Dolby Pro-Logic; only central loudspeaker working Topic is solved

[Radler]

Thanks Neil! I was researching a bit today on the internet and I think I found a couple of intermediate solutions to test the system:

1) Get a DIN-to-RCA adapter to test the amplifier. This way I can feed the amp with any external source like a mobile phone connected through the RCA. I might even be able to perform this check without the adapter, since with the amp open I could directly tap into the +/- pins of a given loudspeaker to send a signal (3rd pic in my first post, grey connector).

2) Connect one loudspeaker directly to the head unit using the DIN output with some wires. I just need to make sure I use a pair of positive and negative pins.

I will also test the DIN cable for continuity since it's a quick check. Not sure I will have time to do this until after Christmas.

[Radler]

Small update: the DIN-jack adapter I bought sadly does not fit the DIN connector at the amplifier, so could not test it that way. But I did check the following:

• At the amplifier end of the DIN cable I could only measure voltage at the central pin, which according to the image I included in my initial post it corresponds to "Remote 12V".

• As I know audio signal can not always be measured with a normal multimeter, I tried connecting a loudspeaker directly to a pair of +/- pins of the DIN cable: no sound.

• I then proceeded to do the same directly on the back of the radio (see image below), to determine if the cable could be the problem. Still I did not get any sound at the test loudspeaker.

From this I infer that the radio is not sending any signal to the amplifier. I will try to get another radio from a V70 I and try and see if it makes any difference.

[scot850]

Remember what I said that the SC900/901 has no internal amplifier. All other SC-XXX unit do. Over here they are pretty cheap to buy from junkyards, so maybe worth considering buying a 'test' radio like an SC-815/6 unit and see how that works as they have inbuilt amplifiers so you can test your radio.

Neil.

[Radler]

Remember what I said that the SC900/901 has no internal amplifier. All other SC-XXX unit do. Over here they are pretty cheap to buy from junkyards, so maybe worth considering buying a 'test' radio like an SC-815/6 unit and see how that works as they have inbuilt amplifiers so you can test your radio.

Neil.

Still, the signal sent by the head unit should be enough to power a simple loudspeaker, right? If I get a test unit with built-in amplifier and I get sound on all speakers (except for the central Dolby) I will still not know if the problem in my case is the head unit or the amp.

SC-900/901's are selling here at around 100-150€.

[scot850]

Fair comment on the price of the replacement unit. I have bought a few SC-900/1 units over the years on Facebook Market Place as the sellers bought them and then found they got no sound when connected up. Zero sound! Naturally, they did not know about the amplifier and additional cables. So having not pulled those they were not available (usually as I had pulled them !). So I got the units for next to nothing for parts, and all of them worked perfectly when connected up with an amp and cable.

I did tell them about the issue and offer to sell them the amp and cable for silly money, but none wanted the hassle to install them.

Neil.

[Radler]

Small update: I got an SC-900 head unit and an amplifier from a scrapyard and I still only get sound from the loudspeaker on the dash. The small connector on the amplifier does not fit the (green) plug in my car, but it also has two pins, so I simply connected them using a pair of wires. I think it should have worked.

I must admit I am a bit lost at this moment.

[scot850]

I would suggest trying to find any cheap Volvo radio SC-805 or SC-815/6 models. Anything with an inbuilt amplifier.

Does your car still have the original connectors undamaged for a radio without an amplifier? They are usually connected behind the radio and covered with foam to stop them rattling. If you do have those connectors undamaged, disconnect the add on harness for the amplifier and see if a more base head unit runs the speakers. If it does not, it is possible someone has some how damaged all the speakers. They do fail with age, but it is not normal for all to fail.

To do a basic speaker test to see if it might be ok, disconnect the connector to it and then use a 9V battery and 2 wires and connect to the 2 connectors on the speaker and see if the cone sucks in or moves. If it does and the cone is not separated then it is a good chance that speaker is ok. Then at least you have a chance most of them are ok, and the issue is elsewhere.

Neil.

[Radler]

I would suggest trying to find any cheap Volvo radio SC-805 or SC-815/6 models. Anything with an inbuilt amplifier.

Does your car still have the original connectors undamaged for a radio without an amplifier? They are usually connected behind the radio and covered with foam to stop them rattling. If you do have those connectors undamaged, disconnect the add on harness for the amplifier and see if a more base head unit runs the speakers. If it does not, it is possible someone has some how damaged all the speakers. They do fail with age, but it is not normal for all to fail.

Today I gave it another try. I took out part of the central panel and found out that there was installed some kind of system to turn of all loudspeaker when getting a call on the phone, except for the two front ones that would be use on the call. I assume we would be talking about a physical phone on the car, which used to bu common in Germany and would explain three holes I saw on the passenger side of the central panel (phone were usually attached that way).

Never mind; I uninstalled the system an reconnected the wires that were cut during the installation. Theoretically now everything is connected as it should, but I still get no sound on the loudspeakers, except for the one on the dash.

I noticed the original connectors for non-Dolby head unit, wrapped in foam as you mention, are actually in use and not free as I expected. The loom connected to the big plug goes somewhere towards the foot well, but the one connected to the smaller plug is connected to something near the steering wheel (last picture bellow). I could not see what it is and could only take a pretty bad photo. Someone mentioned to my that it may be the Dolby Processor, but that they are unlikely to go bad (which in my case it usually is a tell-tail sign of the problem; I always seem to get the weird cases).

Here some pictures I took today.

[scot850]

Sorry, I can't access the photos. You can just disconnect the Dolby processor from the back of the head unit. That cable is just a signal in/out. Without it connected, the system will worked ignoring the center speaker and Dolby unit.

Neil.

[scot850]

Just been testing a SC-900 head unit, amp, cable and Dolby unit for my buddy that he pulled. Good job he asked me to look at it as first dealing with Volvo is painful trying to figure out the code. Luckily the parts guy who helps me with parts issues and identifying is very persistent. Through his own hunting and the car Vin, he was able to find information on the correct security code number!

Radio unit powered up and the cassette left in it played ok. However the CD unit would not load a CD. It tried but then threw an error code.

Having the test system helps here as I removed the top cover of the head unit and after ejecting the 3-CD cassette I could see a CD in the unit. I tried re-installing the CD cassette to see what was happening. Sure enough it could not eject. I killed the power with the CD in unload position and was able to manually push the CD out but it seemed harder than it should be ejecting the CD cassette I found out why!
There were 2! CD's stuck in the unit!! Removing the cassette again I could separate them, and then get access to clean the laser.

Laser cleaned and now it seems to work ok again. I'll now clean all the items and get them ready for re-testing and then my buddy can take them to put in his car. I believe the Dolby head unit does make a fuller sound, even without the center speaker.

In your 4th picture that is the connections for the 2 cables that run to the amp. If you disconnect those the 2 other connectors to the rear of a lower end head unit, even the one without a CD player, you should then get sound to the speakers. That should hopefully remove that nightmare of cables. Unless you really want those things, I'd remove them all, protect the cable damage and try the unit again.

Now you mention the unit for cutting the sound for a phone, I believe that is why you get no speaker sound. When the phone systems cut the sound for the music, the sound only comes out the center speaker, that is why you are only getting center speaker sound. Removing that stuff I believe will restore your door and dash speakers.

Good luck! If your cable is really bad, PM me and I can see if I have an extra I can ship you. I have repaired a cable where someone had cut the whole cable off. It works fine now.

Good Luck!

Neil.