Infinity Basslink constant rumble

IDiggity

Member
2013-01-30 10:46 pm
Hi all,

I'm hoping some of you seasoned veterans on this forum have some ideas on what ails this blasted Infinity Basslink (I). The long and the short of it is that when I turn on my car, the Basslink just "hums" or "rumbles" as though it's receiving a constant bass signal, even with the RCAs unplugged. I can make it stop or start doing that again by striking the casing of the hard enclosure, or if I plug in the RCAs it will try to respond to the music but is drowned out by the constant bass sound. The power LED comes on and stays on, but the servo indicator LED never lights. I've taken it apart, gotten my multimeter out and even fixed a few potential problem components, but it still does it.

Help???

----------------------------------------------------------
For those interested, here's how it got to this point:

The Basslink was installed professionally in a previous vehicle and worked fine for a time. At some point, it started behaving erratically. When the car turned on and the Basslink's integrated amp received the remote turn on signal, about half the time the Basslink would just start blasting as though it was receiving a constant bass signal. It would stop after a few seconds, after which it wouldn't thump to the music, either. It could be remedied by knocking/hitting/rapping on the casing of the subwoofer, after which it would work normally usually until it powered off again. This was a few years back and we no longer have that car, so I didn't think about it for a long time.

Recently, I got interested in hooking it up again. Call it nostalgia. My car has a stock 8" subwoofer, so I used a line out converter to tap into the speaker level inputs to that. Power and remote run down one side of the car, totally separate from audio lines. The unit is grounded to a stock bolt underneath the trunk lining which was used to ground some pre-existing stock component of my car.

So I hooked everything up and it still did the constant on thing. Somewhere in some forum I found someone suggesting that the gain knob was the culprit, and that pushing/pulling on it would modulate the effect. BINGO. It did.

So I took apart the unit, desoldered the existing knob and soldered on a new OEM one from the manufacturer. After I hooked it back up, it still did the constant-bass thing, but was unaffected by the gain knob. I could still make it stop/start by hitting the unit.

So I took it apart again. I noticed on the amp's circuit board that the copper trace that connects to the ground post had been BURNED to the point of having a very tenuous connection. I fixed that by soldering a short piece of stranded copper wire onto the trace and verified that the resistance across the board (from screw to post) on that trace was roughly zero (0.75 Ohms).

I hooked the unit back up, proud of my work and convinced that the broken trace had to have been causing a ground issue. BRRRRRRRRRRRRRRRRRMP! No change in behavior.

I called around and there are no authorized Infinity repairers that work on car audio in my area. :(

Any ideas what to do next? (I know the practical answer is to toss out the Basslink and get a REAL system, but I like fixing things like this because I learn so much in the process.)
 
Last edited:

IDiggity

Member
2013-01-30 10:46 pm
You mentioned a 'servo'. That's generally used to provide feedback from the speaker to the amplifier.It's possible that that circuit is damaged or there is a loose connection on the speaker or feedback device.

I've been looking for it, but not knowing what I'm looking for, I'm not spotting any other problems on the circuit board.

If it helps anyone, here's the service manual for my unit, including the circuit diagrams and PCB layout, etc: Dropbox
 
I have the same document you have.
I have worked on a few of these also and found repeatedly the lower rail supply diodes would fail. This was a very common issue on several I repaired. So Perry is very correct in asking you to check those lower rails first..... I think I still have a complete module around here some where:)
 
Last edited:

Satyr

Member
2013-01-28 3:16 pm
Chicago
Hey there-
I don't know if this helps or not- but we had the exact same issue with a bazooka tube with built in amp. It was configured during install to "sense" the signal from the deck's line level or rca outputs and turn on automatically. (In our case- it was hooked up to the rca's.) The place that did the install didn't hook up the remote turn-on lead, even though the rca's had one in it. So, when the deck was powered off and the car shut off- the amp in the bazooka would stay powered. After a short while boooooooooooom. It was like some sort of weird ground loop reverberation thing. The fix? If the bazooka is hooked up to the remote turn on lead, it shuts down with the deck when the deck is turned off, and since the the amp in the bazooka isn't powered- no boom. I figured this out by experimenting with it- but we still need to hook up the remote turn on lead. Right now- we just have the bazooka disconnected. Hope this helps.
 

IDiggity

Member
2013-01-30 10:46 pm
I have the same document you have.
I have worked on a few of these also and found repeatedly the lower rail supply diodes would fail. This was a very common issue on several I repaired. So Perry is very correct in asking you to check those lower rails first..... I think I still have a complete module around here some where:)

Very interesting.....

I'll admit that I don't know enough about integrated circuits to know exactly what you're talking about when you say "lower rails." I vaguely remember hating problem sets with op-amp problems when I had to take circuits back in college. :mad:

Can I test this with the unit powered off with just my DMM? I'll look at the diagrams and try to make sense of what you say, but in the probable outcome that I can't figure it out, would you mind posting a pic?

Many thanks to all for thinking about this.
 
