Communications headset tester

[skimask]

This is quite a ways out there for a PIC forum, but at least I'm in the Off-Topic section... so here goes....

I do some work on the side for a company fixing communications headsets. Usually, it's the standard stuff, broken wires at the connector, wiring pulled out, dead microphones, etc. And it's really old technology, but it works.

A couple of months ago, I started to get this different type of headset into the shop that I can't wrap my brain around. It's a military type headset used for ground communications and as far as I can tell, the earpieces are your standard 600ohm speakers, the microphone is a dynamic element (150ohm I think, I can't find the spec's on it). There is also a push-button inline with the cord which is a push-to-talk button. Even that's not difficult.

What's wierd about it is that there is a circuit board inline with this switch case. I can't make out what this board does, amplifier, filter, whatever. It's got 2 transistors (1 2N3906 and the other I think is another 3906 but the numbers are scratched up), 3 capacitors, 4 resistors, and a trimmer pot. I don't think it's an amplifier since that would require an external power source (no batteries inside this case).

About a month ago, I found this tester for these headsets laying in the back corner. Of course it didn't work. I was told by a guy that's been there since WAY back when that basically you plug the headset into it, press the push-to-talk button, blow into the mike and your hear yourself in the speakers. But he also told me that the guy that was there before him built it and he (nor anybody) has any schematic's, specifications, or anything for it. So I opened it up ('cause that what you should do right?).

I found 2 LM386's, a single diode, 2 9V batteries, and a pot. So, knowing what I know, I resoldered the connections, replaced the batteries and tried it out. No good. I drew out the tester schematic and found the LM386's were basically wired in parallel. Well, that doesn't work in my mind, so I switched it over to standard setup for using an LM386 (pin 1 & 8 open, gain=20, + input to the pot wiper, one side of the mike to the pot, 3rd pin of pot to ground, other side of mike to ground, - input to ground, output to headphones, other side of headphones to ground, and +18v to Vd, grnd to ground, etc).

I turned on the tester and got nothing but a squeal in the headphones unless I cranked the pot all the way to one side or the other. And of course, when I blow into the mike, I can't hear myself in the headphones.

At the moment, I'm rewiring it and trying various different configurations for the LM386's, including running them in series for more gain, putting them back in parallel (why I don't know). I'm going to try one side of the mike to one LM386 + input, and the other side of the mike into the the other LM386 - input, with each respective LM386 output drive a side of the earphones. I'll also try configuring a single LM386 for a 200 gain instead of 20 as the datasheets read, and so on and so on.

Question is....does anybody out there have any experience with any of these communications headsets at all, maybe seen something in the past, maybe work on them right now, maybe has a clue what this board inline with the switch might do?

JDG

[mister_e]

is this a 4 pins or 6 pins connector at the end (xlr type)? if so, maybe you could compare wit some Clear-Com headset and unit?

those bellow are much recent...
http://www.clearcom.com/Products/party-line/headsets.html

in the past i repaired tons of white and green model.. but none had the circuit you discribe so far. everything was done with the beltpack.

Is there any model # or company name written on?

For sure you must have a base station somewhere to do some tests. So just probe it to get the Headphone outputs wire, mic, and probably you'll discover a Vdd line as well.

[skimask]

is this a 4 pins or 6 pins connector at the end (xlr type)? if so, maybe you could compare wit some Clear-Com headset and unit?

those bellow are much recent...
http://www.clearcom.com/Products/party-line/headsets.html

in the past i repaired tons of white and green model.. but none had the circuit you discribe so far. everything was done with the beltpack.

Is there any model # or company name written on?

For sure you must have a base station somewhere to do some tests. So just probe it to get the Headphone outputs wire, mic, and probably you'll discover a Vdd line as well.

10 pins total at the connector, only 5 pins used, not an xlr type, in fact not even pins and sockets. They're almost more like a BGA chip and the outside shell of the connector does the job of applying the pressure by latching them together. I wish I had one here at the house so I could get a picture.

In forgot to mention in my last post, on the few headsets that I've gotten in that don't have that circuit I described earlier (and they don't work either!), on those 5 pins at the connector, I've got 1 to the braided shield (ground), 2 to the mike, and 2 to the earphones.

No company name or anything on them anywhere to be found, a couple of model numbers on the various plugs and one on the mike and earpiece. The books say to order a new one from Roanwell, which I called and doesn't sell them anymore (in fact they don't know anything about them).

I've got a functional station to test them out at, problem is it's about 12 miles away which is why I kinda want to rebuild this tester for the shop. I've probed it. Nothing spectacular. That same ground pin on the headsets, goes to the chassis of the box. If I touch 5v across the mike contacts at my station, I get clicks in earpieces at another station. If I touch 5v across the earpiece contacts at my station, I get light clicks in the mike at the other station, which is what I would expect. So, that tells me that I've got the right pins for the right job.

Maybe your beltpack and my switch/box/circuit board might have something in common. Next week, I'll try to reverse engineer that board and see what I come with. That'll probably tell me everything I need to know.

