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Author Topic: Relays and Logic  (Read 726 times)
BigSkyAudio
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« on: September 16, 2016, 07:50:12 AM »

Hi,
I have a few problems which I have attributed to the relays and logic area and looking for clues to fix them.
I studied electronics 30 years ago but haven't used that knowledge in a while, so though not in top form I can read, measure and trace ccts.
A few channels will work fine when Vca is in the monitor path but not when the faders are reversed.
Two are the opposite of this, Vca fine when sending to tape but not working in mon path.
Others work fine except with the Vca again in the monitor path, it must be up off infinity for the small fader to pass signal, ie the switching of the mute button is not swapping over.
I have tried changing the Sn7472 and 7400 chips to known working ones, and checked all the reed relay coils and contacts with a multimeter with no power and the read fine.
Hoping someone has some clues that will help me.
BTW, has anyone tried apply 5v and 24 v to a board to trace this stuff without the it being in the extender card so the trace side can be accessed?
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Harrison
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« Reply #1 on: September 29, 2016, 04:13:05 PM »

"Others work fine except with the Vca again in the monitor path, it must be up off infinity for the small fader to pass signal, ie the switching of the mute button is not swapping over."

When the faders are swapped, the mute command is routed to either the channel or monitor mute via the mute select relay. This has a drive circuit that needs to be tested. Or the relay might just not turn over.

You need 2 mute circuits because the faders are at the front of the signal paths, but the mute relays are at the end. So when the big fader is muted, it has to either mute the channel OR the monitor, leaving the other path on.
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Harrison
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« Reply #2 on: September 29, 2016, 04:23:38 PM »

I'll make this a bit clearer.
When the VCA changeover happens, 2 relays are fired; the VCA audio relay itself AND the mute select relay.
The mute select relay steers the DC mute command from the mute logic to the channel or the monitor mute relays.

Check also the solder on the 1N4001 diodes in the emitter of the mute relay driver transistors. They sometimes suffer the same problem as Molex connectors. (The diode is there to effectively increase the Vbe of the switching transistor to improve the switching vs any dc noise on the command signal).
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