Lexicon PRIME TIME Model 93
A mono delay line with two independent taps, a 90dB dynamic range, and an adaptive gain-ranging converter that squeezed fifteen bits of headroom out of a twelve-bit word.
Overview
The PRIME TIME Model 93 is a digital delay processor - a mono delay line with two independent delay taps, built by Lexicon for studio engineers and live performers. Lexicon's own service documentation calls it an outgrowth of two previous generations of digital delays, and the design shows it. It uses Low Power Schottky TTL logic and 4K-bit RAMs for memory. The manual credits these with reduced power consumption, lower internal operating temperatures, and increased reliability. Lexicon was confident enough to print that PRIME TIME would "more than live up to our excellent reputation for reliability."
The headline numbers are a 90dB dynamic range, distortion of less than 0.1%, and flat frequency response. The Model 93 gets there with a 12-bit converter, and the service manual explains the trick at length. Lexicon encoded the analog data as ten bits of straight binary plus a two-bit exponent. The exponent selects one of four gain settings on a programmable gain stage ahead of the ADC. The gain ranges are spaced 10dB apart: 0, 10, 20, and 30dB. After each conversion, gain-selection logic checks whether the word falls between one-quarter and seven-eighths of full scale. If it exceeds seven-eighths, the stage drops 10dB before the next conversion. If it falls below one-quarter, a 25ms time-out begins, and then input gain rises 10dB. The manual is blunt about it: straight binary coding would have needed 15 bits to encode an equivalent dynamic range. The adaptive gain logic keeps the signal at the converter in a range that preserves signal-to-noise ratio despite wide swings in input amplitude. That suits a delay line fed by a console with unpredictable program material.
Delay time runs up to 128ms at full bandwidth, expandable to 256ms with the optional add-on memory. The Delay Multiply control increases the selected delay by a factor of 2, 4, or 8 with a corresponding reduction in band-pass, which is how the unit reaches over two seconds of delay. The Delay Adjust control varies delay continuously from 100% down to 50% of the selected value, so any delay time from 0.25ms to 2048ms is available. Period advertising ran the Model 93 alongside Lexicon's Model 91 in a recurring "Delay System for Every Application and Budget" campaign. It described two time delay outputs independently adjustable from 0 to 256ms, and pitched the unit as combining flexibility and versatility "heretofore unavailable in any system anywhere." That last phrase is marketing copy, not a measurement. The same campaign positioned the Model 91 as the budget option for churches, schools, and community auditoriums, drawing on the company's big-system Model 102 PCM technology. So the 93 sat above the 91 and below the 102.
The routing is more flexible than the "mono delay line" description suggests. The output mixer blends the two delay taps into the Master Output along with either or both of two input sources. Both taps also appear on the rear panel with individual level controls, and phase invert switches let the operator vary phase relationships in the output mix. The input mixer feeds the delay line a mix of the two sources plus frequency-contoured recirculating delays, and that input mix is itself available on the rear panel. Sophisticated elliptical filters improve aliasing performance and eliminate sampling components. The reference section of the manual includes a five-pole elliptical filter frequency response plot, along with pre-emphasis/de-emphasis curves and aperture correction response. The converter itself is a successive-approximation design. The manual walks through the SAR's ten test cycles, which step the DAC output to within 10mV of the true analog amplitude before the ten-bit word is combined with two bits of gain information and stored in memory.
Servicing documentation is unusually complete for a unit of this class, which is a mixed blessing for owners. The Model 93 was built on removable modules - Motherboard, Analog Daughter Board, Memory Board, Memory Extension Board, and Front Panel Board - held in place by either nylon standoff retainers or threaded spacers with nylon screws. The manual warns that removing a module takes care to avoid bending connector pins, and that reinstalling one means aligning every pin with its mating receptacle. Two DAC options were supported: a Motorola MC3410 or an Analog Devices AD561. The circuit board has a dual set of device pads because the two chips are electrically different. Units fitted with the AD561 need no offset adjustment and omit R36 entirely. The AD561's full-scale output varies from 1.5 to 2.4mA with a typical value of 2.0mA, and R37 and R38 are trimmed at the factory to match the actual chip. Swap in a new AD561 and the converter must be recalibrated. Field service guidance begins with the obvious: verify all power supplies. That means the +5, +15, and −15 volt rails plus the +12V regulated supply on the Memory Board and the +7 and −7 volt Analog Board and Motherboard supplies. The unregulated 5V supply, roughly 8.5V, drives the front-panel displays. If the displays don't illuminate on power-up, that supply is the first suspect. The manual also notes that +5 volt ground is isolated from all other voltage grounds when both the Memory and Analog Daughter Board modules are removed from the Motherboard. That will bite anyone probing the unit on the bench without reading ahead.
One warning bears repeating in substance: hazardous voltages sit under both the top and bottom covers, and the power cord should come out before either comes off. Lexicon's warranty ran one year from delivery, covering defects in materials and workmanship. It was voided by abuse, unauthorized repair, operation from an incorrect power source, or, in the manual's phrasing, accident, act of God, war, or civil insurrection. Replacement parts were ordered direct from Lexicon at 60 Turner Street in Waltham, Massachusetts, shipped FOB Waltham, with pricing at whatever was in effect when the order arrived. That address and that parts-department arrangement are long gone.
The Model 93 comes from a time when converters were expensive and slow, and the workaround was clever encoding rather than brute-force bit depth. The gain-ranging scheme, the elliptical filtering, and the module-based construction all reflect that.
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