Multi-brand
PLC Rack Power Supply Repair (Allen-Bradley SLC / ControlLogix, Siemens) — Mail-In
Component-level repair for PLC rack power supplies: Allen-Bradley SLC 500 and ControlLogix series, Siemens S7 PS units, and similar. Rectifiers, capacitors, and fans are the classic failures. Flat $349 with full load testing.
Coverage: SLC 500 (1746-P1/P2/P3/P4) · ControlLogix (1756-PA72/PB72/PA75/PB75) · Siemens PS307/PS407 · similar rack supplies
✓Fixed — or it's free
Unrepairable units are refunded; evaluations cost $0
✓24-month warranty
Every repair covered, parts and labor
✓3–5 business days bench time
Rush (1–2 day) and emergency service available
✓Program preserved
Your application and settings stay in the unit
Symptoms this repair fixes
- ✓Dead rack / no output
- ✓Rack faults when warm
- ✓Output sag under load
- ✓Audible whine or burnt smell
- ✓Intermittent brownouts and processor faults
Honest limits
- ✕Supplies destroyed by direct lightning/surge events across all boards (evaluated honestly — refund if unrepairable)
What you get
- ▸Component-level repair: rectifier, switching stage, capacitors, fan
- ▸Full electrolytic recap where aging is found
- ▸Load test at rated output before return
- ▸Ripple and regulation verified
- ▸24-month warranty
Typical catalog numbers
Part-number pages for this service
How PLC Power Supply units actually fail
Most units we receive fail in one of a handful of well-understood ways. Here is what each fault looks like from the machine, what is happening electronically, and what we do about it at the bench.
Output sag under load
- What you see
- The rack faults, resets, or behaves erratically, yet the supply measures correct when you meter it disconnected.
- Why it happens
- Aged electrolytic capacitors hold voltage fine with nothing connected but cannot supply current. The supply looks healthy on a bench meter and collapses the instant a populated rack draws from it.
- What we do
- Capacitors are replaced and every rail verified under genuine load with ripple measured — the only test that reproduces this fault.
Dead supply — no output
- What you see
- No output at all, sometimes with visible or audible evidence of failure.
- Why it happens
- Failure of the switching device, its controller, or the input section. Continuous operation at panel temperature is what wears these parts out.
- What we do
- The failed stage is repaired at board level and all rails verified under load before the unit ships.
Protection tripping
- What you see
- The supply starts, runs briefly and shuts down, repeating in a cycle.
- Why it happens
- This is usually protection circuitry doing its job against a genuine internal fault, or a supply that has lost regulation and is driving itself into its own limits.
- What we do
- We determine whether protection is responding correctly to an internal fault or misfiring, repair the actual cause, and prove it under sustained load.
Thermal-dependent failure
- What you see
- Works from cold, drops out after the panel has been closed and running for a while.
- Why it happens
- Capacitor ESR rises with temperature, so a marginal supply that just holds up cold falls out of regulation once the enclosure reaches working temperature.
- What we do
- The capacitor set is replaced and the repaired supply is run under load through a warm soak rather than tested only cold.
A power supply is worth more attention than its price suggests, because when it drifts out of regulation it does not fail cleanly — it produces intermittent, migrating faults across an entire rack that get blamed on processors, I/O modules and even software. Load testing is the whole point of this repair: an unloaded bench test cannot reproduce the fault at all.
What happens on the bench
The same sequence on every 5-stage repair, so nothing gets skipped because a fault looked obvious.
- 1
Intake and no-load check
The supply is logged and brought up with no load to establish whether it starts at all, and whether it starts and then shuts down — the second behaviour usually indicates protection operating correctly against a genuine internal fault.
- 2
Capacitor assessment
Primary bulk and secondary output capacitors are measured for capacitance and ESR. This is the dominant failure mode in industrial supplies: they run hot, in enclosed panels, continuously, for years, and electrolytic capacitors age out under exactly those conditions.
- 3
Switching and control circuitry
The switching device and its controller are tested, along with the feedback and protection circuitry that decides when the supply shuts down. A supply that trips under load has often lost regulation rather than lost output.
- 4
Repair and rail verification
Failed components are replaced and every output rail is verified for voltage accuracy and ripple. Where capacitor aging is present the affected set is replaced, since the remaining capacitors have the same thermal history.
- 5
Loaded burn-in
The repaired supply is run under load, because that is the only test that matters. A failing supply will hold its rated voltage perfectly with nothing connected and collapse the moment a real rack draws current — which is precisely the fault your PLC was experiencing.
Check these before you ship
Some of these will tell you the unit is fine and the fault is elsewhere. We would rather you find that out now than pay to ship a good unit.
- □Measure the output under real load, not disconnected. A supply that reads correct at idle and sags in the rack is the single most common industrial power-supply fault, and it is invisible on a bench meter with nothing connected.
- □Check whether the rack is drawing more than the supply is rated for. Modules added over the years quietly push some racks past their original supply budget.
- □Look for the physical evidence: bulged or vented capacitor tops, heat discoloration, or a burnt smell. It is not a diagnosis on its own, but it tells us where to start.
- □Note whether it fails from cold, only when warm, or only under a particular machine state. Thermal-dependent failures point straight at capacitor and regulation problems.
- □Confirm incoming supply voltage is stable and within tolerance — a supply repeatedly failing in one panel often has a cause upstream of it.
Repair or replace?
Power supplies are inexpensive relative to what they take down with them, and they fail in a way that is genuinely worth repairing: predictably, in the capacitors and switching section, with a well-understood fix and a long service life afterwards. The case for repair is strongest where the supply is discontinued, where it is an odd form factor that fits one specific rack, or where you want a maintained spare of a unit you already trust. A repaired and load-tested supply is a far better spare than an untested surplus one.
What to ship
- ▸The power supply module only
- ▸Symptom description with your RMA number
Full packing guidance: mail-in instructions.
Ship your unit to
USPS
Control Board Lab
PO Box 120241
Arlington, TX 76012
UPS / FedEx
Control Board Lab
1009 Oakwood Ln # 120241
Arlington, TX 76012
Write your RMA number on the outside of the box and include it on a note inside. Units over 50 lb (large VFDs, spindle drives, industrial PCs in enclosures) ship by freight — contact us before ordering and we will arrange dock-to-dock shipping.
Frequently asked questions
How do I know it's the power supply and not the processor?
A rack that is completely dark, or that faults when warm, is the classic supply failure. If you have a spare supply, a swap test confirms it. If you are unsure, send the supply — if it tests good on load we tell you, charge only the $95 test-and-verify fee, and refund the rest.
These supplies are cheap used — why repair?
Used supplies of this vintage carry the same aged capacitors yours just failed from. A repair replaces the actual wear items with new components, gets load-tested, and carries a 24-month warranty — usually for a similar all-in cost.
Fix it for $349
Buy the repair now, get your RMA instantly, and ship the unit today.
The Fixed-or-Free Guarantee: if we can't repair it, you don't pay for the repair.
Call (817) 799-7332 · 📱 Text us a photo of your unit · Email repairs@controlboardlab.com