CS1200x driver board bias help

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jbesound
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CS1200x driver board bias help

Post by jbesound » Wed Apr 12, 2017 3:42 pm

I have a CS1200x driver board that is giving me a negative voltage at P7-6. The voltage between P7-6 and P8-7 is 4v which is a little high, but is severely offset to the negative. There are no burnt resistors and I replaced all 4148 diodes and all transistors in the bias circuit. Has me and Grant at Peavey service both stumped. Any help would be greatly appreciated.

Thanks

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Enzo
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Re: CS1200x driver board bias help

Post by Enzo » Wed Apr 12, 2017 6:13 pm

I don't see any transistors in the bias circuit, but there are two dual diodes and two single diodes, so I don;t see 4v as terribly high, drawing says 3.54v. In any case, bias won't cause offset.

The amp is offset, you don't say by how much. Now either there is somethign wrong pulling that bias circuit over to whatever your negative is, OR, it is trying to compensate for offset elsewhere. So go back to the differential, U2B, and what is on pin 7?

Look at the V+ and V- rails as they come onto the driver. See the little 47 ohm series resistors? Is one open?

jbesound
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Re: CS1200x driver board bias help

Post by jbesound » Thu Apr 13, 2017 8:24 am

Ok here are some measurements with the driver board connected to a known good output board. My variac was only at 50% due to the unusually high idle current draw based on the brightness of my 150W bulb in series with the AC mains:

P7-6 -3.57v
P7-8 -5.45v
P8-1 -5.46v
P8-4 -5.44v
P8-7 -7.4v
Q11 E 12.7v, B 11.94v, C -3.56v
Q12 E -12.66v, B -11.91v, C -7.39v
U2 pin 7 -.084v

Based on Q11 and Q12 emitter voltages, the 47 and 150 ohm resistors are ok. There are no signs of heat stress on any of the parts.

jbesound
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Re: CS1200x driver board bias help

Post by jbesound » Mon Apr 17, 2017 9:24 am

CR24 was leaky.

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