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Sub amp silent

Have an NHT sa-3 sub amp. Few years old and went dead with a pop. Fuse did not blow so thought less likely output transistors? Big output capacitors all leaky. Pulled them and cleaned up the board. Replaced with new capacitors. Identical (2) and 2 that had same specs but slightly higher voltage rating (60's vs 50's other specs same). Powers up as before, but still no output. Have not gone over the rest of the output board. Nothing really looks fried. Is it very unlikely to be an output transistor without a blown fuse. How to proceed? I have a volt/ohm-meter. Can return it to NHT's repair co. but what fun is that. Plus they charge $150 plus the cost of shipping both ways and the only parts that cost anything are the Caps I already replaced. Thanks.Jeff Hartford

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  • Jscopus Jan 04, 2009

    I am not a technician, if I was, I would not need help for what is likely a modest issue. When a power amp fails, it is usually one of only a few things. The output resistors (but these almost universally cause a high current a fuse blows), output capacitors (tend to leak/swell and already replaced) diodes, etc. I do not see why a voltage regulator would have failed. The amp is 6 years old.

  • Jscopus Jan 04, 2009

    There is no earphone jack, it is a mono channel subwoofer amplifier. It was playing fine, there was a sharp pop, and than no sound. I checked the transistors and they are fine. Also, it would be very unusual to have an output transistor short without blowing the fuse as there is typically very high current associated with this. It would not make a loud pop if the volume pot died (which does not make sense to me based on the history of how this died.)

  • Jscopus Jan 04, 2009

    Ben-
    Please explain how to do the check your are suggesting. I have a schematic (if I can find it) of the SA-3 and a multimeter V/A/ohms. Do I need something else and exactly what do I do to test? Test with the unit powered on or off? Thanks, Jeff

  • Jscopus Jan 04, 2009

    The unit still powers on. The fuse looked fine and I still swapped it with a replacement fuse. That is why I think the fuse is an unlikely target.
    Jeff


  • Jscopus Jan 04, 2009

    The unit still powers on. The fuse looked fine and I still swapped it with a replacement fuse. That is why I think the fuse is an unlikely target.
    Jeff

  • Jscopus Jan 04, 2009

    Welcomejee-
    That looks like the schematic though I am having trouble finding it. That is the correct pair of output transistors. There were 4 big electrolytic capacitors (2) 10Kuf 80V 85degree Panasonics and (2) 6800uf 105degree 50V or so that I replaced with 63V capacitors with other identical specs.
    Jeff


  • Jscopus Jan 05, 2009

    I had a 5 page schematic that they sent. I may be able to find it but I am sure they can send another. I have a receiver and multiple speakers.

    Jeff


  • Jscopus Jan 05, 2009

    I can get another schematic and I have a receiver and many speakers

  • Jscopus Jan 08, 2009

    Here is the schematics as sent to me again by NHT.

  • Jscopus Jan 08, 2009

    How do you post a pdf? I have the schematics to post? Jeff

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If output transistors are OK and volts are being supplied there then driver stage has no power like 18 Volt, 15 Volt and that circuit needs to be checked.  Please confimr is that schematic in your Amp mentioned below?
Sub amp silent - 193d035.jpg

 

Posted on Jan 04, 2009

  • 3 more comments 
  • Shahid Electronics
    Shahid Electronics Jan 04, 2009

    I can provide you complete schematic as its just part posted here. 
    you can email me .. welcomejee at yahoo dot com and will send you pdf page.

  • Shahid Electronics
    Shahid Electronics Jan 04, 2009

    You are right Benimur mate, if I post complete schematic page here then it would be reduced in size and looks unreadable, thats why I posted its specific part here.

  • Shahid Electronics
    Shahid Electronics Jan 04, 2009

    If you replaced 6800UF 50V caps with 6800UF 63V, its just fine. Cap's working volts must not be less then old cap.

  • Shahid Electronics
    Shahid Electronics Jan 08, 2009

    You can send me pdf on welcomejee at yahoo dot com 


    Let me see whats wrong with amp.

  • Shahid Electronics
    Shahid Electronics Jan 08, 2009

    Ginko, 
    Power supply is working but partially, Main supply would be ok but problem seems in sub-power supply goes to amp-driver section.

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  • 474 Answers

You have replaced the fuses regardless i hope, just to be on the safe side. Some times internal damage to one of the components on the circuite board may not be evident. us a magnifying glass and look closely at all the components. you may see something with slight black marks around it, they would be where something popped. Other than that, it seems like you did alot of the hard work yourself already.

None of the leakage got on any other components did it?