JDG

[mister_e]

Ok,
so i guess it's probably miles away... or not. mmm, all beltpack/base station etc etc, were connected together via 3 pin XLRs. They called that Party-Line.

Try to draw a schematic of your existing tester board. It may reveal some interesting stuff. Same for the transistor board on the headset.

Sure it's doable... If it worked.. it can work again.

Don't give up!

[bcd]

Is it possible the inline box it could be some kind of volume limiter circuit for the headset? I know in European broadcast stations that all the headphones have to be fitted with dB limiters to stop the operators from damaging their hearing.
http://www.canford.co.uk/commerce/item_24-001_2001812.aspx for example.

Most of them use back to back diodes, but if it is military gear they have the money to do things the expensive way...

Bill.

[Luciano]

Hi,

Original headset:

The original headset uses an electret microphone. (Phantom powered).

* * *

Modified headset:

The circuit board is an amplifier for the dynamic microphone.
The circuit emulates an electret microphone.
The circuit is phantom powered.

Best regards,

Luciano

[skimask]

Ok,
so i guess it's probably miles away... or not. mmm, all beltpack/base station etc etc, were connected together via 3 pin XLRs. They called that Party-Line.

Try to draw a schematic of your existing tester board. It may reveal some interesting stuff. Same for the transistor board on the headset.

Sure it's doable... If it worked.. it can work again.

Don't give up!


That's just it. The tester box itself is almost stupidly-simple, just an LM386 amp. Shouldn't be a problem with that right? :) Not so much.... As near as I can figure, the guy that designed/built the tester box is probably around 115 years old by now. Not much of a chance of getting useful info from him.

And the schematics will come next week sometime. It will probably be clear as day as soon as I draw it out.
JDG

[skimask]

Is it possible the inline box it could be some kind of volume limiter circuit for the headset? I know in European broadcast stations that all the headphones have to be fitted with dB limiters to stop the operators from damaging their hearing.
http://www.canford.co.uk/commerce/item_24-001_2001812.aspx for example.

Most of them use back to back diodes, but if it is military gear they have the money to do things the expensive way...

Bill.


Interesting thought. The 2 transistors on the board, while they aren't diodes, can sure be set up to act like it.
JDG

[skimask]

Hi,

Original headset:

The original headset uses an electret microphone. (Phantom powered).

* * *

Modified headset:

The circuit board is an amplifier for the dynamic microphone.
The circuit emulates an electret microphone.
The circuit is phantom powered.

Best regards,

Luciano


Correct me if I'm wrong.
Electret mike - don't need power, just am amp, like the earpiece.
Dynamic mike - Feed power to the mike on one pin, like a pre-bias voltage; connect an amp on the other pin.

I probed the connector without the headset plugged in and didn't get any power on any of the pins (well, a few mV, but it was very jumpy, so I figured it was being induced from somewhere).

How are you guessing that the original configuration might've been electret? Is that 'just the way it's always been' ?


Jeeze...this would be so much easier if I had a datasheet for the things!!!! :)
JDG

[Luciano]

I probed the connector without the headset plugged in and didn't get any power on any of
the pins (well, a few mV, but it was very jumpy, so I figured it was being induced from somewhere).

Did you measure that on the functional station 12 miles away or did
you measure that on the headset tester?

If the station uses an electret microphone then you will have
about 3-9V DC between the microphone line and the microphone GND.


Example of similar circuit:
http://www.tkk.fi/Misc/Electronics/circuits/dynamic_to_electretinput.html

Best regards,

Luciano

EDIT: 3-9V DC with the electret microphone disconnected.
-

[skimask]

Did you measure that on the functional station 12 miles away or did
you measure that on the headset tester?

If the station uses an electret microphone then you will have
about 3-9V DC between the microphone line and the microphone GND.


Example of similar circuit:
http://www.tkk.fi/Misc/Electronics/circuits/dynamic_to_electretinput.html

Best regards,

Luciano

EDIT: 3-9V DC with the electret microphone disconnected.
-



The shop is 12 miles from where they use the headsets. The place where they use the headsets has multiple stations.
The headset tester seemed to be totally dead until I started messing with it :)
And like I said before, now all I can get is some serious squeal in the headsets when I plug them into it (except when I'm at the extremes with the volume pot that's on the tester).

I'm taking notes on all this good stuff for when I go in next week to beat this project up some more!

EDIT:
I just looked at that epanorama site. I remember looking at that site a few years ago, forgot all about it.
Yes, those diagrams (and others in that site under the audio section) look like they might have something to do with what I'm messing with.
My problem with the tester box circuit might be that I'm treating it totally wrong, possibly treating it like it has a dynamic mike instead of an electret mike, which that little circuit might do the 'conversion'.
I should be 'biasing' one input coming (for lack of a better phrase) from the mike with the battery instead of grounding it, and then sending it off to the + input of the LM386.

I'm going to have a good read of those pages, compare notes, etc. Hopefully this thing will work by noon on monday when I get it re-wired.
JDG

[wmarchewka]

Just maybe....

[skimask]

I drew out the schematic for the circuit board inside the push-to-talk switch, and while I probably had a few connections wrong, it looked a lot like a few of those example 'dynamic to electret' converter schematics I saw on .epanorama.

