Technics SA-DA8 audio amplifier overload issue

[sailor83]

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?

[elecdonia]

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.

[sailor83]

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:

Q701:
E: 24.74Vdc
C: 35.53Vdc
B: 25.43Vdc
Q703
E: 25.38Vdc
C: 35.13Vdc
B: 26.04Vdc
Q704
E: 16.72Vdc
C: 26.05Vdc
B: 17.37Vdc
Q706
E: 16.06Vdc
C: 24.33Vdc
B: 16.68Vdc
Q707
E: -17.65Vdc
C: -23.76Vdc
B: -18.35Vdc
Q708
E: -5.57Vdc
C: -14.42Vdc
B: -6,26Vdc
Q723
E: 35.72Vdc
C: 10.58Vdc
B: 35.37Vdc
Q724
E: -14.55Vdc
C: 15.37Vdc
B: -14.53Vdc
Q725
E: 11.06Vdc
C: 15.56Vdc
B: 10.55Vdc

[elecdonia]

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.

[elecdonia]

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.

[elecdonia]

Although I didn't mention it in my previous reply, this Technics SA-DA8 integrated amplifier uses "class H" architecture for its audio power output stages. This feature is embedded inside the RSN313H25-P hybrid IC amplifier modules. Note: The SA-DA8 contains 3 of these hybrid IC modular power amplifier devices. They are labeled IC601, IC602, and IC651 in the schematic.

Brief explanation:
Class-H provides two sets of +/- DC power supply rail voltages to the power amplifier. Because audio signals have a rather high peak-to-average ratio it is useful to supply the audio output stage with reduced +/- DC supply rail voltages most of the time, but (almost instantly) raise them when a large loud peak  comes along. This permits the amplifier to run cooler and saves a considerable amount of power which would otherwise be wasted in the form of heat.

For the Technics SA-DA8 the "low level" rail voltages are +/- 28V.  Then, when a large amplitude signal comes along, the supply rail voltages quickly rise to +/-57V.

The SA-DA8 main PC board contains 2 sets of large "bulk energy storage" electrolytic filter capacitors. The 2 larger capacitors (C703, C704) are for the +/-57V rails. The 2 medium-size electrolytic capacitors (C705, C706) are for the +/-28V supply rails. Near each of the RSN313H25-P hybrid amplifier modules there are 2 large diodes (external to the hybrid amplifier module itself) which supply the +/-28V power supply rails to the hybrid module.

For IC601 these diodes are D601 and D602
     IC602:    D651, D652
     IC651:    D653, D654

A useful test for proper operation of the class-H circuitry is to measure the DC voltages at these 6 diodes. Under a "no-signal" condition the voltage between either end of each of these 6 diodes and chassis ground should be approximately either +28V or -28V.   

However, if the voltage between chassis ground and either end of any of these 6 diodes measures +57V or -57V this indicates the associated RSN313H25-P hybrid amplifier module has failed.

Another way to check this is by connecting the multimeter probes directly across the diode itself (no probe is connected to chassis ground for this measurement). For each of the six diodes, the voltage at the cathode (banded end) should be about 600mV lower than the voltage on its anode.

However, in the case of a failed  RSN313H25-P it is likely that the voltage across the diode will be much larger:  approximately 28V in the reverse direction:  cathode (banded end) more positive than anode. This indicates failure of the associated hybrid IC amplifier module.

Background and history of class-H: The first manufacturers to mass-produce class-H amplifiers were Carver, starting with their M-400 "magnetic field" power amplifier, and NAD with several models they referred to as "Power Envelope." A bit later Hitachi embraced this technology. In fact the "H" in "class-H" stands for Hitachi. It has gradually become a standard technique for increasing the efficiency of linear audio power amplifiers.

[sailor83]

Thank you so much for your thorough reply, I really appreciate it! After the easter break I now had time to check on the amplifier again.

