Aleguitarpro
Well-known member
Hi.
One of the 2 lexicon pcm42 in studio stopped working.
I'm not great troubleshooting digital audio but have experience in DIY and in repair analog stuff to mantain my studio.
Symptoms are:
1. distorted repeats with a glitching kind of tail
2. with feedback at zero repeat and works with little distortion
3. feedback control with no signal applied generate a sort of "feedback loop". If I invert the phase of repeats start a sort of high-pitched whistle/noise
4. noise when feedback control is within the first quarter is a low freq noise that can be modulated by RATE/DEPTH controls.
What I did:
- recapped the unit (since I don't remember last time I used it...), measured EVERYTHING and compare with the other unit
- changed 7805 because I thought that this strange behaviour (and pulsating noise) can be a bad regulator but nothing changed.
I measured the other unit and (I know this is not good) I replaced the digital chips from one unit to the other one.
Nothing changed and the perfect functioning unit is still ok but the other one has the same problem.
Analog part is ok.
But after a couple of days I think to have identified where the problem start.
Without U23 (that is a AM6012DC DAC) at pin 18 I measured +6V that is normal due to the resistors array, without U18/U19/U20.
After R90 I have +10V and after the 2 10k resistors in array I measured +6V.
If I reinstall the DAC U23 I measure -6V.
If I install U18 I measure at pin6 +6V (U18 is inverting).
After U19 at pin 6 again -6V.
Of course this tension arrive at U20 and adding this DC voltage to audio I have distorting and loud repetitions and acting with FEEDBACK this problem rise and rise.
I desoldered every cap around opamps feedback in order to measure them and see if something changes but nothing.
In the other unit without AM6012DC at pin 18 I have the same +6V but mounting AM6012DC this voltage disappear.
I swapped AM6012DC and is not the chip and opamps are all good.
I changed 3039 diode array (checking datasheet I think it's drawn backwards in pcm42 scheme) because has a voltage applied to pin9 even though it is designed as an array of diodes.
Problem is the same.
Attached is the part of the circuit I'm working on but surely there's something I'm missing or don't know.
Thanks for help
One of the 2 lexicon pcm42 in studio stopped working.
I'm not great troubleshooting digital audio but have experience in DIY and in repair analog stuff to mantain my studio.
Symptoms are:
1. distorted repeats with a glitching kind of tail
2. with feedback at zero repeat and works with little distortion
3. feedback control with no signal applied generate a sort of "feedback loop". If I invert the phase of repeats start a sort of high-pitched whistle/noise
4. noise when feedback control is within the first quarter is a low freq noise that can be modulated by RATE/DEPTH controls.
What I did:
- recapped the unit (since I don't remember last time I used it...), measured EVERYTHING and compare with the other unit
- changed 7805 because I thought that this strange behaviour (and pulsating noise) can be a bad regulator but nothing changed.
I measured the other unit and (I know this is not good) I replaced the digital chips from one unit to the other one.
Nothing changed and the perfect functioning unit is still ok but the other one has the same problem.
Analog part is ok.
But after a couple of days I think to have identified where the problem start.
Without U23 (that is a AM6012DC DAC) at pin 18 I measured +6V that is normal due to the resistors array, without U18/U19/U20.
After R90 I have +10V and after the 2 10k resistors in array I measured +6V.
If I reinstall the DAC U23 I measure -6V.
If I install U18 I measure at pin6 +6V (U18 is inverting).
After U19 at pin 6 again -6V.
Of course this tension arrive at U20 and adding this DC voltage to audio I have distorting and loud repetitions and acting with FEEDBACK this problem rise and rise.
I desoldered every cap around opamps feedback in order to measure them and see if something changes but nothing.
In the other unit without AM6012DC at pin 18 I have the same +6V but mounting AM6012DC this voltage disappear.
I swapped AM6012DC and is not the chip and opamps are all good.
I changed 3039 diode array (checking datasheet I think it's drawn backwards in pcm42 scheme) because has a voltage applied to pin9 even though it is designed as an array of diodes.
Problem is the same.
Attached is the part of the circuit I'm working on but surely there's something I'm missing or don't know.
Thanks for help
