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Technics SA-DX750 Receiver

Discussion in 'Repairing Electronics' started by Jordan Haining, Mar 6, 2017.

  1. Jordan Haining

    Jordan Haining New Member

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    Hello,

    I will start by saying I have a fair bit of experience troubleshooting both in past jobs and at home as a hobby but not a whole lot with audio. I am now in school to become a refrigeration and A/C mechanic and have also learned extensively about troubleshooting of refrigeration controls so I am familiar.

    My Technics SA-DX750 Receiver is having issues. During playback it suddenly gets distorted then the audio cuts out. There doesn't seem to be any overloads tripped because everything other then the sound seems to work. It affects signals coming in both the digital and analogue inputs so I don't think it is the digital circuit. Sometimes it will play for over an hour then crap out other times only a couple seconds. I found the repair manual on manualslib and have been reviewing it. It affects all the speakers so that leads me to believe its not either of the main amps (1 for Front L/R and 1 for Rean L/R and Centre). I have followed the traces on the schematic and it appears the 2 components all channels share are IC401 TC9164AN Switch Array. and IC402 NJM4580DD Op Amp. They then go through their respective circuits and to their respective inputs on the main amplifier IC's. The problem is I do not own an oscilloscope and cannot look at the waveform, is one necessary or can I troubleshoot audio circuits with a DMM. Also does it sound like this could be the issue. I just don't have the money to take it to a repair shop or to buy a replacement amp. Any other suggestions would be appreciated.

    Thanks Jordan.

    Edit: I forgot to mention all the caps look good, none of the look like the are broken and the PCB around all of them is clean.
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    As it's intermittent, it makes life rather difficult - and a meter isn't going to help much.

    What will help (and probably more than a scope to some extent) is a monitor amplifier and speaker.

    This could be one channel of another stereo amplifier, or a simple little mono amplifier, such as an LM386 or similar.

    Use the monitor amplifier to monitor the signal before it gets to the power amplifiers, and see if you can find a point where it begins to go faulty - the top of the volume control (assuming a mechanical volume control) is a common first point to check, and work backwards or forwards from there, depending if the signal disappears there or not.
     
  3. Jordan Haining

    Jordan Haining New Member

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    Thanks for the suggestion. I will talk to my electrical instructor tomorrow and see if he has one in class I can use. All the dials and buttons are digital, the only real analogue circuits are the tuner, transformer circuit and the amplifier board labeled as as "main board" on the in the manual. The analogue audio signal enters in the RCA inputs then directly to the switch array, from there through 1 resistor and 1 cap (per channel) to the op amp. from there it goes to the muting circuit which unless activated lets the signal go through another resistor and cap to the inputs of the power amplifier which are each coupled to the SGND line through tiny caps so I am hoping it will be east to figure out. Though thinking about it now it might have something to due with the muting circuit.

    0Thanks again for the suggestion.
     
  4. dave

    Dave New Member

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  5. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Check Caps again to be sure.

    Any slight bulge would indicate gas pressure on the lids of caps which insulates the conductors causing high ESR..
     
  6. Jordan Haining

    Jordan Haining New Member

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    Sorry it was a busy week this week. I have been running the amp waiting for it to trip and its been running good. I have pushed it and its still sounding good. However I did disconnect my centre speaker. My front and rear speakers are Sony 6 ohm speakers but my but my centre is a Kenwood 8 ohm speaker. The centre and front speakers are run off the same amplifier IC (RSN310R36A-P). Could mismatched speakers running on the same IC cause problems? Its been running for 2 hrs now and the heatsink is barely warm. I checked the caps closely and none look like they are bulging
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No, that should be fine - and even using too low an impedance speaker is only a concern when running high powers.

    They are VERY unlikely to be faulty, while duff electrolytic's are certainly the most common fault over the last 20 years or so - it's only in certain specific uses, and almost universally due to the use of sub-standard low quality capacitors. It's rare to get problems in linear and analogue devices, it's almost exclusively in switch-mode devices - where I've changed thousands.
     
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Manual says speaks from 6 to 16 ohms. It was hard to tell if all of the speakers shut down on "protect", but you could monitor the DC voltage on the speaker terminals.
    There's usually some, from a few mV to maybe 750 mV tops. Zero would likely mean the protection relays opened. If they did, you would have to monitor before the protection relays.
     
  9. Jordan Haining

    Jordan Haining New Member

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    It tripped off again so I checked the volate and was getting 1.2mV at the speaker but no sound. I powered it off for a few min then on again and I was getting between 0.8mV-2.4mV with sound.
     