I've made a small mistake by not putting the screws back in the boards after going over some solderjoints that looked dry, therefor having a floating circuit.
After mounting the main board to the chassis and measuring I got the voltages on the output transistors to be:

Q701: E: 14,14Vdc
Q708: E: -16,37Vdc
Q707: E: -28,43Vdc
Q706: E: 5,5Vdc

Which looks to be correct according to the schematics, that you now also have found.

By measuring the testpoints you gave me I found the voltages to be:
SL5:   0,01Vdc
L6:    0,009Vdc
R6:   24Vdc
SR5:   -47Vdc
C5: 0,0086Vdc

This indicates that the amplifier module IC602 is faulty, the main right and surround channel. Am I right?

I also measured the diodes D601,D602, D651-D654, they all show -+24Vdc, exept D652 that shows -48Vdc on the anode pin. Which you said is also linked to IC602.

I've ordered a RSN313H25 from aliexpress, so we'll se how the amplifier turns out in a few weeks!

[Finderbinder]

Sorry, didn't read through all replies, but...
Few days before I had a similar problem with Technics SA-AX7, after some case deformation it showed Overload. Appeared bad solder of Q708. Resoldered it and 

[elecdonia]

Thank you so much for your thorough reply, I really appreciate it! After the easter break I now had time to check on the amplifier again. By measuring the testpoints you gave me I found the voltages to be:
SL5:   0,01Vdc
L6:    0,009Vdc
R6:   24Vdc
SR5:   -47Vdc
C5: 0,0086Vdc
This indicates that the amplifier module IC602 is faulty, the main right and surround channel. Am I right? I also measured the diodes D601,D602, D651-D654, they all show -+24Vdc, except D652 that shows -48Vdc on the anode pin. Which you said is also linked to IC602. I've ordered a RSN313H25 from aliexpress, so we'll se how the amplifier turns out in a few weeks!

Yes, I expect replacing IC602 will put everything into working order.
You may wish to desolder the faulty IC602 and totally remove it from the amplifier while waiting for the replacement IC602 to arrive. It is likely that doing this will permit the unit to come out of “protection” mode. Also the left channel (both main and surround) and the center channel should then all produce audio.

I really hate to need to say this…   But… some Chinese vendors deceptively sell “imitation” replacement electronic parts. Hopefully this won’t be the case for you. Here’s one way to determine whether the replacement IC602 is “real” or “fake:”

1) Do not discard the faulty IC602 after removing it from the amplifier.
2) Get or borrow a small weight measuring scale which can weigh up to 1 kilo (2 pounds) or a bit more (these are often sold as “kitchen scales” to be used for weighing recipe ingredients)
3) Carefully measure the weight of the faulty IC602.
4) Then weigh the replacement IC602
5) Their weights should be identical within +/- 10%
6) Most  “fake” replacement parts have considerably lower weights than genuine parts.
7) Also compare the appearance of the replacement part to the original part. The dimensions, quality of finish, and labeling should be similar to the original part. Post side-by-side photos if you wish.
8 ) If you suspect a “fake” part, do not install it into your amplifier. It is better to contact the vendor and request a refund. Provide the vendor with photos (and the difference in weight if they aren’t the same). Most vendors will issue a refund if you provide them with evidence that you received a “fake” part.

[sailor83]

I've finally got to repair the amplifier. The first chip I recieved was obviously an used one, did nok look the same on the outside, but inside it was the same, and it also looked like it have had some moisture and corrosion as well. Got a refund for that one and ordered a new one from ebay. But I tried the first one, the right channel came and went in pulses. Waited a few weeks for the "new" one and soldered that one back in, it worked! (It was also used, and the front looked like it was sanded down and printed new text on, so 2 of 2 "new" aliexpress/ebay chips are actually old used ones)

The original chip had a burnt leg from one of the mosfets, and it also looked like there was some burnmarks one the mosfet chip itself. Have not tried reparing the thin wire, but have seen other online that have repaired the ic that way.

Thank you so much @elecdonia for your help, I really appreciate it, it made the amplifier come back to life!