Lexicon MPX1 output distortion
- Thread starter morls
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scott2000
Well-known member
Thanks...Much clearer....
Just for reference for someone who may know more,
That section under u10 and 11 is described in the theory of operation in the service manual..... sheet 10
Just for reference for someone who may know more,
That section under u10 and 11 is described in the theory of operation in the service manual..... sheet 10
It's strange, but the service manual parts list includes R97, 98, C76, 78 and others that are on the board, but they don't appear in the schematic...
My next move is to replace the components I've removed before tracing the circuit from U10 and U11 to U8. Here's a closeup of U8 and surrounds:
My next move is to replace the components I've removed before tracing the circuit from U10 and U11 to U8. Here's a closeup of U8 and surrounds:
Attachments
I've replaced the caps and resistors in SOFT_SAT circuit. The issue remains though 
I'm wondering if it could be something to do with C86? This doesn't appear in the schematic, but it is in the parts list as part #
240-00614 description CAP,ELEC,47uf,16V,RAD
The cap I have there is polarised, and I replaced it today, but there is no +ve marking on the board so I went with my photographs as to the polarity. Is it possible this should be non-polar? I think it's part of the SOFT_SAT circuit, as it connects to both C84 and C87.
Maybe this is the issue? I remember recapping this area a while ago, and maybe I've put in a polarised by mistake? This might explain the truncated waveform issue. It's strange that every polarised cap on the board has +ve marked except this one. Yet the parts list doesn't list it as NON-POL.
Would there be any chance of damaging the circuit if I put in a bi-polar cap at position C86 to test this?
I'm wondering if it could be something to do with C86? This doesn't appear in the schematic, but it is in the parts list as part #
240-00614 description CAP,ELEC,47uf,16V,RAD
The cap I have there is polarised, and I replaced it today, but there is no +ve marking on the board so I went with my photographs as to the polarity. Is it possible this should be non-polar? I think it's part of the SOFT_SAT circuit, as it connects to both C84 and C87.
Maybe this is the issue? I remember recapping this area a while ago, and maybe I've put in a polarised by mistake? This might explain the truncated waveform issue. It's strange that every polarised cap on the board has +ve marked except this one. Yet the parts list doesn't list it as NON-POL.
Would there be any chance of damaging the circuit if I put in a bi-polar cap at position C86 to test this?
Capacitors, even polarised ones, are NOT diodes. They may fail internally shorted, but not one-way only.
That being said, i fixed a noisy DBX digital crossover that was noisy, where all the input & output DC-blocking caps were bloated (either CapXon or Lelon, can't recall exactly - crap anyway).
I don't suppose tracing out that part of the circuit (ie. lifting the schematic) is feasible, is it?
That being said, i fixed a noisy DBX digital crossover that was noisy, where all the input & output DC-blocking caps were bloated (either CapXon or Lelon, can't recall exactly - crap anyway).
I don't suppose tracing out that part of the circuit (ie. lifting the schematic) is feasible, is it?
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scott2000
Well-known member
morls said:
Always a chance you can have it in backwards...
SAA7360 data sheet attached......
I've replaced C86 with a bi-polar cap. Fault still present.
Plugged unit in with outputs connected. Fault appeared after 1-2min. tone noted before present.
Performed a software initialise. Fault disappeared while initialise was taking place. Once finished init the unit defaulted to prog 1 MPX blue. The tone was processed. Changing programs changes the sound of tone, so it is being processed by each patch.
Plugged unit in with outputs connected. Fault appeared after 1-2min. tone noted before present.
Performed a software initialise. Fault disappeared while initialise was taking place. Once finished init the unit defaulted to prog 1 MPX blue. The tone was processed. Changing programs changes the sound of tone, so it is being processed by each patch.
scott2000
Well-known member
morls said:Thanks Scott. I think it was backwards - looking at the data sheet the +ve of C86 goes to pin 27 of U8. Have replace U86 according to this. Issue remains.
Yuck...That would be the last thing I'd hope for......
The theory of operation on that sat circuit was interesting in that it mentions addressing something about that adc