  10. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Do all of the speakers trip off or just some of them? The numbers are very good. If you have your center speaker disconnected, remember to check that one too.

    You did check each speaker output, right?
     
  11. Jordan Haining

    Jordan Haining New Member

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    All of then trip off including the sub. I check all but the center I feel stupid. I also didn't check the subs output. Tomorrow I will check them all again.
     
    Last edited: Mar 12, 2017
  12. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Assuming the DC protection is kicking in it will take all speakers out, as it's one single circuit.
     
  13. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    This amp is unusual. The 2-channel amplifier IC's integrates the DC protect circuits and it looks like center and surround are two DPDT relays and would go out together. The front speakers MAY go out separately. I really can't tell unless I print the schematic pages and tape them together.

    I've only worked on one amp of this complexity and it had a design issue. The supply voltage could exceed the absolute maximum voltage for the IC amp. I had been doing some freelance work for a shop and this particular amp came back 2x. I got it on the 3rd time back. All of my others have been stereo.

    The free manuals are available at HIFIengine.
     
    Last edited: Mar 13, 2017
  14. Jordan Haining

    Jordan Haining New Member

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    I have the service manual downloaded. So is it worth it for me to continue trying to figure this out or should I just scrap it?
     
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Helping someone troubleshoot online is really tough. Having an intermittant problem makes it even tougher. The EXACT problem may not have been adequately described.
    e.g. It's likely that one channel gets distorted before it craps out and there is no audio anywhere. I think you first need to determine if the AMPS are going into protect.
    Checking for 0V DC at the speaker or unconnected, is just one way of determining if that's happening. Later, you can refine which dual channel amp is going into protect.
    We're guessing it's an amp problem.

    You can also try to idle the amp with no speaker or input and feel the temperature of the amplifier IC's. See if one side is warmer than the other, Check the DC voltage at the speaker terminals. I've seen too much thermal grease being used.

    With a repair,you would generally check the power supply voltages and ripple. In an amp, you would want to know if it's going into protect and which channel is causing it.
    You would generally use either signal tracing or injection while bisecting the stages.

    Gut feeling, it's related to an amplifier module.
     
  16. Jordan Haining

    Jordan Haining New Member

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    I understand, and it makes it even harder when the troubleshooter has never worked on audio circuits. I am gonna ask around and see if anybody I know has some equipment I can use. In the mean time I will try idling it like you said and see if one amp heats up more than the other. I will also re-check the DC voltage at the speakers again. I was looking for datasheets on the amplifier IC's but couldn't find any. I might put this on the back burner until I can get a hold of some equipment.

    Thanks for the help.
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    You need to check the voltage at the outputs BEFORE the protection relays, to see which (if any) is going faulty.

    There's no point checking on the speaker outputs, as the relay disconnects them when faulty.
     
  18. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Nigel's response does make more sense. In any event, I was suggesting that a big fat zero would mean that a protection relay opened, providing your meter does read zero.
    I now agree with Nigel, that it's likely that all channels would go out, but not sure with this amp.

    The module data may have gone away with the merger on Sanyosemi with onsemi. At least the modules seem to be available for repair. There were suggestions to look for cold solder joints and/or burnt spots on the boards.

    It might make sense to monitor the RLY pin on the modules. It looks like if would be >9V or so when OK.
     
  19. Jordan Haining

    Jordan Haining New Member

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    After school today I plan on focusing a fair bit of time on it working to figure it out.

    The strange thing is there is no relay click when it cuts out just a bit of distortion then nothing and when I did test the output at the speakers I got a steady 1.2mV on the output. I will monitor the relay voltage and see what happens. I will also look for cold joints and burns, I'm gonna need to get more flux and reheat the joints if any are cold.
     
  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Multiple relays are used and you probably won't hear them. The distortion might be one channel only. You could try "turning off" the un-related channels using left/right and balance to see if the distortion is a single channel phenomenon. The other thing to do is monitor the pin labeled RLY on each of the AMP modules before and after it cuts out.
     
  21. Jordan Haining

    Jordan Haining New Member

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    I finally got a chance to test my relays voltage before trip it was 9.6V and after the trip it was still the same, I also didn't notice and change in the relay voltage as it tripped. This is a little more intricate then I am used to working with so I think I might just wait until I can afford it and buy a new one. with 3 kids I don't always have time to work on it. Maybe salvage parts from it. Looking at the price of the receiver online its probably just as cheap to buy one as it is to take it for repair.
     

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