Communications headset tester

[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


(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

[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

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