D108 and D109 according to my documents they attach to the base terminals of the pass transistors Q118 is2SB649A and Q117 is 2SD669A. look at the lower right hand corner of the print in your document. They are located right there on the print. these are your lower rail regulators circuits.
Perry wanted you to verify the +&-15 volts DC out of these two regulators. If its not there then ohm out the zener diodes they tend to short out by my experience on several of these....hope this helps some...
 

IDiggity

Member
2013-01-30 10:46 pm
D108 and D109 according to my documents they attach to the base terminals of the pass transistors Q118 is2SB649A and Q117 is 2SD669A. look at the lower right hand corner of the print in your document. They are located right there on the print. these are your lower rail regulators circuits.
Perry wanted you to verify the +&-15 volts DC out of these two regulators. If its not there then ohm out the zener diodes they tend to short out by my experience on several of these....hope this helps some...

Interesting... all these elements are actually on a smaller board that connects to the main board by a set of 14 pins and a set of 10 pins plus a whole bunch of goopy glue.

Ohming out the diodes is frustrating with my DMM because it will start out at a random number for resistance, sometimes negative, and count up incrementally until it hones in on a number. It seemed like the DMM would keep counting up indefinitely in some cases - not sure what to make of that.

That said, only some of the diodes on the board would measure a resistance in both directions whereas some would show the intended ~infinite resistance in the "wrong" direction.

D108, D109, D112, D114, and D159 allowed measurement of resistance in the "wrong" direction. :/

D111 and D113, on the other hand, immediately showed infinite resistance in the "wrong" direction.
 

IDiggity

Member
2013-01-30 10:46 pm
This may not be easy to diagnose without power applied.

What sort of goop is on the board?

Is the noise a constant tone or an intermittent/random sort of a noise?

Some sort of yellow-ish glue, old and hardened. :whacko:

The noise is a constant and loud tone, as though the unit is receiving a constant signal. It does attenuate/chirp to the beat and starts making some interesting noises if the RCAs are plugged in.

I have access to any number of oscilloscopes at work - not sure about a DC power source...
 
Interesting... all these elements are actually on a smaller board that connects to the main board by a set of 14 pins and a set of 10 pins plus a whole bunch of goopy glue.

Ohming out the diodes is frustrating with my DMM because it will start out at a random number for resistance, sometimes negative, and count up incrementally until it hones in on a number. It seemed like the DMM would keep counting up indefinitely in some cases - not sure what to make of that.

That said, only some of the diodes on the board would measure a resistance in both directions whereas some would show the intended ~infinite resistance in the "wrong" direction.

D108, D109, D112, D114, and D159 allowed measurement of resistance in the "wrong" direction. :/

D111 and D113, on the other hand, immediately showed infinite resistance in the "wrong" direction.

The diodes in question will most likely be shorted and read a dead short on your meter or something near zero ohms. Bouncing readings are normal when there are capacitors near by charging and discharging as you reverse the probes. On dead shorted diodes they will read dead short or some very low readings like 20 ohms or less, usually zero in most cases. On the rare situation where the diodes go open they also sometimes are broken or exploded on their case visibly. You might want to test with live voltage each pass transistors output to see of +&- 15 volts are being delivered like Perry suggested if the readings are causing confusion.

And perry is correct that fixative they use should be removed carefully with dental tools. It tends to harden and turn colors as its useful lifespan is used up. It can in some cases cause problems like shorts it it carbonizes too badly.
 

IDiggity

Member
2013-01-30 10:46 pm
Thanks, all, for the replies.

I'm told I might have access to a suitable power supply at work. I'll see if I can test voltages in a live power setup.

In the meantime, I thought you all might get a kick out of the response from tech support:

TechSupport said:
Before I refer you to a service center, being that you already replaced the volume pot and you obviously know what you are doing, let me make a suggestions. Judging by what you have written, it almost sounds like the problem is a bad ground or cold solder joint. Please try tightening everything inside the amp, and go over all solder terminals to let the solder flow again. This may fix your problem.

Me said:
I am actually quite a novice at soldering on PCB boards. Do I understand you correctly that I should re-flow every solder joint on the circuit boards, or are there specific joints I should be looking for?

I checked the resistance on the internal trace that connects with the exterior ground connection - after my fix job, the resistance is less than 1 Ohm (about 0.75 Ohm).


A car audio expert on an online forum suggested that the diodes tend to short out in these units. Could that possibly be the cause?

TechSupport said:
Yes, a good idea is to let the solder flow on every solder joint. Of course you don’t want two joints to flow together unless they were already that way. While I have not heard of a diode issue on these amps, to be honest, anything is possible. But the two suggestions I made is always a great starting point when you have a problem like the one you have described.

There's gotta be maybe hundreds of solder joints on multiple boards that are soldered and glued together. :crazy:
 

IDiggity

Member
2013-01-30 10:46 pm
Update: I've poked around enough now to find out that I have access to a bunch of benchtop electrical equipment at work.

The easiest items to access:
HP 54600A oscilloscope
RSR FG-32 sweep generator
Tenma 72-2085 DC power supply (up to 24V, up to 6A)

Will the above power supply provide enough current to do the kind of testing I will likely need to do? I'm sure something that can provide the amps that a car battery can would be better, but I'm just looking for "sufficient for testing" here.

Experts please weigh in. Many thanks.