Pop Goes The Audio
Posted 25 November 2013 - 02:00 PM
"Without music, life would be a mistake" - Friedrich Nietzsche
Posted 26 November 2013 - 12:02 PM
I was playing around with my Mesa preset today, and I scooped the mids and turned up the treble (it's ok when no one is watching), and I heard a faint popping noise, and then the sound faded out to nearly nothing. I switched presets and heard a louder pop, then the audio was normal until I switched again. Upon rebooting it was fine until I further tweaked, and it repeated the same cycle. Any suggestions as to the cause of this issue?
Two things are common for all the control pots that tweak the volume, mids, highs, lows, FX level, presence, etc. Those are the +5V analog supply and U10 on the left user Interface board, which is a TLC1543CDW, a analog to serial digital converter chip which samples the voltages on all the control pots along with temperature for the graphic LCD and sends that data in serial digital format to digital processing by U27 the system processor on the main board via a couple of latch chips U5 and U7.
Unless the problem is confined to just the volume where I'd look at the pot itself, I'd start looking first at that 5VA supply first, then at that U10 IC, if the 5V supply is looking good. If it's just one of the volume pots alone you can probably either clean it or just replace it.
As usual a loose board interconnect can cause the same problems so give those a good shake before you go digging in too far.
There is a built in self test that can be used to check the pots.
To enter the self test mode,
With the unit off and while holding down the "REVERB" button power up the unit to enter the self test mode
Press the TREMELO button to decrement though the tests or press the GATE button to increment though the test routines.
Press the COMPRESS botton to execute the test you're at.
Test 1 is the SRAM.
Test 2 is push button switch test
Test 3 is the LED test
Test 4 isn the Character LCD test
Test 5 is the Graphic LCD test
Test 6 is the Encoder test
Test 7 (the one we want here) is the POTS test. Press Compress to start the test. Roate each of the pots. When the pot is fully CCW the right LCD should display 000 when the pot is fully CW the display should read 127.
Press REVERB to exit the test.
The tests go though 16 for doing other things such as reloading the software, resetting patches, testing the MIDI and FX interfaces, etc
Be very very careful when in the test mode. DO NOT execute any of these tests unless you absolutely know what you are doing as some of the tests are used to upload and erase the system software and or patches and you could even convert you amp to a boat anchor. When in doubt ABSTAIN. BE CAREFUL, DO NOT proceed unless you are sure, very sure.