A retrofit that flickers is usually not a bad batch of lamps. It is the transformer, and you cannot tell which kind you have by looking at it.

Identify which kind of transformer you have before buying anything

This is the whole decision and it is invisible from across the yard. A magnetic transformer is an iron-core or toroidal unit stepping voltage down at line frequency, and it is heavy for its size. An electronic low-voltage transformer is a switching converter rather than a transformer in the strict sense, and it is much lighter. Read the label and the weight rather than guessing from the enclosure, because that single fact determines whether LED lamps are likely to behave. One supplier is blunt that the electronic type presents a much bigger challenge for LED retrofit lamps, and there is no adapter that resolves it.

Flickering after a retrofit has a documented cause, and it is not the lamps

Worth knowing because the instinct is to return the lamps and try a different brand, which usually reproduces the problem. The mechanism is described in patent literature: pulse-width modulation in an electronic transformer produces a much faster voltage rise and fall than a 60 hertz sine wave, causing current spikes twenty-five thousand to one hundred thousand times per second, which stress the rectifier diodes and bulk capacitors in the lamp and can trigger the transformer’s own over-current protection into shutting down. The listed symptoms are flickering, flashing, dim output, or the lamp not lighting at all. If a retrofit does all of that with two different lamp brands, stop buying lamps.

There is a slow version of the same failure, and it looks like bad lamps

The more expensive outcome is the one where it appears to work. The same source notes that where the transformer functions and the lamp does operate, the lamp may overheat at its input components and suffer early life failure from those current spikes. So a retrofit that is fine for a season and then starts losing lamps one at a time is not a quality problem with the lamps, it is the same incompatibility expressing itself slowly. This is worth telling a customer up front, because the alternative is them concluding LED landscape lighting is unreliable when the actual issue is a fifteen year old switching converter.

Sources disagree about magnetic minimum load, so read your own label

This is a case where the confident general answer does not exist. One lighting supplier states that magnetic transformers usually have no meaningful minimum load and tend to be more LED-friendly. Another states that many older magnetic transformers were designed for high-wattage halogen bulbs and have a minimum wattage requirement to function, often twenty watts or more. Both are suppliers in this trade and they cannot both be generally right. The only trustworthy source is the label on your unit, and this matters specifically for a conversion because the load collapses: one supplier illustrates the scale by noting that a given transformer runs roughly twenty-five to thirty modern four watt LEDs or about six traditional twenty watt halogens. A three hundred watt system can land under fifty watts, which is exactly where a minimum load requirement would bite.

Most retrofit lamps rectify on board. Tape light does not

A concrete fork that catches people extending a system rather than just relamping it. Halogen filaments are content with AC, and LED chips need DC. Most 12 volt LED retrofit lamps handle that internally, with MR16 LED retrofits described as accepting 12 volt AC and rectifying it on board. LED tape light does not: it requires DC, and a 12 volt AC landscape transformer will not run it regardless of wattage headroom. So a plan to add a run of tape under a step or a bench alongside existing path lights is not a wiring extension, it is a second power supply. Check what each product actually wants before assuming one transformer covers the yard.

The transformer probably did not fail

Before buying a replacement, work the list, because the diagnosis is usually elsewhere. One source puts it directly: most of the time a transformer that failed did not fail at all, and the real cause is that the load is too high, water got in, a terminal corroded, the timer stuck, or voltage drop is making perfectly healthy fixtures look broken. Overloading in particular is a thermal problem that can trip the internal thermal protector and shut the system off, which presents exactly like a dead unit. Work through those checks and replace the transformer only when testing actually points at it.

Test by walking the run, not by standing at the transformer

A reading at the transformer output tells you the transformer is alive and nothing else. The useful test is to walk the run with a multimeter reading voltage at each fixture in turn. If the readings fall progressively along the run, the answer is a higher tap on a multi-tap unit, heavier cable, or rewiring the run as a hub or T-pattern instead of one long daisy chain, none of which is a new transformer. And the safety line is worth quoting rather than paraphrasing: the input side of a landscape transformer is 120 volt line power, and anybody not comfortable working around live line voltage should not open or probe the input terminals. The 12 volt output side is safe to work with; the 120 volt input side is not.

Specify low-load capability and multi-tap on any replacement, and add surge protection

If the conclusion is a new transformer, the specification follows from everything above. Units labeled no minimum load, with multi-tap outputs in the 12 to 15 volt range, are described as the best choice because they hold consistent brightness even on small LED loads and give you the taps to solve drop without recabling. Tru-Scapes states the rule for conversions plainly: verify the transformer supports low-load operation, or replace it with a modern unit designed for LED compatibility. And while you are at it, add surge protection, because a nearby lightning strike or grid switching event can drive a transient down the supply and flash over the primary winding, which is described as the genuine high-voltage-stress failure mode and usually kills the unit instantly with no repair possible. A whole-house protector at the panel, or at minimum a plug-in unit on the outlet feeding the transformer.