ReVox A720 Preamplifier Tuner With Gorgeous Nixie Tube Display

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General:
One of the coolest Preamp-Tuners made in the 1970's.

Plus it's featured on The Vintage Knob website so you know it's something very special...!

Utilizes Nixie tubes for the FM tuner frequency display which is super neat. Also has a unique preset design which is very clever and well engineered.

Construction quality is top notch which is what we've come to expect from ReVox.

We performed a light service on this unit, checked and cleaned all controls, and confirmed that everything works as it should.

Plus is has the ability to work at just about every input voltage which we also tested.

SkyFi Service Notes:
• Tuner works beautifully
• Removed all covers and cleaned faders and switches

Click below to add our recommended matching cables from Kimber Kable, all brand new as SkyFi is an official Kimber dealer. 

Summit Series Silver Streak Analog Interconnects (PAIR) - RCA Ultraplate or WBT Connectors

More from The Vintage Knob:
Rare ultra-high-tech digital "preceiver" (preamp + tuner) holding everything that was the future in 1973. Beautiful beast, too.

Most fascinating are five radio stations presets:
Fiddle with the left pots, memorize with the buttons to the right.

Quartz tuning, 6-gang varicap, FET-charged amps, 8 passive filters, 23ns delay demodulator, PLL 76Khz oscillator for the MPX section.

And four beautiflly glowing tubes for display.

The preamp section also has just about everything:
Adjustable inputs (within 20dB), lo/hi filters, loudness switch, bass /presence /treble tone controls, "null" balance for headphones listening, tape dubbing and monitoring, two preamp outputs with "power-on" triggers, two headphones' outputs and a wired remote for balance, loudness and volume.

Fot the ultimate in mid 70's frills, the A720 could be hooked to the high power and fan-cooled A722 mid-size power amp and the A724 Quadriphonic SQ / RM / QS 4-channel decoder / power amp - now you understand why the balance can be remote-controlled on the A720 :)

The power-on triggers of the A720 work with an optocoupler + triac and can automatically power-on several power amps and (active) loudspeakers.

More from ReVox:
In the Revox A720 the advanced circuit technology of a digital FM tuner is combined with an audio preamplifier section of outstanding characteristics.

By not incorporating a power amplifier section, extreme flexibility with regard to its combination with external equipment could be achieved.

In this digital FM tuner preamplifier, complex and unusual circuit solutions have been realized through a high degree of integration, thereby ensuring exceptional operational reliability.

Outstanding frequency stability in station selection is achieved through crystal referenced synchronization of the local oscillator.

To ensure this high quality and accuracy in performance, each unit has to pass through a series of tightly controlled production tests plus a careful final inspection step.

ReVox A720 Preamplifier Tuner With Gorgeous Nixie Tube Display - Manual

The SkyFi Testing Process for Preamplifiers:

We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect the PCB’s for discoloration from resistors or transistors that may have been running hot. On vintage units we often spot check select capacitors for value and ESR.

When we first power on a preamplifier we connect its RCA output to a Sencore PA81 Power Analyzer which simulates real world loading conditions and gives us an oscilloscope interface. The first order of business is checking that the volume control works smoothly throughout its entire range with acceptable channel balance. This is accomplished by feeding a 1KHz sine wave into one of the preamp’s line level inputs while monitoring the preamp’s output on an oscilloscope. We then switch to a 1KHz square wave to test the tone controls, loudness function, and filters where applicable. During this step we are watching for equal alteration of the test signal by both channels. This also helps us identify dirty controls that will need treatment. Once the basic line stage functions are verified, we test each input individually. This is especially important for devices that use relays to select their sources. If the preamp is equipped with a phono stage we test that as well. We use an inverse RIAA filter which allows us to feed a reference test signal into the phono input with the proper RIAA equalization and level. A square wave or sine sweep is used to verify that the device’s phono stage is faithfully reproducing the RIAA curve. If the preamp under test has balanced inputs and/or outputs these are tested as well.

We finish up our bench testing with a listening test with our bench amplifier and reference speakers. During this test we check for hum or hiss that may not have shown up in earlier testing. We also check that all of the tone controls and filters perform as expected. If the preamplifier has remote control functions these are also tested. Preamps with tube circuits or complicated power supply topologies are connected at our long term test rig for extended stress testing under real world conditions.

The SkyFi Testing Process for Tuners:

We start with a visual inspection of all internal components to make sure that there are no signs of heat stress or damage. Capacitors are checked for telltale signs of predictive failure including bulging, shrunken wrappers, or physical leakage. We also inspect the PCBs for discoloration from resistors or transistors that may have been running hot. On vintage units we often spot check select capacitors for value and ESR. Vintage analog tuners also have moving parts related to the tuning gang and dial string. These parts are inspected for smooth operation.

If the unit passes visual inspection it is bench tested for a handful of key performance parameters using a Sencore SG80 AM/FM Stereo Analyzer. The SG80 allows us to “simulate” an ideal radio station using precision test signals instead of music. This device, in conjunction with an oscilloscope allows us to properly evaluate the following parameters:

1.  AM Reception (Where Applicable)

2.  FM Mono Reception & Tuning Meter Function

3.  FM MPX Reception (Stereo)

4.  Dial Tracking - How accurately the tuner dial or display indicates the actual frequency of the broadcast being received.

5.  Stereo Separation - A properly working stereo tuner will have minimal crosstalk between the left and right channel.

6.  Sensitivity & Signal Strength Meter Function - By lowering the output of the SG80 we can simulate weak stations and determine how well the tuner will be able to pull in weak distant stations. This adjustment also helps us verify signal strength meter function.

If the tuner has acceptable performance related to the parameters above we connect the unit for listening tests with a simple dipole antenna. We listen for audio reproduction quality of local stations and evaluate how many stations the tuner can receive while we sweep through the dial. We are looking to verify that the tuner can decode stereo on strong local broadcasts and pick up a wide variety of local stations at the bottom, middle, and top of the frequency band. At this point we also test convenience features such as muting, filters, built in oscilloscope function, etc.

We finish up with an extended listening test on our long term test rig. We tune in a strong local station and monitor for drift over a 2-3 hour period.