I recently Had a Blood Sugar machine that leaked battery fluid and i cleaned it up and it still didnt work, so I stuck it in the fridge and tryed again later and it worked :)


Not that it will for you but it was kind of cool.

I would take your volt/ohm-meter and test the out put side and work your way back to where you get a good reading and that should narrow it down, then replace what you can. Some smaller components that are machined on to the board at the factory may not be so easy to replace though.

Good luck.

Posted on Jan 05, 2009

  • Chris Busch
    Chris Busch Jan 09, 2009

    When you checked with the Volt/ohm meter, it should be on and it should be outputting I.E. playing music, regardless if you can hear it or not. While its playing music, or not, but on, and you know music should be playing, then take your meter and test the all the hot wires. Now, im asuming that you have a volt/ohm meter because you know how to use one.



    Set it appropriatly, before testing. If you dont know hoe, look it up first, its simple, theres a setting you use for say a car which is like 13 amps and one for your home. The Meter has its own fuse so if you connect it to something and it blows, its just a fuse.



    test all the hot connection and wires and everything systematically. Use the specs that you have to know where the hot lines are.



    When you get to a spot that should be hot, but is dead, then you found where the problem starts and you can start looking there.



    A pop usually results from a fuse or capacitor, especially if its really loud. But since you changed them all, its hard to tell know what it could be. Does anything actually smell burnt?



    If theres any burnt smell, you can bet there would be evidence around what popped. If there isnt, then its a Fuse or capacitor.



    When repairing my friends GPS i used a large magnifying lense and found that it was a tiny little connector for the antena wire and the soder underneith had snapped, but it was so tiny we couldnt really see it. After that we where able to repair it.



    Well good luck. Sorry we cant be any more help



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  • 448 Answers

Can they send you a schematic? Do you have a home stereo with rca inputs and a speaker laying around/

Posted on Jan 04, 2009

  • bob collins
    bob collins Jan 05, 2009

    If you have a home stereo with rca input. You are welcome to test individual parts without electricity but if it were me, operation is what you are looking for. Take a set of rca leads, plug the rca into the input of your home system, touch the leads with your hands and you will hear a hum on the house speakers.Strip the end of them. Clamp the ground side of the rca lead to the chassis of the amp. With the amp turned on, touch the positve lead of the rca to the input of the amp. You should hear it on your home unit whether amp works or not. Then take move this lead to the various audio outputs on the amp. Your audio should get louder on your home amp. Be careful because if you get it working and your gain is cranked and things work all the way to the end, your home system will boom with loud noise. I work with DMX audio systems all the time and using a preamp to check a preamp is the way I would go.

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  • Master
  • 5,158 Answers

You did mention the fuse not blowing. did you only visually check it our use a metre to check continuity on it ?

Robert

Posted on Jan 04, 2009

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Even if you are not a technician you know more than many contributors posting their comments here.

The pop, as you said, is often given by a final transistor blowing. But if you found few caps that have blown, that may had been the original cause.

If by fuse you are referring to PSU fuse, that has nothing to do with the transistor blowing. The output transistor may still be defective, and the main fuse be OK.
the transistors have low ohms resistor or fuses (not PSU one, but fuses or resistor near the on the final amp section board) that are designed to blow when there is a problem.
Test all resistors around the output transistors.
The transistors can be tested by reading continuity between base, and alternatively collector and emitter contact.
If the transistor is working this will return approx same impedance.
Test also the rest of resistors, and inspect the board for broken joints, starting from the area where you replaced caps, and if the unit is powering up, concentrating on the final, where you get output trans.
Also inspect the unit to find out if there is a short to the chassy, or on the board, this also cause unit not responding, and components reading fine.


On testing output trans see:How to test Transistor

See also :
Sci.Electronics FAQ: Repair: Power Amps Repair

If you still have troubles, order service manual from Factory.

Posted on Jan 04, 2009

  • Ginko
    Ginko Jan 04, 2009

    You said "Powers up as before, but still no output." That means the power supply is working.

    Ensure that soldering at the base of capacitor is fine, you can test also that there is power getting to caps, using the multimeter, but this is not a safe operation if you do not have enough practice.

    Limit the testings to continuity reading and observation, that should be enough for now.

    Test the output transistor, as suggested already, see see:How to test Transistor .


    Better not test caps from continuity, unless you discharge them by shorting their pin. Do all tests with power unplugged.


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  • 6,966 Answers

Hi and welcome to FixYa,

Initially, I am inclined to think that the capacitors are not output capacitors but rather power supply filter capacitors.

