Relays, circuit boards and brushes

What It Is

The switching electronics in the power unit. Older units use a separate step-down transformer (120/240 V to ~24 V) plus a relay whose 24 V coil closes heavy contacts carrying motor current (MD sells a 28 A relay that 'replaces all relays in 110-240 V systems' and must be mounted upright). Modern units combine transformer, relay and often a mini-breaker on a single circuit board rated by voltage, amperage and motor count (MD boards: single-motor 15 A/18 A 120 V, dual-motor 15 A/28 A 120 V, 7 A/12 A/18 A 240 V; Alutron A517-02 'Control Board 100550' 15 A 250 V for Beam/Eureka/Electrolux/Honeywell). 'Universal relay control boxes' are stand-alone relay+transformer enclosures that let a unit with a dead board, or a shop-built unit, be controlled by standard low-voltage inlets.

What It Does

Takes the low-voltage signal from the inlet contacts and switches line voltage to the motor(s). Boards may add soft-start, run-on delay, dual-motor staging and a thermal mini-breaker.

What Fails

• Relay contacts wear/pit (unit will not start or chatters); relay sticks closed (unit runs continuously even with the low-voltage wires removed).
• Transformer open winding (no 24 V, no click).
• Mini breaker (10/15/20 A) burns out after repeated motor overheating.
• Cracked solder joints or burnt traces on the board.

How To Test

• Unplug the unit. Disconnect the low-voltage pair at the unit, plug back in: if it still runs, the relay/board is stuck - replace. If it stops, the short is in the wire or an inlet (coin/screw bridging contacts).
• Unit will not start: use the manual switch. A click with no motor = relay is being energized, motor is dead. No click = check cord, then transformer/relay.
• Transformer: with 120 V on the primary, briefly touch the two low-voltage leads together; a small spark means the transformer is good (MD).
• Relay: applying 24 V to the coil terminals should produce a click; polarity does not matter.
• Board bench test (MD): wire neutral to NTL, line to 'power in', plug in, short the low-voltage terminals with a jumper. Spark + click = transformer and relay work. No spark = transformer bad. No spark, no click = relay bad, replace board. Spark + click but no motor = plug a 110 V lamp across 'Motor Neutral' and 'Motor Power'; if it does not light, the relay contacts are bad.
• Choose a replacement board by voltage (120/240), single vs dual motor and motor amps (board rating must exceed motor draw); MD warns that some listed titles and descriptions disagree on amperage, so confirm before ordering.

Compatibility Checks

• Match board to the number of motors and total amperage.
• Separate relays need a transformer sized for them (MD 28 A relay requires the large transformer).
• A universal relay box works with any 2-wire low-voltage inlet system and any 120 V single-motor unit within its amp rating.

Common Mistakes

• Replacing the motor when only the relay/board failed (or vice versa) - test the relay click first.
• Ignoring a continuously running unit (burns the motor and bag/filter).
• Using a 15 A board on a motor that draws more than 15 A.

When To Replace

When the tests above show no click, no spark, or no output; when a mini-breaker trips repeatedly even after brush replacement; or when visible burn marks appear. Boards are often cheaper than a transformer+relay pair.