumination may reveal traces in photographs that are invisible to the naked eye.

Step 2 — Map Continuity Groups

Set your multimeter to the lowest ohms range and zero the leads. Clip one test lead to any transformer wire and systematically check every other wire for continuity. When you find a pair (or triplet, indicating a centre tap) with continuity, tag them as a group and record the resistance. A winding with three leads showing mutual continuity has a centre tap — the CT will read approximately half the total end-to-end resistance from either outer lead. Continue until all leads are grouped. There should be no continuity between separate windings.

Step 3 — Identify by Resistance Signature

Once all leads are grouped, the windings can be identified by their relative DC resistance:

Resistance	Leads	Identification	Why
Highest	3 (CT)	HV Secondary	Most turns, finest gauge wire
Second highest	3 (CT)	LV Secondary	Fewer turns than HV, heavier gauge
Medium	2 + 2	Primary A & Primary B	Two separate identical windings
Very low	2 + 2 + 2	Three filament windings	Fewest turns, heaviest gauge wire

Step 4 — Variac Voltage Verification

Once windings are grouped by resistance, confirm their identity by applying low voltage through a Variac. Connect the Variac output to one of the identified primary windings, start at zero volts, and slowly increase. Measure the output voltage on each secondary winding. The HV winding will produce the highest voltage, the LV winding will produce the next highest, and the three filament windings will produce low voltages. At rated primary voltage (115V), the 6.3V winding will read slightly higher than the two 5V windings, and the 6.3V winding will typically show marginally lower DC resistance due to its heavier gauge wire (it carries more current to the associated equipment).

⚡ HIGH VOLTAGE SAFETY WARNING

Even at reduced Variac input, the HV secondary will produce dangerous and potentially lethal voltages. Keep all secondary leads properly insulated and secured. Do not touch any exposed conductor while power is applied. Use clip leads and take measurements with the Variac at zero before repositioning meter probes. The 516F-2 is a high-energy supply — treat every wire as live until proven otherwise.

Recovering Colour Traces

Before stripping wires, try carefully cutting back the corroded outer insulation near the transformer body with wire strippers. Original colour is often preserved underneath where it was protected from heat and UV exposure. Strong side-lighting, a magnifying lamp, or even UV/black light can reveal colour differences that are invisible under normal workshop lighting. Photograph suspect sections — digital cameras sometimes differentiate colours better than the human eye in degraded insulation.

Distinguishing the Two Primaries

The two primary windings should have nearly identical DC resistance. To confirm correct phasing for parallel operation (115V), connect them in series across a Variac and slowly bring up voltage. If correctly phased, the secondaries will produce approximately half their rated output at full primary input. If incorrectly phased (series-opposing), the secondary voltages will be near zero. Reverse one primary’s connections to correct.

Which 5V Filament Goes Where?

The two 5V rectifier filament windings are essentially interchangeable — both supply 5V to their respective rectifier tube filaments. The V1 (5R4) filament winding floats at the full HV potential, and the V2 (5U4) filament winding floats at the LV potential. Both windings are insulated for these voltages. In practice, either can be connected to either tube socket without issue.

Rewiring with Modern Wire

When splicing new leads onto the transformer stubs, use high-temperature silicone-insulated wire rated for at least 200°C. Solder joints should be mechanically secure before soldering, then covered with adhesive-lined heat-shrink tubing. For leads carrying HV secondary voltages, use wire rated for at least 1kV insulation. Dress the new leads away from the transformer core and secure them to prevent vibration fatigue.

Consider the W0IY Service Bulletin 1 Kit

If you are already inside the transformer rebuilding leads, this may be an ideal time to install Barry W0IY’s Service Bulletin 1 PCB kit, which replaces the ageing electrolytic capacitors, the selenium bias rectifier, and optionally the tube rectifiers with solid-state diodes. The kit documentation includes the most detailed wiring table and T1 connection map available for the 516F-2, which will be invaluable during reassembly regardless of whether you install the PCB. ^[6]