Restoration of a Yamaha A-760 vintage integrated amplifier and matching T-760 tuner

Last modified: Thu Feb 15 15:08:10 2007

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Presentation

The Yamaha A-760 is a stereo 80W/ch. integrated amplifier manufactured in Japan by Yamaha and sold worldwide only in 1981. I have the European model. The A-760 and its slightly bigger brother (100W/ch.) the A-960 are the successors to the renowned CA-1000/CA-1010/CA-2010 integrated amplifiers.

The Yamaha A760 and its signature lighted square buttons


Yamaha advertised the A-760 and A-960 as featuring a new "X" power supply, which would adjust the power supply's line voltage to the level required by the output stage; Yamaha used the same technology in their R-1000 and R-2000 receivers, and in the TOTL M-60 and B-6 ("pyramid" design) power amplifiers. Apparently Bob Carver held a patent for exactly this sort of technology and threatened Yamaha with a patent infringement lawsuit. In 1982 Yamaha came out with revised A-760II and A-960II models that I assume did not feature an "X" power supply anymore.

The A-760 features:
  • Bass and Treble controls.
  • Balance control.
  • Continuously variable loudness control.
  • Speakers selector switch: off, A, B, A+B.
  • High filter and mono/stereo switches.
  • MM/MC phono switch and red indicator LEDs.
  • Input selector: Aux, Tuner, Phono, Tape 1, Tape 2.
  • Rec Out selector: Aux, Tuner, off, Phono, Tape 1, Tape 2.
  • Disc (phono direct) lighted switch.
  • Main Direct (tone control bypass) lighted switch.
  • Listening level (power) monitor with red LED indicator.
  • Dimensions: Width = 43.5 cm Height = 11.2 cm Depth = 36.5 cm.
  • Weight: 9.1kg.

Technical specifications

  • Output power (8 ohms, 20Hz-20kHz, 0.01% THD): 80W/channel.
  • Continuous output power 8 ohms: 100W/channel.
  • Continuous output power 4 ohms: 150W/channel.
  • THD (20Hz-20kHz):
    Phono MM to Rec Out, (5V output):   0.003%
    Phono MC to Rec Out, (5V output):   0.006%
    Aux/Tape/Tuner to Sp Out, (40W):   0.005%
  • Intermodulation distortion: 0.01%.
  • Damping factor (8 ohms, 1kHz): better than 55.
  • Power bandwidth (8 ohms, 40W, 0.02% THD: 10Hz to 50kHz.
  • SNR Aux/Tape/Tuner (150mV): 103dB.
  • Phono MC input:
    RIAA deviation:   +/- 0.3dB.
    Signal/Noise Ratio (500uV):   76dB, IHF A.
    Impedance:   100 ohms.
    Sensitivity:   250uV.
  • Phono MM input:
    RIAA deviation:   +/- 0.2dB.
    Signal/Noise Ratio (10mV):   98dB, IHF A.
    Impedance:   47kOhms.
    Sensitivity:   2.5mV.
  • Semiconductors: 47 transistors, 4 ICs, 8 FETs, 54 diodes, 3 LEDs.
  • Smoothing capacitors: 4 x 6800uF/63V.
April 1981 advertisement for the A-760 and A-960

Matching tuner: the Yamaha T-760

Life is a box of chocolates, so here is what I found recently on the bidding site:

The matching T-760 on top of the A-760.


There was a bad contact in the frequency display circuitry, fixed by resoldering about two dozen solder joints on the printed circuit board. Apart from that, the tuner is working perfectly. It easily tunes into my favorite FM station (TSF Jazz, 89.9MHz) and sounds great. There is a button to select DX and local modes.



The T-760 features:
  • AM/FM 5-Station Random-Access Preset Tuning.
  • Computer Servo Locked Synthesizer Tuning System.
  • Dynamic Auto-Blend Circuit.
  • Selectable DX/Local Tuning modes.
  • DC NFB PLL Multiplex Demodulator.
  • Digital (4-digit) Frequency Readout.
  • IF stage uses 4 ceramic filters.
  • Recording calibration switch (333Hz @ -6dB tone).
  • 300 Ohm (bal.) and 75 Ohm (unbal.) coax antenna inputs.
  • Dimensions: Width = 43.5 cm Height = 7.2 cm Depth = 26.25 cm.
  • Power Consumption: 11W.
  • Weight: 3.1kg.

Technical specifications

  • 50dB quieting sensitivity (stereo/mono, IHF): 27/4uV.
  • Usable sensitivity (98MHz 30dB quieting): 2/1uV.
  • Image Response Ratio (98MHz): 62dB.
  • IF Response Ratio: 100dB.
  • Distortion:
    Mono, 1kHz (Local/DX):   0.05%/0.3%
    Stereo, 1kHz (Local/DX):   0.05%/0.5%
  • Signal to Noise Ratio (at 85dBf):
    Mono (IHF/DIN):   82dB/76dB
    Stereo (IHF/DIN):   79dB/73dB
  • Stereo Separation @ 1kHz: 55dB.
  • Frequency Response, 30Hz to 15kHz: +/- 0.5dB.
April 1981 advertisement for the T-560 and T-760


The green digits of the T-760's frequency display are a good match for the green lights on the A-760:



I'll add more pictures of the T-760 internals soon.

