Talkback system malfunction

Talkback on my console has EXTREMELY low gain. It used to be fine, and could be heard OK despite the number of busses, auxes it was routed to.

Now, with no other audio being played, I can just barely hear when I tap directly on the mic. I have an oscilloscope (newly acquired), and can see a 1K sine wave arriving at the opamp when I inject it at the mic leads. I probed the other pins on the opampm and don't see the signal on any other pins. I think my next step is to verify that the opamp has V+ and V-. If it does, can I assume that the opamp is dead?

As you can tell I'm pretty new at this. I do have a schematic of the master section and the talkback circuitry is outlined on the PCB. All devices are clearly markled on the PCB as well, so if I can just remain logical in my approach, I should be able to find this.

> can I assume that the opamp is dead?

Maybe its output is shorted. Or the wire to the TB gain control broke. Or a cap dried up.

If the chip is socketed, pull it out, inject signal to the opamp's output pin-hole. If that squeals all your TB outputs, check opamp power pins, then replace the chip. There are logical ways to go about this, but since the total TB system cost is about $1.50, and there is no "vintage sound" that needs preservation, blind wholesale parts replacement is a sane option.

Welcome to the lab, Larry.

You don't say what type of console; -that might help. Other pertinent information might be what type of mic is it? -Electret is a decent assumption, though it would be nice to have that info. (nothing stops stabbing round in the dark quicker than turning a light on! :wink: )

I've just had two Amek 9098i's where the mic itself died (in one case) and went low-output (in the other case). Luckily on the 9098i's, the talkback mics are plugged in using on XLRs on the meterbridge, so I was able to quickly track it down without so much as a screwdriver being wielded.

If it's an electret in a vulnerable position such as an SSL (in a place where ash, dust and curious pencil-erasers in the hands of idiot inquisitive producers can penetrate) then you might want to consider that possibility. Local surplus stores have good replacement electrets for about $1, or you can buy panasonics for under $5, essentially the same as used in the Earthworks mics that sell for much more than the price of the capsule might suggest, for what it's worth.

Keith

Thanks, for the pointers!

PRR: I'll probe around a bit more, now that I've got the pinouts for the opamp, it's a thru hole type - no socket, but I guess it'd be OK to just inject onto the output pin itself, right?


SSLTECH: It's a DDA DMR 12.
I'll look for specs on the mic, but it's got 2 leads on it, so if electret, I should see a voltage source to power it, correct?

I should see a voltage source to power it, correct?

Click to expand...

Yes. welcome to the lab. you will find we are friendly folk around here and dam right smart some of us. ME learning right nicely from being around here. but welcome.

The signal might be superimposed on the Voltage powering the mic... If you see more than 1.5V DC at the signal out of the mic, it'll almost certainly be an electret.

Keith

yep

wot the others said ... welcome to the LAB

Signal flow is from preamp to Push to talk and from there to a matrix of parallelled switches which route signal to the auxes, buses and the mix. I can probe any of those switches and there's no signal.


So far I've seen signal on the inputs and output of the TL071 and it's being amplified (had to change v/div setting to keep trace on screen).

Signal appears at the push to talk switch, and appears on additional pins when switch is pushed, HOWEVER, I can't find signal on any of the switches in the routing matrix, so I'm thinking I've got a bad joint on the output of the Push to talk switch.


Sound reasonable?

[quote author="Larry Sheehan"]Signal appears at the push to talk switch, and appears on additional pins when switch is pushed, HOWEVER, I can't find signal on any of the switches in the routing matrix, so I'm thinking I've got a bad joint on the output of the Push to talk switch. [/quote]

into switch good
out of switch good
into switches in the routing matrix - bad

it there any active ecectronic between push to talk and the routing matrix ?

No devices between the push to talk and the matrix. The matrix switches are paralleled and none of them read a singal arriving. I'm convincing myself as I type.

I think I'll try a jumper from the PTT Sw's output to an input on the matrix switches and see what I find out.

UPDATE: It was a broken trace from the switch to the matrix. I also found that somewhere along the diagnostic trail I had loosened a punchdown on the connector from the mic&VU Meters to the card.

ALSO discovered that my PCB soldering skills are pretty bad. I was thinking of building some inexpensive kits (maybe a function generator) and then something more involved.

Any recommendations?