Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at max

[beetle1303]

Hi,
I am totally new in Audio/TV repair and Electronic in general, but keen to learn basic electronics in TV and Audio systems.

I have been investigating this Soundspace 5 system for a few weeks now, doing some very basic check but heading no way...
Here's what I know:
1) Main system is good - Radio and CD Changer works, and clear sound when using headphone, and volume control works.
2) Both powered speakers sound level very low - almost no sound at mid-level volume, and audible (but low) at full volume.
3) I have opened the main unit and powered speakers. Visually inspected and using DMM to check power and signal sources, resistors and capacitors values, and traced a few circuits, all appeared to be good. Where some resistors and capacitors seemed to be above/below specs, I have pulled them out of circuit and measured OK with DMM.

*soundspace 5 service manual here: https://elektrotanya.com/nakamichi_s.../download.html

I have been focusing on the powered speaker's AMP, simply because it's easy access.. The main board seems too complex to even try...

Any suggestion on where else to check and trouble shoot further?
Thank you.

[R_J]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

There are two seperate audio output ic's, correct? one for left and one for right channels, I doubt both are bad. so it is likely something common to both, Check the voltage on the plugs going to the speakers, Do you have B+ on cn903 pin2 ? What is the voltage on pin5 ? this is a mute pin.
Check the solder connections on the plugs for the speakers on the mmain board and the speaker jack pcb.

Also check the headphone jack switch, It looks like that when the headphones are pluged in, that mutes the speakers. On the headphone jack board there is a switch, so check the resistance across pins 1 & 2 of CN608 with the headphones plugged in and when they are unplugged. it should be short in one position and open in the other.

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

Thank you R_J.
Yes, each powered speaker has identical hardware.
Here are the outcomes:

Main unit in standby mode:
1) CN903 Pin2: +20v (although the service manual stated +15V)
2) CN903 Pin5: 0V

Main unit in power-on mode:
1) CN903 Pin2: +18.3v
2) CN903 Pin5: +9.3V
*both speakers audible in full volume

Main unit in power-on mode with HP plugged in:
1) CN903 Pin2: +20v
2) CN903 Pin5: +0V
*both speakers no sound, and headphone volume is very loud (louder than speakers volume when headphone is unplugged)!

I set DMM to continuity test on headphone board:
1) Headphone unlougged - Pin1 and Pin2 shorted, speakers volume audible
2) Headphone plugged in - Pin1 and Pin2 open, speaker no sound.

Kindly advise if I have missed out any tests.
Thank you.

[redwire]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

Did water get spilled inside? The PCB top looks black and corroded by transistor Q302.

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

I gotten this unit from a seller who claimed it was used regularly and was working all along, and suddenly silenced when power on...

Can you point me to the transistor Q302?
Thank you.

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

I found the location of Transistor Q302, and it appears dry and clean. See pic.

I can't read the label clearly off the diagram - for my education, what does this transistor do, and this portion of the circuit do?

[budm]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

The Headphone jack is NOT driven by the Power amp IC TA8225 (the output for speaker is bridged mode, speaker output is not reference to circuit ground so it cannot be used for driving the headphone which is ref, to circuit ground).
If you look at schematic page 40, the headphone is driven by IC U304 (NJM4556).
When the headphone is plugged in it will put the speaker power amp section into standby mode as you can see the Voltage on V_STBY pin 5 of CN903 drops down to 0V as you reported in your post #3:
Main unit in power-on mode with HP plugged in:
1) CN903 Pin2: +20v
2) CN903 Pin5: +0V
Look at the schematic page 41 and check to see if Pre Mute line stuck high (>2V), or not.

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

Thanks budm.

"Look at the schematic page 41 and check to see if Pre Mute line stuck high (>2V), or not."
I am such a novice and not able to relate "premute" in the schematic to the physical part... Could you kind point out how do I do that?

[R_J]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

Regarding Pre-mute, can you locate the transistor Q509, or R507? The pre mute line will mute the audio going to the speaker amps.

If you look at page 40, you will see 3 transistors, Q301R, L & A. when they are turned on by the pre mute line they basically kill the audio signal going to the speaker amps.
The pre mute line should be low, near 0 volts.

[redwire]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

I found the location of Transistor Q302, and it appears dry and clean. See pic.
I can't read the label clearly off the diagram - for my education, what does this transistor do, and this portion of the circuit do?

This is from your original pictures, it looks like the glue went conductive and turned black. It should be scraped off.
Q302, Q303, Q301 (on heatsink) are the 11.5V regulator, looks like B+ for the CD player at CN804 and also PREMUTE power.

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

This is from your original pictures, it looks like the glue went conductive and turned black. It should be scraped off.
Q302, Q303, Q301 (on heatsink) are the 11.5V regulator, looks like B+ for the CD player at CN804 and also PREMUTE power.

The glue was more yellow/brown and black color was actually shadow.. I scrapped it off, re-tested but unfortunately no effect on the low volume..

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

Regarding Pre-mute, can you locate the transistor Q509, or R507? The pre mute line will mute the audio going to the speaker amps.

