Neotek Talkback "Pop"

[idylldon]

I've got a Neotek Series II that I've been using for years without needing the talkback but I now find it would be handy if I could get it working right.  It works, but when I release the TB button I get a rather irritating "pop" in the signal wherever it is routed (headphones, in-studio monitors, slate).  I've done the mod that you see in one of the attachments and have also recapped the whole board, including the talkback section.  I've also replaced Q105 (2N5638) and it's still there.  I've attached the schematics.

Anyone have any ideas on how to minimize or eliminate this problem?

Thanks,
--
Don

 

[idylldon]

The other schematic.

 

[fazeka]

Don,

Saw your post on the Neotek list.

I'm thinking a cap would help tame the pop but I'm hoping someone else can chime in as I'm merely a hack... 

I have a Series 1E that has a very slight pop but I don't find it to be a problem.

Chris

 

[sodderboy]

I would want to check the FET Q105.  Popping is usually a leaky FET.
Is there a pop if you TB to slate rather than to cue?  That answer will help narrow the location of a bad FET.
Mike
PS: Does that console have the provision for a remote TB pedal connection?  Somewhere on one of the ELCO's?

 

[idylldon]

I would want to check the FET Q105.  Popping is usually a leaky FET.

I've already replaced that FET and there wasn't any change.

Is there a pop if you TB to slate rather than to cue?

Yes.

PS: Does that console have the provision for a remote TB pedal connection?  Somewhere on one of the ELCO's?

No Elcos on this console and there isn't a connection on the punchdown blocks for one.

Thanks,
--
Don

 

[Brian Roth]

You might take a look at the voltage ramp up/down on the gate of the FET with an analog meter or a scope.

Bri

 

[idylldon]

Brian,

It swings from 17 with the TB switch open to 0 when it's closed.

Cheers,
--
Don

 

[Brian Roth]

What I meant to say was to look at the voltage and see if it has a "ramping" of the control voltage, up and down, versus any abrupt jumps.  That is where the scope could be more helpful vs. a meter.

Bri

 

[idylldon]

I didn't see any fluctuations in the voltage with the scope.  It was dead steady.

Cheers,
--
Don

 

[JohnRoberts]

Divide and conquer...

What happens if you temporarily shunt a few circuit nodes in that circuit to ground , or through a large cap. Of course the talk back audio doesn't pass, but do you still get pop...

The gate voltage is changing a bunch but the pop could be coming through one of several paths.

Find the pop, kill the pop...


JR

 

[Brian Roth]

I didn't see any fluctuations in the voltage with the scope.  It was dead steady.

Cheers,
--
Don

So, when TB is activated/deactivated do you see the gate voltage ramping up/down or does it jump abruptly up or down?  You'll have to set your scope's sweep to a pretty slow rate in order to see how the transitions look.  I would think you should see what looks like a capacitor charging/discharging on the gate, and NOT any abrupt changes.

I was just wanting to eliminate the gate drive as a possible trouble source.

Bri

 

[idylldon]

So, when TB is activated/deactivated do you see the gate voltage ramping up/down or does it jump abruptly up or down?

Thanks for hanging in there, Brian! 

The voltage jumps abruptly.  It doesn't ramp up or down. 

Cheers,
--
Don

 

[idylldon]

OK, file this under stuff I should have checked but took for granted someone before me did it the right way. 

As I mentioned, the TB never worked right in the console and I never bothered to work on it until now because I didn't really need it.  After a bit of head scratching, I decided to disconnect the onboard TB mic preamp (a TL071 design) from the rest of the circuit and replace it with an outboard pre.  It worked and the pop was gone.  OK, so I figured I'd check out the preamp section to see what might be amiss.  After looking over the schematic and checking the parts, I discovered the TL071 was installed BACKWARDS by a previous "tech."  Amazing it worked at all, but it did just enough to make me think the problem was in the switching circuitry rather than the preamp circuitry. 

Oh, well, live and learn.  I now have a TB system that's dead quiet when it's switched.

Cheers,
--
Don

 

[Brian Roth]

Glad you got it figured out!  My next step was going to be having you probe around the other FET terminals in search of some errant DC voltage.

Bri