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[DCT]

Searching for answers here. I've got a TAC Matchless that has developed a radio station on the monitor buss, that bleeds to the master L/R outs. Here's what's odd:
If I have nothing connected to the monitor out jacks, all is well. No monitor capability, but the master outs are clean. If I just connect a cable to the monitor outs-radio. -This is whether or not the other end of the cable is connected to gear. I've tried isolating possible ground loops, different cables, no other outs on the console have this issue. My workaround has been to use a pair of aux sends, which works fine but I want to get it taken care of.
I'm fairly sure at this point that the problem is in the master module itself. But I'm open to suggestions. What would be the suspect parts that could've gone bad? Caps, chips, cold solder joints?
Any help would be great.

[Paul Frindle]

DCT wrote on Mon, 01 November 2004 20:51

Searching for answers here. I've got a TAC Matchless that has developed a radio station on the monitor buss, that bleeds to the master L/R outs. Here's what's odd: If I have nothing connected to the monitor out jacks, all is well. No monitor capability, but the master outs are clean. If I just connect a cable to the monitor outs-radio. -This is whether or not the other end of the cable is connected to gear. I've tried isolating possible ground loops, different cables, no other outs on the console have this issue. My workaround has been to use a pair of aux sends, which works fine but I want to get it taken care of. I'm fairly sure at this point that the problem is in the master module itself. But I'm open to suggestions. What would be the suspect parts that could've gone bad? Caps, chips, cold solder joints? Any help would be great.

This is usually caused by partial instability and possible design issues within the output stages. What happens is that the radio freqs - which are too high to be suppressed by internal feedback loop - gets injected into the output. Since the freqs are too high for the amps to respond, instead the RF appears on the amps input via the feedback circuit and is immediately demodulated and audio programme content arrives amplified at the output

If it were me I would put an oscilloscope on the output to see if it's taking off in the 100 - 200KHz region. If it is then perhaps you have a fault internally that might respond to fixing. If it isn't then it's most likely to be a design issue.

You are them faced with either getting the RF out of your environment by various means or modifying the system to cope better with RF.

I had to modfiy the entire mix amp sections of a large Trident console in Trident studios during the 70's because BBC Radio 1 was permanently audible from the main outputs!!

[DCT]

Quote:

This is usually caused by partial instability and possible design issues within the output stages. What happens is that the radio freqs - which are too high to be suppressed by internal feedback loop - gets injected into the output. Since the freqs are too high for the amps to respond, instead the RF appears on the amps input via the feedback circuit and is immediately demodulated and audio programme content arrives amplified at the output

Thanks Paul, I think there is a failed section/instability as I've had this console in the same location for awhile, with no problems. I did however have the pots cleaned recently...perhaps on the R&R of the master module, something went south. I'll try your scope test to see if there's anything taking off up in the 100-200k range.
Thanks for the suggestions.

[Brian Roth]

Hmmmm!  On a TAC Matchless desk in my area, various "RF" problems were eventually traced to a broken "pin 2" lead from the output XLR "jack bank" back to the PC board.

Just a thought....

Bri

[Paul Frindle]

Brian Roth wrote on Tue, 02 November 2004 07:33

Hmmmm!  On a TAC Matchless desk in my area, various "RF" problems were eventually traced to a broken "pin 2" lead from the output XLR "jack bank" back to the PC board. Just a thought.... Bri

And another possibility is if a ground has become disconnected internally, either at the output on on the PCB itself. In this case the RF is finding a way in via common mode.

[DCT]

I'm pulling the master module tonight. Me thinks it's probably that header that connects the outs to the main board (up by the XLR's). Have had issues with it previously.
going to replace it with a snap lock header if possible.
Thanks for the TAC insights.
What chips are you running in your TAC Brian?

[Brian Roth]

It's a client's desk, and is fully "stock"....no hot-rodding.

Bri