The "burned" part: a Rifa 0.47uF 250VAC RFI capacitor

Upon opening the cover, the "burned" part was pretty obvious. The Rifa RFI capacitor had overheated, shorted and exploded, spilling oil over the surrounding components and printed circuit board. The smell of burned oil was very strong.

A 0.47uF 250VAC X spec'd part


Note this is an X spec'd part (the capacitor is specified for use across mains lines), rated 250VAC.

Capacitor catastrophic failure


Ahem... a catastrophic failure. Note the burned fuse - not really surprising! The spilled oil is visible too. The blue component to the left of the capacitor is a Yamaha-specific part, very much unobtanium nowadays. Ouch!

Looks bad, smells bad: it's bad!


One dead cap... And here the replacement, physically slightly smaller, rated 275VAC, X2, same capacitance; cost: 2,20 euros.


The replacement capacitor


Here the control board as I found it:

The control board with the exploded capacitor


And with the new capacitor; note the increased empty space, this will also allow for better airflow between the parts:

The control board with the new capacitor


Under the hood

There are four screws and washers holding the metal cover; once removed, the cover can be lifted and put aside.

The Yamaha A-760, under the hood


The general layout is strikingly similar to that of the Marantz PM450: The wiring and the sheer number of components belie an expensive and complex amplifier circuit, and I can't help comparing the Yamaha internals to those of the Marantz PM450 on which I was working just a few days ago. Both amplifiers achieve their power ratings with a single complementary pair per channel, but to get an extra 40~50W RMS at a slightly lower distortion, the Yamaha engineers resorted to a very complex design.

The back panel

The back panel has a standard layout for 1980's amplifiers in this class:

The back panel, standard layout


Yamaha equipment, made in Japan, certifications and serial number:

The serial number and certifications


Speaker terminals

This type of spring-loaded speaker terminal is considered obsolete nowadays, but was standard in 1981.

Speaker terminals for 2 sets of speakers: A and B


RCA inputs

Note the gold-plated Phono input.

RCA inputs


Transformer and primary "X" Power circuit

The primary "X" Power circuit can be seen to the right of the massive transformer. An opto-coupler is used to control a triac circuit that enables an extra winding in the transformer primary circuit.

Transformer and X Power control circuit


The primary X Power control circuit works fine temporarily, without the RFI capacitor.

The primary X Power control circuit


RIAA preamp board and input switches

The RIAA preamp is based on a JRC NJM4559D (2V/us slew rate variant of the 4558) dual op-amp, with each half preceded by a pair of low noise high transconductance SK170 N-channel JFETs in a differential amplifier configuration. The 4559 also gets its own voltage regulation (seen on the lower right part of the board).
Note the high-quality selector switch that allows one to choose either the MM or MC circuit on the top right, and the two similar input and rec out selector switches on the left.

The RIAA preamp and input switches


Output stage

Each channel of the power output stage is driven by a pair of bipolar complementary transistors Toshiba 2SB755/2SD845 mounted on a massive extruded aluminium heatsink:

Toshiba power transistors 2SB755/2SD845


Protection circuit and listening level indicator drive circuit

The A-760 has a protection circuit that drives a relay that disconnects the speakers in case there is a short or DC voltage present at the speaker terminals. It uses one half of a 4558 opamp, and the other half of the opamp is used to drive the listening level indicator LED.

The protection circuit and relay


The (in)famous lighted buttons

No doubt the lighted white/green switches on the A-760 and A-960 give them a distinctive look:

The lighted switches give the A-760 a distinct look.


But incandescent electric light bulbs burn out. Edison found that out some time ago (like 1879). Actually he found a way to make them last longer, and he patented it.
Nowadays we have white LEDs, but when Yamaha engineers designed the A-760, white LEDs didn't even exist in the labs (they were invented by Shuji Nakamura while working in Japan at Nichia Corporation in 1993); so Yamaha engineers used miniature incandescent light bulbs.
So far so good, except the specified 14.5V 80mA miniature light bulbs are pretty much unobtanium. So I replaced one of mine (the "Power" switch light bulb) with a white LED. Things are not that simple though, because Yamaha is running the small bulbs on AC, and in series:

Detail from the schematics, the light bulbs in series


So I had to build a little circuit that would provide the current to the LED but also allow the other two light bulbs to go on working as before. Here is what I came up with:

Little LED circuit to replace light bulb in Yamaha A-760


I am running around 20mA through the white LED, which is its rated current, so it should last another 20 years or so. The 1N4004 is there for safety reasons. The two resistors are power resistors, one is 2W and the other is 3W, and they get hot to the touch. I sleeved the resistors and 1N4004 diode with black fire-proof insulation and secured the sleeve with a nylon tie.

The LED circuit is sleeved in black insulation


Datasheets (PDF)

These are the Toshiba complementary power transistors used in the power output stage: The N-channel JFETs used in the RIAA preamp: The A-760 uses 4558/4559 op-amps for the tone controls and the RIAA preamp. The JRC NJM4558D/NJM4559D was a decent part in the early 80's, but modern op-amps perform a lot better. Just compare the datasheets:

An elegant view of the A-760 and T-760