Additionally:

  • pls confirm if the NHT SA-3 uses a pair of 2SA1302/2SC3281 power transistors for the output driven by 2SA1837/2SC4793;
  • check/post the +/- voltages across the new capacitors;
  • check the collector of Q15, 2SC3281 for the + voltage;
  • check the collector of Q16, 2SA1302 for the - voltage;
  • check for presence of the +/- 13VDC.
Pls post back results.

Good luck and Thank you for using FixYa.

Posted on Jan 04, 2009

  • 2 more comments 
  • Louie  Role
    Louie Role Jan 04, 2009

    Appreciate the direct reference on your comment/postback.

    Voltage checks are done with the power on whereas resistance/continuity check are performed with the power off and preferably the component out of circuit.

    You posted that the fuses have not blown and therefore you are assuming that the output transistors are not defective. Pls confirm this by performing a check on the base to collector to emitter of the output transistors. This page would illustrate how to check a transistor using a VOM. The output transistor once removed from the circuit (pls mark which goes to where), laying flat with the pins/feet pointing towards you would be base, collector and emitter. Although the real outputs if 2SA1302/2SC3281 would be larger.

    Most NHT sub amps are standard design amplifiers. To my knowledge, NHT amps (but not all) uses a unique design to supply power to the output transistors. The fuses does not tie directly to the output transistors from the +/- of the power supply. Instead the fuse goes to several power MOSFETS (IRF640) that regulate/control/monitor the current/voltage supplied to the output transistors. If any of the IRF640 is blown, the output would have no voltage even if the fuses are good.

    To be more specific and to confirm the above, would appreciate if you could post/share the schematic diagram.

    May I also inform you that there are posts made by other members of the community (our co-experts, I am aware that you have provided solutions yourself in some other posts) that would be of additional help. ginko has posted the method/manner by which you could test the transistors and other components, while bunnydawg addressed the fuse not blowing. There were however and I would expect more posts that are too general/generic to be of any real use to you. I would leave it to your discretion to filter through the flood of information.

    As a site policy, once you rejected a solution or give it a "Thanks for Trying", that particular post/expert would no longer be viewable. For your benefit, may I suggest that you copy and paste the "workable solutions" to a document that you would have a record of previous or no longer viewable postings.

    Cheers.


  • Louie  Role
    Louie Role Jan 04, 2009

    Oopps, missed the posting of welcomejee with the amp schematics of an NHT Subtwoi.

  • Louie  Role
    Louie Role Jan 04, 2009

    And now I doubt if Jscopus could still see my posts.

    welcomejee, further left of the amp schematics, the 6 X IRF640 in series with the +/- line after the fuses, and the 2 inductors/coils, pipes the +/- supply to the outputs, right?


  • Louie  Role
    Louie Role Jan 09, 2009

    Hi again,

    Any updates/developments? Were you able to check for the presence of +/- voltages on the collectors of the output transistors?


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Hi 1) Make sure a bridged load of no less than 4 ohms is applied.
2) make sure both the left & right channels are working.


or sound like the subs were blown, whether it makes any humming noise

Posted on Jan 04, 2009

  • ABHISHEK.C
    ABHISHEK.C Jan 04, 2009

    The amp is only 2ohm stable in stereo mode and 4ohm stable in briged mode. 

    EX: If you connect a 2 ohm sub to the amp in briged mode.
    1. It will cause the amp to get hot and possibly cause the amp to stop working.
    2. It can cause the voice coil(s) to get hot and detach from the voice coil former.

    Check the preamp output on the RCAs on the radio and what the amp is capable of handling. If the radio is more or less than the amp can handle it can cause the circit for the RCAs inputs tobe damaged over time. If the gain/level is turned up past 3/4 of the way it can shorten the life of the amp extremly. Also if the amp is starving for power it can damage the amp. Check the peak and rms on the amp and sub(s) if the amps peak or rms is higher than the amp it can damage both the amp and sub(s).Check to make sure all the connections on the amp are tight. Then check to be sure the connections to the input termainals on the box(s) and sub(s). It can cause the sub(s) to pop.



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  • 201 Answers

I would say measure between the legs of the TRansistor if you get a short between the legs its faulty, they are usually designed with a relay to protect when the-res a short... I would replace all of them if 1 is shorted its located on a heat sink. Check also following

  1. Make sure your earphone jack is clean and ok, had once a amp I replaced almost all the components when the earphone jack was dirty causing no sound to output
  2. Check your volume Pot
PLease let me know

Thank you for using Us

Posted on Jan 04, 2009

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Double Check open circuit. Check the voltage rectifier if it still has continuity. You might as well replace that.

Posted on Jan 03, 2009

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