Allen & heath Saber talkback issue

[bleedingears]

I've just picked up an old A&H saber PA console. Everything seems to be working fine, but when I hit (or release) the talkback button there is a very loud click in my monitors.

I am using it in a recording studio, and my monitors are running out of the monitor output. I tried it out of the main outs, and there was no click - even with TB routed to L+R. There was also no click out of the headphone output. I haven't tried the aux's yet, as I haven't finished wiring in the headphone amp.

I'm pretty new to electronics DIY (built a couple of pre amp kits is all), but I am hoping this will be easy to fix.  Will it just be the case of fitting a new switch or something else.

If anyone could give me 'an idiots guide...' to a fix, I would be eternally grateful.

 

[JohnRoberts]

Click or thump, or both click and thump?

Typical cause would a DC blocking cap that is leaking, or not being properly discharged to 0V because of a bad solder connection to a bleed resistor.

Some amount of click may always  be there, a loud thump should not. I am not familiar with that board so don't know how quietly they switch when working to design. 

Talkback circuitry that doesn't print to L/R master is usually a lower priority for sound quality, but it should not be very nasty. 

JR

 

[bleedingears]

Its a click, no thump.  Problem is, it comes out at full volume. I have my active monitors at max gain and use the console to control levels. With all faders, and the TB volume down, this click is still at max volume.

The master outs have no volume trim pot so the output is too hot for my set up.

I did remove the master module and had a look in the area of the TB switch (not that I know what I'm looking for). The only thing that looked weird was 2 resistors soldered together over another resistor (so in series, but with these 2 raised from the board). I don't know if this is normal or not.

 

[musika]

Look for the schematic here:

http://www.allen-heath.com/media/SABER-MK1-SERVICE-MANUAL.pdf

I  have attached what I think is the TalkBack portion.  It's a very large document.

 

[JohnRoberts]

From a quick glance there are several electrolytic caps that could be introducing DC, but even in an ideal world, the gate of the JFET used as a switch toggles from -16V to 0V when you press the talkback switch so that 16V step will cap couple through to the output (a little). That's why I asked click or thump..? if it gate logic leaking through that will be a pretty thin click, it is is DC being switched in that will be more of a thump.

The easier path is to readjust your gain structure so you amps aren't WFO and the noise floor of the control room switching is relatively attenuated.


You can check that there isn't any DC where there shouldn't be, but that will be more thump than click. 

JR

 

[bleedingears]

Thanks for your help on this.

I've tried running out of the main outputs and turning my monitors down (nearly all the way to get a reasonable level), but its pretty obvious that this isn't how the desk is meant to be run. In using these outputs, my solo/PFL's don't do anything, and talkback doesn't dim the mix.

Tested the talkback going through aux 1+2 to a headphone amp and there is no click there either, with the talkback working fine. It seems its only present on the monitor outs.

These desks have 2 different master modules available to them. I have the 'live' master, and there is also a 'studio' master. Not too sure of the differences, but mine doesn't have an oscillator so I guess thats the schematics for the 'studio' module.

Either way, with everything else working fine, I think my best bet is to have a mic plugged in to channel 40 sent to aux 1+2 but not L+R and use the mute button for TB. Of course the mix won't dim, but I'll get the use of my solo/PFL buttons back.

I really have no idea what I'm looking at, and would hate to screw something else up. I appreciate the help thats been given here. Feeling a little out of my depth.
 

 

[JohnRoberts]

Actually it's a matter of deductive (?) reasoning. If it doesn't click/thump when sent to other outputs that suggests that the talkback circuit itself is Ok, or the other circuits are blocking any DC that is causing clicks.

Your schematic doesn't show the entire path from talk back preamp to monitor out, but I would trace that out and look for possibly a leaky (leaking current not leaking spoo) DC blocking capacitor in that path. OR an open/missing resistor to ground used to discharge any voltage build up on that DC blocking cap.

Not rocket science, but science. Eliminate what it isn't, and what's left is what it is.

JR

 

[bleedingears]

Thanks, that makes a lot of sense. I think I  will open her up and have another look tomorrow.