Linn LK280
Eighty watts a side, a green LED on the fascia, and a protection circuit that makes every repair hard work.
Overview
The LK280 is the last and best of Linn's LK2 series of power amplifiers. You tell it from its predecessors by the power LED. The LK280 has a green one, while the earlier LK260 and LK275 both used red. Linn's own service documentation warns that this indicator isn't fully reliable, since LEDs are easily swapped. The only sure method is to pull the sleeve and read the boards inside.
Under the hood, the LK280 is a different animal from the two amps before it. The LK2 series ran through three versions. The first, a 60-watt-per-channel design built on PCAS 003 boards, is called the LK260 in Linn's service literature. The second, a 75-watt version with improved protection circuitry and better sound, is the LK275. It uses the same PCAS 003 board family, refined. The LK280 is the third type, rated at 80 watts per channel into 8 ohms and built on the PCAS 020 board. Linn's manual is explicit that the PCAS 020 is not a modified PCAS 003. It's a completely different design that happens to share the same approximate size, shape, and layout. According to Linn's own documentation, the LK280 "sounded considerably better than either of its predecessors."
The 80-watt figure doubles into 4 ohms, where the amplifier is rated at 160 watts per channel. Voltage gain is 26 dB and input impedance is 3 kilohms. The chassis measures 12.6 inches wide, 2.95 inches high, and 10.4 inches deep - 32 by 7.5 by 26 centimeters. Each channel module carries its own power supply, sharing only the common transformer, and each module is easily replaceable as a unit if service is required. When something goes wrong, you're swapping a card, not rebuilding an amplifier. The rear apron carries two XLR input jacks. Audio (April 1989, p.91) listed the full manufacturer's specification set for the LK280 as part of the magazine's equipment profile of the LK1 preamp and its matching power amp.
The LK280 was reviewed as half of a system, and that's how most of them were sold. HiFi-Stereo (January 1989, p.58) covered the LK1 preamplifier and LK280 power amplifier together in a Hirsch-Houck Laboratories test report, and the pairing was the intended configuration. The LK1 brought volume-level presets for each input, the ability to listen to one source while recording another, a keyboard lock-out to prevent unauthorized use, and remote control, at a price of $1,050. HiFi-Stereo (November 1988, p.77) noted that Linn's LK2 power amp, rated at 60 watts per channel into 8 ohms, had just been upgraded. That upgrade produced the LK280. Julian Hirsch's lab found the LK1's gain was typically set so that an input of 0.5 volt produced a 1-watt output from the LK280, and observed that the preamp's margin was less than it should be for a unit of that quality. The amplifier itself ran warmer than most, with its output devices mounted along the sides close to their heat sinks.
The fault-finding is where the LK280 gets interesting. Linn's service manual is candid: the LK2 circuitry is "notoriously difficult to fault-find on in the traditional way," meaning signal tracing and voltage measurement will get you nowhere. The culprit is the Trip protection circuit, which shuts down the entire board the moment it detects anything that might damage the amp or the speakers. When Trip engages, no power reaches the amplifier section at all, so you're diagnosing a dead board with the power off. Linn's advice is to consult the fault tables first and work from there.
The failure modes are well documented. On the LK260 and LK275 - not the LK280 - roughly 60 to 70 percent of faults trace to just four components. Linn published a reliability modification to replace them and prevent recurrence, and recommended it even for working boards as preventative maintenance. The LK280 avoids that weakness because of its different board design. It has its own vulnerability, though: the U3 hybrid card, a small board mounted at 90 degrees to the main board beside the big capacitors. When an LK280 trips, U3 is a prime suspect. It's a Linn-only part, IC 030, not available elsewhere. Linn's guidance is to replace U3 and, while you're in there, check or replace the regulators U1 and U2 (LM317T and LM337T respectively) and the big 0.1-ohm, 2.5-watt resistors R37 and R46 at either side.
Power transistors are the other recurring problem. Depending on the version, the output devices clamped to the heat sink can fail with an internal short across one or more junctions. Linn notes that a transistor may measure fine on an ohmmeter and still be faulty, and sometimes the only way to confirm is to change it and see. For the LK280, the parts are 2SA1386 PNP and 2SC3519 NPN devices, both rated 160 volts at 15 amps in MT100 packages. Earlier LK260s used TIP36C and TIP35C transistors instead. Regulator failures - U1 or U2 - can also cause a channel to drop out, sometimes intermittently. Those are standard LM317T and LM337T parts, easy to source.
Buyers should know one wrinkle. Linn sold upgrade kits in large quantities to let owners convert early LK2s to the latest specification, so a great many units are not what they appear to be. A fascia reading "LK2" may well be an LK280 inside. A red LED may have been swapped for a green one, or vice versa. Serial numbers, checked against Linn's product history, tell you what a unit was sold as, not necessarily what it is now. The only reliable identification is to remove the sleeve and read the boards.
Linn's service manual was written for retailers and distributors, with the goal of repairing as many LK2 faults at the repair centre as practical, and it names Paul O'Neill at Linn Products as the contact for suggestions and comments. That level of manufacturer transparency is rare now and was uncommon then. It means anyone owning an LK280 today has access to genuine factory guidance, which helps, because these amplifiers are old enough that the hybrid card, the regulators, and the output devices are all fair game for failure.
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