Electronics > Repair
Technics SA-DA8 audio amplifier overload issue
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! :D 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! :D
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:
--- Quote from: sailor83 on April 11, 2023, 06:06:50 pm ---Thank you so much for your thorough reply, I really appreciate it! :D 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! :D
--- End quote ---
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! :D (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! :-+
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