Digital Test
Metrotech 4510  

Manufacturer: Metrotech
Model: 4510
Year: 1972
Schematic: RE
Functions: 24-hour Clock, Audio Time-Code Generator
Display: RCA Numitron
Digits: 6
Logic: TTL

This is a time-code generator for use in audio tape logging systems, this unit was used for police call logging. Standalone, it functions as a 24-hour clock. The Metrotech 4520 is the associated decoder. These units also have Dictaphone and "Dictachron Time Code Unit" labelling on them.

For the time-code generation, time stamps are continually output in serial form as two binary levels amplitude-modulating a 300Hz audio tone. Each time stamp is a sequence of 25 bits: a start marker bit, a 0 bit, 5 BCD digits of 4 bits with the LSB of the unit-seconds first, 2 bits for the tens-of-hours digit, and a 1 bit. Each bit occupies 40mS. The 40mS is divided into thirds, the first third is always occupied by 4 cyles of 300Hz carrier to mark the start of the bit, the second third will be empty for a logic-0 and 300Hz carrier for a logic-1. The third third is used to indicate the beginning of a time stamp: normally it is empty, the presence of 300Hz carrier marks the start of the 25-bit sequence. At 40mS per bit and 25 bits, a time stamp occupies precisely 1 second.

The format, and decoder design, are such that a serial time stamp can be decoded when received in either direction, allowing the time stamps to be displayed during rewind of the tape.

Numitrons (RCA Part No. DR-2000) were a 7-segment incandescent display device produced by RCA starting around the late-1960s/1970, just prior to 7-segment LED displays. Numitrons were packaged in a vacuum-tube-style envelope with a standard 9-pin (Noval) base. While Numitrons - distinct from Minitrons or other 7-segment incandescent displays in flat packages - were well-enough known, this is one of the few pieces of production equipment I have seen that actually uses them.

Date codes in this unit and the companion decoder span the years from 1972 to 1980. The use of Numitrons, original (not LS) SSI/MSI TTL, and the overall design suggest the design as being from the early 1970s. The later time stamps were likely the result of maintenance over the service life of the units.

Closer view of the Numitron display.

Internal view.

The Numitron display board and the board containing the TTL counter ICs forming the clock.

IEC power connector installed to replace the original 163-type.

The 4510 generator with the 4520 decoder.

- Unit Log -

Serial Number: 2347-12
Year of Manufacture: 1972 (Component date codes of 7221 to 8008, caps on main board 7221).
Date of Receipt: 2014 Jan
State upon Receipt: Unit will reset and preset but will not count. Top cover missing.

Date: 2014 Jan
Procedure: Count/Hold switch not conducting. Contact cleaner seeped in through throat. Unit now counts.

Date: 2014 Jan
Procedure: Old 163-style power connector replaced with IEC standard.

Date: 2014 Jan
Procedure: Cleaned and refurbished. Switches rough, lubricated with small drop of CC oil in throat. Mounting block on one corner of power transformer has stripped 4-40 thread, replaced.

Date: 2014 Jan
Procedure: Top cover constructed. Front panel aluminum casting drilled and tapped for two screws for new top cover. Tap broke in left hole, punched out and retapped. Some threads lost but adequate.

Date: 2019 May
Procedure: Burned out numitron segment: minutes-units segment c. Numitron replaced with DR2010. Replacement had solder on pins so apparently previously used.

Date: 2019 Jun
Symptoms: Display brightness drops after a few minutes of warmup.
Solution: Power supply CR9 1N4001 intermittent open, replaced with 1N4004.

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Early Digital Test Equipment