If you look at page 40, you will see 3 transistors, Q301R, L & A. when they are turned on by the pre mute line they basically kill the audio signal going to the speaker amps.
The pre mute line should be low, near 0 volts.

Yes, I located the transistor Q509, and the resistor R570 (not R507) next to the transistor. I measured the voltage across R570, both standby and power-on mode show +9.6V!

So this is the problem I presumed. I'm really excited!! Where and where do I check next?

[budm]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

The glue was more yellow/brown and black color was actually shadow.. I scrapped it off, re-tested but unfortunately no effect on the low volume..

When the glue changes from yellow to brown, it is needed to be removed because it becomes conductive and corrosive which will eat away copper trace and component legs, so if you see more glue turns brown you should remove them even the one that is still yellow so it will not cause problem in the future.
BTW, when you are taking the Voltage readings, are the board mounted back in place with all the board's mounting screws?

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

When the glue changes from yellow to brown, it is needed to be removed because it becomes conductive and corrosive which will eat away copper trace and component legs, so if you see more glue turns brown you should remove them even the one that is still yellow so it will not cause problem in the future.
BTW, when you are taking the Voltage readings, are the board mounted back in place with all the board's mounting screws?

Looking at the before and after pictures, the 2 black cable rubber at the connection to the board is exposed after scrapping off the glue. Could these be the cause of the PREMUTE line high at 9.6V ?

I dismantled and lifted the whole base board up vertically, and reconnected the faceplate, speakers, and antenna to do the measurement. The power-supply board is still attached to he main casing though.

Is there a grounding issue? The symptoms are exactly the same as before dismantling anything..

edit: added a pic showing a ground wire on the incoming power source to the faceplate.

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

Not sure if this is correct or helpful - I measured voltage at pin8 (MUTE) of U502, it was 0V at standby, and 0.1V when powered up.

[R_J]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

That line is the MUTE which is likely changed by the mute button on the remote. You can check that by monitering that mute pin and pressing the mute button to see if it changes.
The pre-mute could be driven by the (CDC pin30) This line also connects to Q507 which controls Q508 and then Q509. The CDC line also connects to Q105 whcih controls the mute lines of U101.
If the PREMUTE line is 9.6v it is because Q509 is turned on. maybe a leaky C516.
Check the voltage on Q507 base and then emitter. check the voltage on Q508 base and collector, check the voltage on Q509 base and collector

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

Unfortunately I don't have the remote control to test..

On checking out Q507, Q508 and Q509, I must declare that I am really not sure which pin is call what, and how to measure the voltages in these transistors. I google and attached the pinouts for reference. I placed the DMM black lead at chassis ground, then placed the red lead at each of the transistor leads, and here are the readings:

Q507: pin3(base) 2.3v, pin2(emitter) 1.03v, pin1(collector) 13v
Q508: pin1(grd) 1.03v, pin2(in) 12.96v, pin3(out) 1.03v
Q509: pin1(grd) 12.96v, pin2(in) 10.25v, pin3(out) 12.93v

Also, I do not know what's the right way to measure the "PREMUTE line", the resistors reading with black pin of chassis ground, and red lead on:
R569: 10.30v in / 1.03v out (across is 9.26v)
R570: 12.96v in / 3.28v out (across is 9.68v)

Finally, before I placed black lead on chassis ground to measure all the transistors, I was probing the transistors with black lead on pin1(grd), the red on pin2(in) and pin3(out), and I might have also done black lead on pin2(in) and red lead on pin3(out).. It BLOWN 1 of the fuse in power supply pack! I though it was due to short circuit when I accidentally touched wrong pins.. I went out to buy 2 of those fuses, replaced it and tried the same probing around, and the fuse blown again! Not sure if what I did is dead wrong, or Q508/Q509 is bad...

[R_J]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

Is the fuse still blowing or only when you try to measure voltages? Q508 emitter is connected to ground, you should not measure 1.03 volts, it could be that the metal chassis is not connected to ground.

Also, I do not know what's the right way to measure the "PREMUTE line

Use your meter. The premute line should be near 0 volts to get audio, when it is high (12v) the amp is muted.
What you can try is to remove one end of R570 and see if you get audio from the speakers
Have you checked that cap (C51?) 1µf/50v

[beetle1303]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

I measured again Q508, it fluctuated a little and stayed 0v. And yes, the fuse still blows and just blown when I re-measuring Q508 Q509. And when it blown, I think my red lead touched positive lead of C516...

So I also removed C516, tested on at 1.15uF. I bought a new one and soldered it back, but still the same - no sound.

Then finally while trying to remove 1 end of R570 as suggested, I grounded it instead because I don't think I can put it back later.

And VOILA!!! Perfect stereo sound from both speakers!!

Thank you so much R_J for suggesting to remove 1 end of R570, it didn't find the fault but it certainly made it WORKS!!

[budm]

Re: Nakamichi Soundspace 5 - headphone works normal, both speakers volume very low at

You will have to watch out if the PRE_Mute is no longer in the circuit, it can get loud pop on the speakers and damage the speakers.