No cabinet sound on DE SW

stangbat

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I have no cabinet sound on my DE SW. Left and right sound works fine. Here's the run down:

-Sound board version 520-5050-00 Rev. C
-I have continuity from the sound board to the speaker so the wiring is good
-The speaker is good
-Voltages to the sound board check fine. If anything, the +12v is high (14.11). 5v is 4.94.
-On the MB3730A amplifier for the cabinet sound I get 10.9v at pin 7 (VCC) with respect to ground, this is the same as the other two MB3730A amplifiers for the left-right sound.
-On pins 5 and 6 (the output of the MB3730A for the cabinet sound), I get 10.42v with respect to ground. On the other MB3730A for LR sound I get 4.65v. So this obviously a lot higher.
-On the +/- speaker outs for the LR sound, I can measure AC voltage between the the connections (obviously or there would be no sound). On the +/- out of the cabinet sound I measure no AC voltage between the connections.
-The MB3730A amplifier for the cabinet sound gets noticeably hotter than the other two amplifiers.

Any suggestions on where to go next? Any other measurements I should take? Should I replace the MB3730A amplifier? They are $7.50 at GPE. Check and replace capacitors?

Thanks!
 
Check the Speaker first, then....

Hi,

I have had a number of problem with DE sound systems. First I have found multiple speakers that had less than and even a short that cause the amp chip to smoke.

Measure across the speaker terminals, I believe this measurement should be 4 ohms. If this measures less than 4 ohms, replace the speaker. Then you will likely have to replace the amplifier chip.

If all measures OK at the speaker, make sure you don't have a short somewhere on the speaker wires, check the resistance at the sound board cable for the cabinet speaker to verify there isn't a short.

I have only had to replace speakers and the amplifier chips solve most of the DE sound problems.

Barry
 
Thanks for the response. The speaker and wiring are good. The speaker measures 8 ohms and I've also played sound though it with no problems. I've also tested continuity of the wiring and played the other channels through the cabinet speaker so I know the wiring is good.

From looking at the schematic and due to the fact that the left and right sound work, I'm wondering if the amp chip is bad. I guess I can try replacing it first. It makes sense to try that but I don't want to be randomly throwing darts.
 
If you have a signal generator or can produce an analog tone of say 1kHz you can inject the signal in and determine if the final amp stage is functioning and move inward if you find all is well. Or you can use a small transistor radio to generate a signal, ground the appropriate side of the speaker input to the DE sound board, attach a meter probe to the other lead, and poke to determine stage functions. Or you can take a mono-amp, ground the appropriate lead to the sound board, and probe listening for sound at the output of the various states of the circuit to determine where the sound stops thereby identifying the faulty component.



Barry
 
If you have a signal generator or can produce an analog tone of say 1kHz you can inject the signal in and determine if the final amp stage is functioning and move inward if you find all is well. Or you can use a small transistor radio to generate a signal, ground the appropriate side of the speaker input to the DE sound board, attach a meter probe to the other lead, and poke to determine stage functions. Or you can take a mono-amp, ground the appropriate lead to the sound board, and probe listening for sound at the output of the various states of the circuit to determine where the sound stops thereby identifying the faulty component.



Barry
I have signal generator software on my laptop. To be clear, are you talking about grounding one of the outputs from the laptop's speaker out to the sound board and using the other speaker's lead to probe the amp's input?
 
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