Headset wiring query

Thread Starter

Alec_t

Joined Sep 17, 2013
12,309
I've been given two audio headsets to repair, of the same but unidentified brand. On each, the moulded-on 3.5mm TRRS plug was damaged, so I chopped both off, stripped back the insulation on the free cable ends, and tinned the ends.
Each cable has only three 'wires'; one each for the Left and Right signals and a screened one for the Mic.
My DMM shows low Ohms (~16 on one headset, 25 on the other) between the L and R wires and ~1.8k between the Mic and its screen (Gnd). The mystery is that there is a DC open circuit between L and Gnd and between R and Gnd, on both headsets. So how are the L and R circuits completed? Coupling capacitor?
The headset schematic seems to be this:
Headsets.jpg
 

BobTPH

Joined Jun 5, 2013
4,030
Your description is confusing, because you say there are only 3 wires, but you show 4 in your diagram. I assume you mean red and blue unshielded wires plus a coax for the mic.

I would try putting some audio between the red wire and shield and then the same for the blue wire and see if that activates the two earpieces.

Bob
 

Dodgydave

Joined Jun 22, 2012
10,085
So you're measuring between the Red and Blue and you get 16 to 25 Ohms, but when you measure between the Red or Blue to Gnd it's open circuit?
 

Thread Starter

Alec_t

Joined Sep 17, 2013
12,309
I would try putting some audio between the red wire and shield and then the same for the blue wire and see if that activates the two earpieces.
Thanks for the suggestion. Just tried that, with no luck. I suspect that the return connection to ground inside the headset, for both L and R earpieces, has broken on both headsets. That would be a coincidence but could be explained by the cables having been wrenched on both (which I know is what caused the plug damage).
 

Marcus2012

Joined Feb 22, 2015
424
If it's using the shielding for the return path for all three components could that shielding have broken between the mic and speakers?

Edit: just caught the previous post mb
 
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Audioguru again

Joined Oct 21, 2019
3,828
You are assuming that the earphones are stereo. But stereo headphones are separate, not joined.
Therefore the two earphones are connected in series and produce mono not stereo.
 

Thread Starter

Alec_t

Joined Sep 17, 2013
12,309
You are assuming that the earphones are stereo.
I am. They're gaming headsets (now identified as Logitech brand) and the earphones are clearly labelled L and R. I'm assuming that each earphone winding is intended to connect between its respective L or R terminal and Gnd, so inside the set they might well go to a common Gnd point and from there via a single conductor in the cable to the plug. I suspect the connection from that common point to that conductor is now missing. If each earphone still connects to that common point then the two stereo windings are indeed in series as measured externally.
 

Thread Starter

Alec_t

Joined Sep 17, 2013
12,309
I lifted the bonnet and found I'd transposed the L and R wire colours in the cable. I can now see L is red and R is blue.

IMG_1677a.jpg
IMG_1678a.jpg
But the mystery deepens......
1) L and R inside the headset each have a screened wire. Their terminations at the ciruit board are labelled L+, L-, R+, R-. The two screens (L-, R-) are commoned (as I expected) but don't connect to the Mic screen (which I didn't expect).
2) Including the screens there are six wires coming from the circuit board and going into the headset-end of the cable, but only four wires emerging from the plug-end of the cable (the whiskery bits around the red and blue wires are non-metallic fibres)! Magic!
I'm tempted to short the L/R input commoned screens to the outgoing Mic's screen to get things working. Any likely problem?
 
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Marcus2012

Joined Feb 22, 2015
424
Could they have connected them inside the cable? Is there any inline moulding at all in the cable, for mute switch or anything? (or is that what I see in the PCB?) I suppose the only way to be sure is to desolder the cable from the PCB and do clean continuity checks on it.
 

Marcus2012

Joined Feb 22, 2015
424
Oh this is probably nothing but I just noticed that the left most cable in the PCB picture (the Black one) is marked as R+ on the PCB and the Blue as R-.
 

Thread Starter

Alec_t

Joined Sep 17, 2013
12,309
Could they have connected them inside the cable?
I think they must have, but now there's no connection.
Is there any inline moulding at all in the cable
Not that I can see.
The sleeve of the TRRS plug must have all the ground-shields connected together on it.
The plug had no sleeve. It was a bare minimum with a single ground connection to the second ring (standard PC pinout). I'll get a replacement plug with a sleeve, to provide strain relief.
(the Black one) is marked as R+ on the PCB and the Blue as R-.
Thanks. I hadn't spotted that. Not sure of the significance.
 

Thread Starter

Alec_t

Joined Sep 17, 2013
12,309
Update. I soldered in a link between the commoned L/R screens and the Mic's Gnd screen and the headset earphones now work fine. New plugs on order.
I still haven't fathomed out why the cable has 6 wires at one end but only 4 at the other!
 

Marcus2012

Joined Feb 22, 2015
424
I still haven't fathomed out why the cable has 6 wires at one end but only 4 at the other!
Glad it's all sorted. But yeah I'm also lost as to why they did that. Is it 6-core or 4 -core? if 6 why spend the money, if it's 4 where do the spare 2 connect. So many questions for logitech here, but mainly why lol.
 

Thread Starter

Alec_t

Joined Sep 17, 2013
12,309
Is it 6-core or 4 -core?
6 at the headset end, 4 at the plug end !! Check the pics.
Maybe two cores at the plug end originally connected to some semiconductive screening layer on the inside of the cable sheath, but my DMM doesn't show the sheath having any significant conductance to act as the signal return for the earphones. Those seemingly non-metallic wispy fibres around the cores at the plug end don't have a low enough resistance either.

Edit
For anyone doing repairs on Logitech headsets, note that each core in the cable comprises 7(?) thin copper strands individually insulated with a varnish layer. This varnish needs to be removed, by a vey hot iron or flame, before the strands can be tinned.
 
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