Lexicon M224 (1978)

A 1978 digital reverb built from discrete 74LS logic, four output transformers, and a power supply that runs on a three-amp slo-blo fuse. The M224 is what a reverb looked like before anyone decided it should be convenient.

Overview

The M224 is a Lexicon digital reverb from 1978, and the service documentation tells you a lot that a spec sheet wouldn't. Crack the lid and you find a machine built the way computers were built in the late 1970s. The motherboard carries eight 30/60 and eight 43/86 PC edge connectors, with individual boards for the arithmetic unit, the floating-point converter, and the rear-panel display. This is not a single-chip reverb with a power supply bolted on. It is a rack of discrete logic, and the parts list reads like a TTL catalog: 74LS00, 74LS02, 74LS04, 74LS20, 74LS157, 74LS163, 74LS175, 74LS367, 74LS194, 74LS244, 74LS377, 74LS86, 74LS109, 74LS123, and a single 74S287 PROM. The Arithmetic Unit board alone carries ten 74LS00s, five 74LS163s, six 74LS194s, four 74LS670s, and ten 74F283 adders. Those adders are the giveaway. The M224 does its math with 74F283 four-bit binary full adders, the same logic family found in minicomputers and arcade boards of the era.

The Floating Point Converter board is the other half of the story, and the name is not marketing. It carries eight 74LS163 counters, eight 74LS194 shift registers, two 74LS244 buffers, two 74LS377 latches, and a 74S287 PROM. That is a hardware floating-point conversion stage built from discrete logic, sitting between the analog conversion and the arithmetic unit. In 1978, that was how you did floating-point math in a rack-mounted audio product: one chip at a time.

The analog side is just as serious. The output stage uses four transformers, part number 470-00261, marked "XFORMER, OTPT, M92". That is one per output channel, so this is a transformer-balanced output design. The input and output connectors are PC-mount XLRs: two 3-pin female (510-02806) and four 3-pin male (510-02807). The service notes are explicit that the pin assemblies and shells are retained from the chassis rather than the board, so a repair involving the XLRs means pulling the shell, not just the board. Around the analog section sit four 220µF 25V non-polar electrolytics, eight 150pF and eight 510pF 500V ceramic caps, and twelve ferrite beads on the board. That is a lot of RF and decoupling attention, which suggests Lexicon expected the M224 to live next to tape machines and consoles, not on a shelf.

Cooling is handled by a single tube-axial fan, part 410-01529, rated 4-1/8 by 1-1/2 inches and 50 CFM. Power comes from a dedicated transformer, 470-01475, with an AC and RFI filter (490-00396) and a separate power-fail sense connector (490-00812). The mains fuse on the 115V option is a 3AG slo-blo, 3 amp, 250 volt part, number 440-01624. That is a lot of current for an audio processor, and it tells you how much iron is in the supply.

The rear-panel display board is its own assembly, part 710-01444, and it is dense with indicators. It has fifteen HP 5082-4480 LEDs (CR1 through CR15), three 5082-7613 seven-segment displays (D0 through D2), sixteen illuminated digit switches (453-01508, 1P2T, 0.5W) and six more of the higher-rated 453-01509 type (1P2T, 0.69W). A flex-jump cable, 670-01506, 27-conductor at 0.1 inch pitch, connects the display board to the rest of the machine. The front panel of an M224 is not a screen and a knob. It is a bank of illuminated pushbuttons and a three-digit LED readout, wired to the mainframe by a ribbon cable.

The M224 was part of a family, and the parts lists show how the pieces relate. The 115V fuse option kit comes in three versions: kit #023-03021 for the M224XL, #023-03706 for the M224X, and #023-03028 for the M224. The 25-foot control cable option (680-02045) is listed as picking and adding to fin kit #024-03700 for the M224X or #022-03061 for the M224. The 50-foot cable option (680-02055) follows the same pattern, with fin kit #022-03700 for the M224X or #022-03061 for the M224. The shared kits and cable options confirm that the M224, M224X, and M224XL were a coherent product line with common accessories. The M224 is the base model of that trio.

What the documentation leaves out is just as telling. The service manual and parts lists do not state a sample rate, a bit depth, a frequency response, a dynamic range, a price, a production run, or a country of origin. There is no published spec for reverb time, no memory size, no algorithm list. The M224 is documented here as a collection of boards, chips, connectors, and mechanical parts, and that is all that survives in the archive. Anyone claiming precise reverb times or converter specs for this machine is working from somewhere other than the service documentation.

For collectors, the M224 is a maintenance proposition more than a plug-and-play purchase. The electrolytic capacitors are decades past their design life. That means the four 220µF 25V non-polars on the output board, and the 4.7µF and 22µF tantalums scattered across the FPC and ARU boards. The tantalums in particular are the parts most likely to fail short. The fan is a wear item. The XLR shells are retained from the chassis, so a dropped unit with a cracked shell is a chassis-level repair, not a connector swap. The 3-amp slo-blo fuse is easy to find. The 74F283 adders are still available but no longer cheap. None of this is unusual for a 1978 digital product.

The M224's place in the archive is as a document of how Lexicon got to the PCM-series machines that followed. The floating-point converter, the discrete arithmetic unit, the transformer-coupled outputs, the illuminated-switch front panel — these are the engineering choices of a company building digital reverb before the tools to build digital reverb cheaply existed. The M224 is not the most famous Lexicon, and it is not the most collectible. It is, however, one of the clearest windows into what the company was actually doing in 1978, and the parts lists preserve that in more detail than any brochure ever did.

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