AND...somehow I found an old schematic for a headset tester. The paper is all beat up and barely readable, so I could be wrong on a number of points.

It looks to me like power (18v, 2 9v back-to-back) comes in, goes thru a 1K resistor (and a 1K pot for volume control I assume) and feeds one side of the electret mike (which is actually the output from the dynamic mike run thru the converter circuit mentioned above), then across a small cap, connected to the other side of the mike, which is also grounded.

The 1st output of the mike (nearest to power thru the 1K resistor) is fed directly into the + input of the LM386 thru a coupling cap. The output of the '386 back to the headsets thru a coupling cap. Nothing on the gain pins, '386 negative input to ground, bypass open.

Now that's the way I figured it would run. However the actual tester unit that I have, has 2 LM386's in it (and again, it was ruined by another shop, not me). If I had to guess, I would say that the output from the 1st '386 fed the input of the 2nd '386 and back out to the headsets from there.

Shouldn't the negative input of the '386s be connected to the output or is that just when using op-amps, since the '386 has a gain of 20 with nothing connected to 1 & 8. Or am I wrong again....

JDG

[mister_e]

i guess the guy used 2 LM386 in a bridge configuration to have more Juice at the output. So i guess each LM output go to the headset.

OR one is the mic preamp, the other one the power amp...

download the LM386 datasheet, you'll see what i mean. Maybe it's something copy/paste from there...

[Luciano]

Hi,

If you are interested, I can post a circuit
based on the TBA820M audio amplifier.
You can use this circuit for the headset tester.

IC TBA820M (8-DIP).
Digi-Key Part Number: 497-3025-5-ND
Price: USD 1.35

Circuit description:

Input: Electret microphone. (Phantom powered).
Output: 4 to 600 ohm.

Used components:

One IC TBA820M
One 9V battery
One on-off switch
Six resistors
Seven capacitors

* * *

Let me know and I will post the schematic of the circuit.

Best regards,

Luciano

[skimask]

Hi,

If you are interested, I can post a circuit
based on the TBA820M audio amplifier.
You can use this circuit for the headset tester.

IC TBA820M (8-DIP).
Digi-Key Part Number: 497-3025-5-ND
Price: USD 1.35

Circuit description:

Input: Electret microphone. (Phantom powered).
Output: 4 to 600 ohm.

Used components:

One IC TBA820M
One 9V battery
One on-off switch
Six resistors
Seven capacitors

* * *

Let me know and I will post the schematic of the circuit.

Best regards,

Luciano


I grabbed the datasheet. I'll see what happens.
JDG

[Luciano]

Hi,

Here is the schematic.
You can use this circuit for the headset tester.

IC TBA820M (8-DIP).
Digi-Key Part Number: 497-3025-5-ND
Price: USD 1.35

http://img6.picsplace.to/img6/24/thumbs/TBA820M.JPG (http://img6.picsplace.to/img6/24/TBA820M.JPG)
(Click to enlarge the picture)

Best regards,

Luciano

[mister_e]

in theory it should work... but frankly i don't like the 10uf cap in parallel with the Mic... kind OF BIG BIG low pass filter. Something is bad in it... i'll try editing in MsPaint or... wait wait wait...

like... this
http://www.mister-e.org/Pics/Rev2.JPG

[skimask]

in theory it should work... but frankly i don't like the 10uf cap in parallel with the Mic... kind OF BIG BIG low pass filter. Something is bad in it... i'll try editing in MsPaint or... wait wait wait...

like... this
http://www.mister-e.org/Pics/Rev2.JPG



That's the same 'basic' circuit I had going with the LM386, and I actually myself when I blew into the mike a couple of times. But after those first few noises, I couldn't get it to do it again, thought it might've been flat batteries, no fix, burned up the '386? Nope, tried 3 more....UGHHH!!! Then a little while later, I was able to blow into the mike and get some serious distortion at the earpieces only when I was blowing into the mike. And yet again, after a couple of minutes of thinking I was getting somewhere....nothing.... I've almost given up on trying to get it to work. 5 straight days and a number of half days on it...too much time used up. Time to cut the losses I think...

But on the same note, I did have a stroke of luck. I found a number and called Roanwell Corp. in New York and actually get ahold of one of the guys that actually helped design and make that model of headsets back in the 50-60's. He says he'll call back with spec's for me in a day or two. We'll see how that pans out.

Thanks for the info guys. It's all stored/printed/kept/etc just in case. We'll see what happens in the coming couple of weeks...
JDG

[Luciano]

Hi Steve,

Thank you!

Yes, you are right the 10μF shouldn't be there.

The original circuit was using a 3-wire electret
microphone. A 3-wire electret microphone has a
dedicated connection for the power supply so this
is why this capacitor was there. (In the original
circuit the connection between the voltage divider
and the microphone output was not present).

About fifteen years ago I built two of them and I have
been using them everyday since as home phone intercom.

Best regards,

Luciano

[mister_e]

Good! yup 3 wire electret mic remind me something. So of course it make sense.