Stereo Head Unit Test

[Wavey1]

Yes, I searched first :->

I have my console opened up for other work, so it would be easy to pull the head unit and bench test at this point.

My OEM stereo is producing no sound in the left channel. I had the head unit rebuilt a couple of years ago (Willman's) and everything worked great for about a year. Now the left channel is out. I suspect it's the speaker amp but I don't want to open the door up to check that, if I can easily check the output at the head unit.

I have the diagram for the terminals at the back of the head unit, so I know which terminals to jump. My question is, what am I looking for? Just continuity between the left channel + and -, or a certain ohm reading, or what?

Thanks!

 

[JLCoolman]

Connect the left channel output of the head unit to the input of the right channel amp/speaker.
If you get sound out of the right speaker, then more than likely the amp went bad on the left side.

Also, connect the right channel output of the head unit to the input of the left channel amp/speaker and see what happens.


I'm on a second head unit with BrianK repaired amps and am starting to get a similar issue to my original head unit of faint sound on the left channel.
I originally just did the amps without really troubleshooting. After I received the repaired amps, I was still experiencing some issues, so with the amp/speaker enclosures uninstalled and the door panels still off, I swapped the left and right enclosures to test things and was able to determine the issue was the head unit's output on the left channel.
Now that I'm experiencing the issue again, I'm wondering if there might be an issue with the wiring of the left channel (wiring between amp/speaker enclosure and output at head unit). That or it is just coincidence that my second head unit is failing in the same manner as my original.


And like you, I really wish to experience the original OEM sound system at its best. I really like the way it sounds as is, even with my faulty left channel. Most people can't even tell there's an issue with it until I adjust the balance to demonstrate.



One last tid bit, is the issue consistent for all modes of the head unit (radio/tape deck vs. cd-player)?
I experience different sound qualities between the selected modes, with radio/tape deck acting the same in comparison to the cd-player.

Which reminds me, that I started to wonder if there might also be an issue in the cable connecting the cd-changer to the head unit.


Sort of funny how many factors there are to a 20+ year old sound system.

 

[Wavey1]

Thanks, just tyring to bench test it at this point. It's out of the car and the battery is out of the car for a while so I can't power it up or jump it in the car. I'm just not sure what I should be looking for off the speaker output pins with a multimeter. Anyone?

 

[JLCoolman]

I'm not sure if there's really any use in trying to bench test it without any power going to it?

Considering all the circuitry behind the outputs, I wouldn't think you would get any continuity between the positive/negative of a specific channel.

Maybe find a way to hook it up to a 12-volt power supply and run wire from the output pins to the harness in the car?

Good luck.

 

[Old Guy]

As JLCoolman notes, there is pretty much nothing you can do in terms of checking for the presence of an audio signal without the head unit being powered and even if it is powered, a multimeter is not an appropriate tool. You really need an oscilloscope to check the audio signal path. If you don't want to follow JL's original suggestion, which would probably be the easiest and least error prone, you could try the following. Power up the head unit on the bench using a 12 v battery or regulated power supply (do not use a battery charger!). You will need to connect up an antenna to the head unit or an external audio signal source so that you have an audio signal present. If you are using the head unit radio to generate the audio signal, you need to tune it to a known radio station so that you have an audio source. The signal out of the head unit to the external amps is low level. However, in the absence of an oscilloscope, if you can find a high impedance earphone, the head unit probably has a high enough output to drive an earphone. Use the earphone to listen for the presence of an audio signal on the right channel which you said works. This confirms that your test set up is working. Then connect the earphone to the left channel. No signal or impaired signal indicates that the head unit has a problem. Equal signals on the left and right channels indicates the problem is elsewhere!

JL's suggestion is really much easier!

 

[BTL3.0]

Just now i notice mine starting to have similar symptom. Left channel sometime work, sometime dont. If i turn the radio off then turn it back on then it will work for a bit. Then if it does go out while music playing, i found that a slight tap below the volume know will get it to work again. Maybe a loose connection?

My setup:
Old Grom ipod/mp3 adapter
Himbot bluetooth adapter connected to grom via 3.5mm audio jack

I stream bluetooth audio from my phone to the himbot then himbot through 3.5mm jack to grom then it tied to the radio. Finally radio out to speakers.
Work great for me. Sound quality is good enough for my use.

 

[Wavey1]

Right - I have it powered up on the bench with a 120V primary > 12V secondary transformer, and an antennae plugged in, no problem there. If it was a normal head unit I'd just connect a speaker to each channel to test, but because it's designed to send the signal to a speaker amp, I wasn't sure what to look for. It's just easier to bench test it at this point because I have the time, it's all set up and it will be a while before the battery is back in the car. I'll try the headphone approach. Thanks!

 

[jwmelvin]

I have it powered up on the bench with a 120V primary > 12V secondary transformer

I'm guessing you mean a 12V DC power supply, not just a transformer (which provides 12V AC). If not, use a power supply before doing anything else.

 

[Wavey1]

Correct.

 

[Wavey1]

The simple answer I was looking for, from Daryl Willman: Just jump it to any small speaker. Did that and got a signal from both channels, so my problem is almost certainly the left door speaker amp, or possibly the wiring head unit-to-door. Thanks!

 

[JLCoolman]

Glad to hear you were able to diagnose the issue.


How did the signal sound through the small speaker?

 

[Wavey1]

Ha! Like crap, but all I wanted to do was confirm the head unit was functioning on both channels.

I think I'll send both door amps (and maybe the floor amp) to Brian for rebuilding. Although Willman has brand new door and floor units for $250 ea.

While the enclosures are out, is it possible to replace the OEM 4" speakers with better aftermarket, in the original housings? Thinking JL or Pioneer or something. I don't want to get into a separate amp and aftermarket 4X6s.

 

[JLCoolman]

Yeah, I guess at least you confirmed there was a signal coming from both channels. What kind of signal is still in question though, lol.

Unless you want brand new, you can't go wrong with BrianK.

Not sure about replacing the OEM speakers. I think the sound quality will definitely be different (for better or worse is up in the air) since the amps and speakers and enclosures were tuned together specifically for the NSX's cabin.

 

[Wavey1]

Right, that's my concern re: the speakers. I'm also not sure if new speakers would actually sound better, assuming the originals aren't cracked or anything. Seems like a high-quality speaker with a done tweeter would have to be an improvement. The secondary signal off the amp would be just like a signal from a normally amplified system, correct? But maybe not given that Bose designed the system for the car.

The other work is done on the car and the battery is back in, so I can do a quick speaker jump (left channel to right door) to verify the head unit is indeed OK before it goes back in. I know I'll be replacing it at some point with Willman's retrofit but I'd prefer to stretch that out if possible.

I had Brian do my AC unit and aspirator fan, and he is great to work with. Thanks for the advice!

 

[jwmelvin]

The secondary signal off the amp would be just like a signal from a normally amplified system, correct? But maybe not given that Bose designed the system for the car.

Amar Bose's whole design philosophy was using a variety of acoustic and electronic methods to balance limitations in the electromechanical system. So I strongly suspect that you would not get good results by replacing only the speaker drivers.