Shure M267 mixer limit light stays on no output

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westcoast44
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Shure M267 mixer limit light stays on no output

Post by westcoast44 » Wed Apr 02, 2014 8:40 am

Hi im having an issue with my m267 mixer...the little red light is on all the time..no output...after testing some voltages on the board i notice all the voltage reading according to the schematic from the manual read normal..except everywhere there should be 15.7 volts i get around 2 volts...not sure if thats the issue but im at a loss to where the 15.7 volts originate from...becuase every point that is suppose to get 15.7 volts is only getting 2 volts..any help would be appreciated.
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Deep Thought
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Re: Shure M267 mixer limit light stays on no output

Post by Deep Thought » Wed Apr 02, 2014 9:35 am

U205A generates the 15.7 volt bias supply (half the 30 volt supply). I'd start troubleshooting there, but a lot of that box uses the voltage for one thing or another and any of them could be shorting the supply.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

westcoast44
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Re: Shure M267 mixer limit light stays on no output

Post by westcoast44 » Wed Apr 02, 2014 9:58 am

Deep Thought wrote:U205A generates the 15.7 volt bias supply (half the 30 volt supply). I'd start troubleshooting there, but a lot of that box uses the voltage for one thing or another and any of them could be shorting the supply.
coulda swore i checked that...U205 pins 1 2 and 3 are at 15.7 volts pin 4 is at ground....pin 5 6 and 7 are at like 2 volts :( so the problem is possibe short or bad IC?

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Re: Shure M267 mixer limit light stays on no output

Post by Deep Thought » Wed Apr 02, 2014 11:19 am

R249 and R252 form a voltage divider to reduce the 30 volt power supply to 15.7 volts, which is used to bias other circuits to the midpoint so the op amps can operate from a single-ended power supply. C221 provides some filtering and stiffens the divider. U205As job is to lower the source impedance, providing a current source buffer between the divider and the rest of the circuit. If you have 15 volts at pin 1 that is working.

U205A pins 1, 2 and 3 should be at or close to 15.7 volts. Pin 4 is ground. Pin 8 is the 30 volt supply to U205. The other pins on U205 are part of the limiter circuit. Pin 5 at 2 volts is not good. What is the voltage at either end of R248? It should be 15.7 volts or close to it. If not, something has indeed shorted that bias supply buss somewhere downstream of R248. Also, what is the voltage on the other non-pin-5 end of R247?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

westcoast44
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Re: Shure M267 mixer limit light stays on no output

Post by westcoast44 » Wed Apr 02, 2014 11:26 am

Deep Thought wrote:R249 and R252 form a voltage divider to reduce the 30 volt power supply to 15.7 volts, which is used to bias other circuits to the midpoint so the op amps can operate from a single-ended power supply. C221 provides some filtering and stiffens the divider. U205As job is to lower the source impedance, providing a current source buffer between the divider and the rest of the circuit. If you have 15 volts at pin 1 that is working.

U205A pins 1, 2 and 3 should be at or close to 15.7 volts. Pin 4 is ground. Pin 8 is the 30 volt supply to U205. The other pins on U205 are part of the limiter circuit. Pin 5 at 2 volts is not good. What is the voltage at either end of R248? It should be 15.7 volts or close to it. If not, something has indeed shorted that bias supply buss somewhere downstream of R248. Also, what is the voltage on the other non-pin-5 end of R247?
one end of r248 is 15.7 the other is 2 volts..both sides of r247 are at 2 volts

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Re: Shure M267 mixer limit light stays on no output

Post by Deep Thought » Wed Apr 02, 2014 11:50 am

Well, something on that bias supply line is shorted. I would look at everything with a direct connection to that point (not through a resistor), but that 2 volts makes me think it is a shorted active component input somewhere. Check C218 and C235 too.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

westcoast44
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Re: Shure M267 mixer limit light stays on no output

Post by westcoast44 » Wed Apr 02, 2014 11:51 am

Awesome thanks for the help!

westcoast44
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Re: Shure M267 mixer limit light stays on no output

Post by westcoast44 » Mon Apr 07, 2014 7:21 am

replaced ribbon cable..fixed it..must of somehow shorted in the ribbon cable

rftranstv
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Re: Shure M267 mixer limit light stays on no output

Post by rftranstv » Mon Apr 07, 2014 10:36 am

I have a few of these mixers as well,some of the pots are going bad i think, anyone know where to get them? or something compatible?

scootchu
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Re: Shure M267 mixer limit light stays on no output

Post by scootchu » Mon Apr 13, 2015 9:02 am

Hi folks,
First time post and sorry to revive and tag onto a year old thread, but I have an M267 that recently had the same thing occur. We use it for town meetings and the camera op said he heard a high pitched pop/squeal and then no output with the red limiter light on.

My voltages on U205 are:
pin 1 - 15.6v
pin 2 - 15.6v
pin 3 - 15.6v
pin 4 - 0 grd
pin 5 - .1v
pin 6 - 28.7v
pin 7 - 28.7v
pin 8 - 30v

R248 has 15.7 on one side - 0v on the other
R247 has .1v on one side and 0v on the other
R249 has 15.6 on one side and 30v on the other
R252 has 15.6 on one side and 0v on the other

Just looking I don't see any caps bulged. I expected to find one given the description of how it happened.

Thanks for the help!
Scott

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