Electronics > Repair

Technics SA-DA8 audio amplifier overload issue

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I've been trying to figure out this Technics amplifier. It shows overload, and no output.  There is an area on the mainboard which shows sign of a lot of heat. Also the heatsink gets very hot from the transistors.

The transistors that generates a lot of heat is Q701, Q707 and Q708.

Have been measuring the voltages according to the schematics from the service manual. On pin Q707 one says 0V, here I measure -33V, is this an issue or should it be like that? the voltages on Q701 and Q708 is close to the values on the schematics.

Any tips?

Regarding the schematic you posted: Most of the voltage readings shown for E, B, and C next to the regulator transistors are totally incorrect. This is unusual for Panasonic/Technics (most of the time their schematics are accurate), but certainly some of the voltages shown on this schematic are wrong. For example, the emitter of Q703 is connected directly to the base of Q701. Yet Q703 emitter is said to be 0V while Q701 base is shown as 27.26V This cannot be correct!

Question: Does this amplifier have a front panel display?
If so, does it light up when the amplifier is turned on?

The output voltages shown on this schematic look reasonable to me:
There are +14V and -14V outputs. These probably provide power to IC opamps used for the line-level sections of this amplifier such as phono preamp, tone controls, etc.
A 5.6V output is derived from the +14V rail. This is likely used for a system control MCU and other logic-level digital circuitry.
The -27V output is commonly used to operate a VFD (vacuum fluorescent) front panel display.

Please measure the voltages at these 4 output terminals ( +14, -14, +5.6, -27) and post them.
     A deviation of +/- 1 or 2V for the +/-14V and -27V outputs wouldn't be out of range.
     The 5.6V should be within +/-200mV.
     If any of these outputs measure 0V this indicates failure.

Relationship between output voltages and regulator transistors is as follows:
     Q701 regulates the +14V output (I would expect this transistor to run rather warm under normal operation)
     Q708 regulates the -14V output
     Q706 regulates the +5.6V output
     Q707 regulates the -27V output  (If this output fails the VFD display will not light up)

For all 4 transistors: Their collector voltage should be from 5 to 15V larger than the expected output voltage. Because these are all standard BJT devices the base voltage (B) should always be about 600-700mV larger than the emitter (E) voltage.

This circuit has an overcurrent detection circuit consisting of two 1 ohm resistors: R723 for the +14V (shared with 5.6V regulator), R724 for the -14V regulator.
Please check the resistance of these 2 resistors. If both are reasonably close to 1 ohm then they are OK. Next measure the voltage drops across each resistor (R723, R724) while the amplifier is powered up.  I would expect <200mV, which corresponds to a current of 200mA.

Thanks for the reply!

The front panel lights up, everything works there. It just says overload, and since there was some heatmarks around this circuit I figured I would start there.

Both R723 and R724 shows 1.16Ohms, just cables to multimeter 0.22Ohms.
Voltage drops over R723 260mV, and R274 170mV.

I've been taken some measurements:

E: 24.74Vdc
C: 35.53Vdc
B: 25.43Vdc
E: 25.38Vdc
C: 35.13Vdc
B: 26.04Vdc
E: 16.72Vdc
C: 26.05Vdc
B: 17.37Vdc
E: 16.06Vdc
C: 24.33Vdc
B: 16.68Vdc
E: -17.65Vdc
C: -23.76Vdc
B: -18.35Vdc
E: -5.57Vdc
C: -14.42Vdc
B: -6,26Vdc
E: 35.72Vdc
C: 10.58Vdc
B: 35.37Vdc
E: -14.55Vdc
C: 15.37Vdc
B: -14.53Vdc
E: 11.06Vdc
C: 15.56Vdc
B: 10.55Vdc

Your resistors R723 and R724 test good for ohms and the voltage drops across them look OK.

But the voltages you report for the transistors look all wrong.
All of these voltages should be measured with the minus ( - ) terminal of your DC voltmeter connected to circuit ground (earth) which is usually the metal chassis of the amplifier.
Please check this. Maybe your - probe on your voltmeter wasn’t connected to ground?

Try this: Locate zener diode D705 on the PC board. The end with the marking band on it should measure about 6.2V. The other end (no band) should measure 0V.  If you don’t get 0V at this terminal then there might be a disconnected ground wire between this PC board and the amplifier chassis. I have seen PC boards where the ground connections are made by the screws which attach the PC board to the metal chassis. If the screws are loose or missing then you might have an open circuit.

There is another zener diode on this PC board: D707. For this zener diode the end with the band should measure 0V and the end without the band should be about -28 to -30V.

I located a full set of schematics and PC board layout diagrams for this Technics SA-DA8 integrated amplifier.

It is a “home theatre” style unit with 5 power amplifiers capable of driving speakers:
      Main left and right speakers
      Surround left and right speakers
      Center channel speaker

There are 3 large “hybrid IC” type audio power amplifier modules in this unit.
        Part # RSN313H25-P

Each power amplifier module contains 2 independent audio power amplifier circuits. These are assigned as follows:
     IC601 is main left channel and surround left channel
     IC602 is main right channel and surround right channel
     IC651 is center channel (the other amplifier circuit inside this module is not used)

The most likely reason for this unit to stay in “protect” mode is due to failure of one of these modular hybrid IC audio power amplifier devices.
These modules were a proprietary Panasonic/Technics part. They were used in many Technics receivers and integrated amplifiers throughout the 1990’s. Twenty years ago these modules were still readily available as replacement parts. I do not know if any are still available today.

To identify which module failed: Locate these test points on the main PC board:
     TP L6 and SL5 are for IC601.
     TP R6 and SR5 are for IC602
     TP C5 is for IC651

Each test point should measure less than +/- 500mV DC to chassis ground.
When a module has failed there will be a large DC voltage at the test point which will typically be near the positive or negative main DC rail. For this unit the “high” supply rails are +57V and -57V.

The module with a large DC output is the bad one.
If you locate a faulty module then carefully desolder all of its pins from the PC board and remove it.
This may result in the unit coming out of protection and being able to produce audio on the remaining channels.


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