Restoration of a Yamaha A-760 vintage integrated amplifier and matching T-760 tuner
Last modified: Thu Feb 15 15:08:10 2007
- 15-02-2007: And then there were two... I bought on the bidding site a matching Yamaha T-760 tuner. It sounds (and obviously looks) great with the A-760. The cost was 6 euros, plus 16,20 euros for shipping from Austria.
- 31-12-2006: Put back an X2 RFI capacitor, cleaned all the switches and pots with De-Oxit, re-checked and readjusted bias and offset and power supply voltages per the service manual, visually re-inspected the circuitry, and now it's setup for some serious listening with a pair of Yamaha Pianocraft bookshelves. So far performing (and almost looking) like new!
- 24-12-2006: Replaced a burned lamp with a couple of resistors, a diode and a white LED. Checked the power transistors, cleaned up the amp, fired it up and it works!
- 23-12-2006: Opened the cover, found out what was burned, took some pictures. See below. A quick visual check of the other components does not reveal any obvious damage.
- 23-12-2006: got the amplifier this morning, directly from the previous owner, with the original owner's manual. Was warned about the smoke that came out of the amp when it burned. It does smell bad!
- 20-12-2006: found the service manual for the A-760 on the Web. Am I lucky or what?
- 19-12-2006: won the bidding on a "burned, sold as-is" Yamaha A-760 amp on eBay: 15 euros including delivery!
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.
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.
- 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):
|Phono MC to Rec Out, (5V output):
|Aux/Tape/Tuner to Sp Out, (40W):
- 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:
|| +/- 0.3dB.
|Signal/Noise Ratio (500uV):
|| 76dB, IHF A.
|| 100 ohms.
- Phono MM input:
|| +/- 0.2dB.
|Signal/Noise Ratio (10mV):
|| 98dB, IHF A.
- Semiconductors: 47 transistors, 4 ICs, 8 FETs, 54 diodes, 3 LEDs.
- Smoothing capacitors: 4 x 6800uF/63V.
Matching tuner: the Yamaha T-760
Life is a box of chocolates, so here is what I found recently on the bidding site:
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.
- 50dB quieting sensitivity (stereo/mono, IHF): 27/4uV.
- Usable sensitivity (98MHz 30dB quieting): 2/1uV.
- Image Response Ratio (98MHz): 62dB.
- IF Response Ratio: 100dB.
|Mono, 1kHz (Local/DX):
|Stereo, 1kHz (Local/DX):
- Signal to Noise Ratio (at 85dBf):
- Stereo Separation @ 1kHz: 55dB.
- Frequency Response, 30Hz to 15kHz: +/- 0.5dB.
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.
Note this is an X spec'd part (the capacitor is specified for use across mains lines), rated 250VAC.
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!
One dead cap... And here the replacement, physically slightly smaller, rated 275VAC, X2, same capacitance; cost: 2,20 euros.
Here the control board as I found it:
And with the new capacitor; note the increased empty space, this will also allow for better airflow between the parts:
Under the hood
There are four screws and washers holding the metal cover; once removed, the cover can be lifted and put aside.
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.
- Transformer (shielded) at back left.
- Inputs at back right and RIAA preamp circuit right at the RCA jack.
- Volume pot at front right and tone controls behind the front panel.
- Output stage in middle. The Marantz engineers used a single heatsink running parallel to the front panel, whereas Yamaha engineers decided on two separate heatsinks (one per channel) at right angles to the front panel.
The back panel
The back panel has a standard layout for 1980's amplifiers in this class:
Yamaha equipment, made in Japan, certifications and serial number:
This type of spring-loaded speaker terminal is considered obsolete nowadays, but was standard in 1981.
Note the gold-plated Phono input.
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.
The primary X Power control circuit works fine temporarily, without the RFI capacitor.
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.
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:
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 (in)famous lighted buttons
No doubt the lighted white/green switches on the A-760 and A-960 give them a distinctive 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:
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:
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.
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: