OEM headunit fader bypass- electrical gurus needed!

[afterthisnap]

The left side sound cut out on the oem 1990 tape/aux head unit. Jiggling the fader control worked for a while to restore full sound, but now the left side is totally dead.

The front/back fade setting works fine.

Treble/Bass seems to be functional.

 

I've read that you can bypass the fader/potentiometer by replacing it with resistors rated at half resistance of the fader/potentiometer.

 

I opened up the unit and the fader connections are a bit more complex than I anticipated. Here are the pics:

 

I think the FUBAR condition is in the 1,2,3 connections.

 

Checking resistance with all settings in the middle, I get

1-3: 45 ohms

1-2: infinite

2-3: 70 ohms

 

With the left/right fader turned all the way clockwise, I get

1-3: 45 ohms

1-2: 60 ohms

2-3: 5 ohms

 

With the left/right fader turned all the way counterclockwise I get

1-3: 45 ohms

1-2: 7 ohms

2-3: 55 ohms

 

 

I'm not entirely sure what to make of this, as when the fader is turned full counterclockwise (right volume cutout), I don't get any sound from the left speakers. I don't get any sound from the left speakers in any setting anymore.

 

I know it's easy enough to put in an aftermarket unit, but this is a quest of curiosity to see if I can rig a fix.

I don't need fade/balance, so I thought perhaps a 20 ohm resistor from 2-1 and 2-3 would bypass the fader and give me full left/right sound again. Please let me know if I'm missing something as circuitry is not exactly my forte.

Thanks!

[afterthisnap]

Apparently I don't know how to use a multimeter...

All these values were taken at a 10x setting. At 1000x I get different values but they are relatively the same.

Also, I get different values if I test the same connections but switch the black/red probe of the multimeter.

[idosubaru]

it's most common for the pots to simply be bad. sometimes you can get away with cleaning them, i've done it before though never to an automotive stereo but still the same thing.

 

does the pot look accessible/cleaning?

[kanurys]

Just putting a resistor across 1-2 and 2-3 will be putting it in parallel with the potentiometer. You would first need to remove the old pot. It is more common to have resistances in that higher range than lower stuff because it reduces the noise floor in a circuit. Anyways, there are two likely pin-outs.

 

Voltage divider 1:

1 - ground or hot

2 - reference (voltage control signal sent back to the head unit)

3 - hot or ground (opposite of pin 1)

 

OR

 

Voltage divider 2:

1 - right voltage control signal

2 - hot

3 - left voltage control signal

 

Either way, when set to "center", 1-2 and 2-3 should read the same value, give or take probably a small percentage of error. This is not necessarily true while the part is soldered to the board. As you turn the pot slowly, the values should move in a trend, not jump around a lot.

 

The most accurate way to test the pot is to remove it with a solder sucker ($10 at radioshack and $0 how-to on google) or solder wick. Then do your test. If it is bad (which it appears like it is) search for a replacement online and solder it back in. Don't run your iron too hot or touch it to the pins too long, as you can damage the component.

 

You could also hook up a bread board to the board and test with a variety - but related to the original resistor - of values or just get a working 50k pot from radioshack to play with. Then you would know that the problem is isolated to the component instead of somewhere else in the head unit.

 

I hope this helps. Have fun, and remember to have everything you need to make a good sandwich on hand.

Edited July 8, 2012 by kanurys

[kanurys]

As GrossGary said, you might try cleaning the pot, first. DeoxIT is a good product for this purpose.

[hankosolder2]

Your measurements seem logically inconsistent.

 

IF the 1 2 3 terminals are indeed the balance pot, it is a single section device. As such, the "wiper" (middle terminal) should be connected to ground. There are fixed series resistors in the circuit which provide the attenuation, and which ever end of the pot is 'closer' to ground loads down the signal in that channel more to provide a balance effect. IF this is the style circuit you have, you can defeat it by disconnecting all the terminals of the balance pot.

 

I'm wondering if you have misidentified which section of the pot is for the balance, or if the problem is not even in the balance pot. If the volume knob is concentric with the balance, sections 456 and 789 are probably the volume sections, one group for left, one group for right. I'm wondering if you might actually have a faulty volume pot and when you're fiddling with the balance pot, you were flexing the connections to the volume pot. Sometimes the elements in pots will crack or the riveted connections to the solder terminals will loosen. You should be able to perform resistance checks on the volume sections to verify their integrity. The tests will work better if you do it out of circuit.

 

Finally, finding replacement parts for this kind of stuff is EXTREMELY difficult. A dual (stereo) volume pot + a concentric balance pot is fairly custom. None of the standard generic parts suppliers carry stuff like that.

 

 

Nathan

[afterthisnap]

Thanks, guys.

 

Cleaning the pots didn't help, and I'm pretty sure the innards of the pots are mangled based on how crunchy they now feel.

 

I'll get some more pics up tonight.

I should have mentioned that connections 1 through 9 are connected to the balance/fade pot. You pull the knob out (up in the pic) to adjust balance, push in (down) to adjust fade.

 

10-15 are connected to the treble/bass pot.

 

Nathan-

I've only really used multimeters to test car parts and batteries. It's entirely likely that I'm using it wrong to test circuit boards.

The volume control knob is on a separate board but is connected to the one it the photos via a 4 wire connection. The volume wires connect via the 4 vertical connections at the bottom of the first pic.

 

Replacement pots have been impossible to find (unless I order 800 or more).

 

Thanks!

[afterthisnap]

Also, the #2 connection does not have continuity with the steel case of the radio, which I assume would also act as ground.

I should have a better multimeter available to me tomorrow. I'll post up some better readings.

[afterthisnap]

Here are some more pics if it helps clarify anything:

 

[afterthisnap]

New fancy autorange multimeter says:

 

Middle Setting

1-3: 9.6 kOhms

1-2: 12.4 kOhms

2-3: 12.53 kOhms

 

Clockwise setting:

1-3: 9.6 kOhms

1-2: 9.61 kOhms

2-3: 9.3 Ohms (no kilo)

 

 

Counterclockwise:

9.6 kOhms

7.6 Ohms

9.62 kOhms

 

All these were taken with the pot on the board. I'm about to take it off now.

 

4-6: 10.2 kOhm

4-5: 13.6 kOhm

5-6: 13.6 KOhm

 

7-9 had identical values

 

Thanks.

 

Edit:

Desoldering pumps are cool!

 

Off the board I get:

Middle setting:

1-3: 39.7 kOhm

1-2: 19.9 kOhm

2-3: 20.0 kOhm

 

Clockwise:

1-3: 39.7 kOhms

1-2: 39.7 kOhms

2-3: 10.4 Ohms

 

Conterclockwise:

 

1-3: 39.7 kOhms

1-2: 7.7 Ohms

2-3: 39.7 kOhms

 

 

I'm wondering if the heating from desoldering did anything to normalize the middle setting. Regardless, I'm still into the idea of doing the bypass. It looks like the pot+/- resistance for 1-3, 4-6, and 7-9 are all about 40 kOhms.

 

Will six 20 KOhm resistors do the trick?

Edited July 9, 2012 by afterthisnap

[kanurys]

I like my solder sucker. I'm looking for an old fridge compressor to convert into a power sucker:Flame:

 

Since there are several controls on one knob, you might want to find a way to test the unit with several resistor values on a bread board before soldering your final setup. I'm sure you could find close single resistor values to the stock numbers, but several in combo would get the job done, too. You could even make your own balance knob if you feel creative. Anyways, if you do any mods, make sure you disconnect/bend/clip the pins on the original knob so that portion of the multi-function knob is disconnected.

 

I glanced on Google for the circuit schematicvfor that radio. No luck. Does anyone have a resource for this?

[afterthisnap]

Update:

 

 

DID NOT WORK.

 

I had half sound for a few seconds before everything went dead. Luckily, I found a replacement unit online and now everything sounds great!

[kanurys]

Correct me if I'm wrong (I've been up late doing research) but it looks like you put the resistors in the wrong spots. They should only have involved connecting "1, 2, and 3." It looks like they're partially on 1 and 3 and on the other pins, as well.

 

Since it reacted strangely when you turned it on and then went dead, something might have fried. I appreciate your persistence, though. I'm going to get some sleep and look at it with fresh eye tomorrow...