800b input channel problem

Founded in 1973, Soundcraft first made its mark with the launch of the Series 1, the first mixing console in a flightcase.

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800b input channel problem

Postby ChristianB » Wed Jun 26, 2013 9:34 am

Hi,

one of my 8011 input channels make some trouble. When I turn the low mid and mid potis clockwise (only them, the others are working), the channel will overload and the signal fails. I've changed the TL072 without any effect and so I presume that one of the caps (C19 and or C20) is damaged. My knowledge of circuit design limited, so I want to ask the experts for help.

By the way I want to change the electrolytic caps as described in some of the threads. But I didn't found an explicit tip for the two transistors T1 and T2. Originally SC used the 2N4403, in the later 8021 input they used a pair of PN4355. Which one do you prefer, or do you have a better alternative?

Thanks in advance,
Christian
ChristianB
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Re: 800b input channel problem

Postby Jim Williams » Wed Jun 26, 2013 10:15 am

Both input transistors are PNP devices. The best choice is either the 2SA1083/4 or the slower 2SD737. Those are ECB so adjust the pinout when replacing them.

I suspect you either have a broken EQ pot, sweep pot or the solder pads are loose. Resolder them and if it doesn't fix it, replace the mid EQ 10k linear pot and the 100k dual reverse log sweep pot. Parts are available from Soundcraft, Northridge, CA. Ask for Frank Rodriguez in the parts dept.
Jim Williams
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Re: 800b input channel problem

Postby ChristianB » Tue Jul 02, 2013 5:25 am

Jim,

thanks a lot for your idea with the pots. Two solder pads of the sweep pot were damaged, and now its all in a perfect order.

The 1084 are ordered in the United States, no chance to get the in Europe with a good price.

The TL 072 I want to replace by the TLE 2072, but I've seen that there'll 4 different types of this chip:
ACP, AIP, IP and CP. Do you have an idea which one is the best choice?

Thanks,
Christian
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Re: 800b input channel problem

Postby Jim Williams » Tue Jul 02, 2013 10:40 am

Any other opamp than an 072 or LF353 will oscillate without extensive circuit mods. You will need to add feedback phase compensation caps and local psu bypass caps to get stability.
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