Vintage Synth Repair

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Quazimodo
Sent: Saturday, October 27, 2012 5:07 PM
To: vintagesynthrepair@yahoogroups.com
Subject: [vintagesynthrepair] Akai S900 voice issues

 

  

Hi folks,

I have an Akai S900 sampler which has three dead voices (5, 6 & 8)

I managed to get the service manual downloaded and I've discovered abnornmal
voltages (compared to the good voices) on a couple of pins of the BA9221 D/A
convertors. All three voices seem to be identical faults.

I have Googled "S900 dead voices" but there is literally nothing out there
which leads me to believe it is probably not a 'common' fault. And I have
three a once...!

Just checking if I could be barking up the wrong tree...? Has anyone ever
had similar problems it these Akai samplers?

Cheers,
TOM

>
> What pins have the abnormal voltage readings on the 'good' and 'bad' DACs?
>  Schematic of that area?
> 
> Hopefully this will lead us to figuring out if it is the DACs or some other
> part of the circuit.
> 
> OK, Hugh
>

>
> Thanks Hugh,
> 
> OK I've uploaded a JPeg of the relevant part of the circuit.
> I've called it S900_DAC... it's in our photo's section here.
> 
> So I am getting around 3volts on pins 14 & 19 of the BA9221 (IC107)
> on faulty voice channels.
> 
> On good voices there is just millivolts on those pins.
> 
> Would you suspect the BA9221....?
> 
> Cheers,
> TOM
> 
> 
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, Hugh Vartanian <bouncev@> wrote:
> >
> > What pins have the abnormal voltage readings on the 'good' and 'bad' DACs?
> >  Schematic of that area?
> > 
> > Hopefully this will lead us to figuring out if it is the DACs or some other
> > part of the circuit.
> > 
> > OK, Hugh
> >
>

>
> Hi Tom,
> A few things:
> 1. Please verify that the voltages on the good chips is the millivolt reading and the bad chips is 3V. (sorry, just checking!)  If indeed the good chip has millivolts on these pins, then there must be 6V/220ohm=25ma or so going somewhere and I don't know where.  The pin input current is specified around 1ma on the chip datasheet.  So something is rather strange here.
> 1a.  How many millivolts?  Was the reference pin 13 or pin 15? (see #3) 
> 2. I assume the other voices' circuits are the same and they don't share any part of this circuitry.
> 3. Where does pin 15 go?  What is its voltage?
> 4. What is the voltage on pin 18, for the good and bad voices?
> 5. What is the 6V supply reading on the right side of the 220ohm resistor (R121)?
> 6. In general, please verify that all the other voltages are the same, give or take (on the right side of the chip)?
> 7. Are any of the resistors in the circuit, like R102 or R103, special temperature dependent parts?
> 8. Any chance that capacitor C142 is shorted?
> 
> OK, we'll get to the bottom of this!
> Hugh
> 
> --- In vintagesynthrepair@yahoogroups.com, "Quazimodo" <noddyspuncture@> wrote:
> >
> > Thanks Hugh,
> > 
> > OK I've uploaded a JPeg of the relevant part of the circuit.
> > I've called it S900_DAC... it's in our photo's section here.
> > 
> > So I am getting around 3volts on pins 14 & 19 of the BA9221 (IC107)
> > on faulty voice channels.
> > 
> > On good voices there is just millivolts on those pins.
> > 
> > Would you suspect the BA9221....?
> > 
> > Cheers,
> > TOM
> > 
> > 
> > 
> > 
> > --- In vintagesynthrepair@yahoogroups.com, Hugh Vartanian <bouncev@> wrote:
> > >
> > > What pins have the abnormal voltage readings on the 'good' and 'bad' DACs?
> > >  Schematic of that area?
> > > 
> > > Hopefully this will lead us to figuring out if it is the DACs or some other
> > > part of the circuit.
> > > 
> > > OK, Hugh
> > >
> >
>

>
> Quazimodo, how are you doing with the DAC debug, last round of measurement
> questions?
> Hugh
> On Oct 28, 2012 10:55 AM, "Quazimodo" <noddyspuncture@...> wrote:
> 
> > **
> >
> >
> > Thanks Hugh,
> >
> > OK I've uploaded a JPeg of the relevant part of the circuit.
> > I've called it S900_DAC... it's in our photo's section here.
> >
> > So I am getting around 3volts on pins 14 & 19 of the BA9221 (IC107)
> > on faulty voice channels.
> >
> > On good voices there is just millivolts on those pins.
> >
> > Would you suspect the BA9221....?
> >
> > Cheers,
> > TOM
> >
> > --- In vintagesynthrepair@yahoogroups.com, Hugh Vartanian <bouncev@>
> > wrote:
> > >
> > > What pins have the abnormal voltage readings on the 'good' and 'bad'
> > DACs?
> > > Schematic of that area?
> > >
> > > Hopefully this will lead us to figuring out if it is the DACs or some
> > other
> > > part of the circuit.
> > >
> > > OK, Hugh
> > >
> >
> >  
> >
>

>
> Hi again Hugh,
> 
> Sorry my fault. I should have made more notes and investigated further...
> 
> *The 6volts at the junction of R102 & 103 is present on all voices
> 
> *Pin 14 is at the full rail volts of 6v on bad voices - it is 1 or 2 millivolts on good voices
> 
> *Pin 19 on bad voices is at around -3volts - but on good voices is around +3volts
> 
> *The capacitor C112 (22uf/16v) is good on all voices
> 
> *Pin 15 is labelled elsewhere as "A.GND"... so that's a grounded pin
> 
> Again thanks for your help...
> 
> Cheers,
> TOM
> 
> 
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, "hughvartanian" <bouncev@> wrote:
> >
> > Hi Tom,
> > A few things:
> > 1. Please verify that the voltages on the good chips is the millivolt reading and the bad chips is 3V. (sorry, just checking!)  If indeed the good chip has millivolts on these pins, then there must be 6V/220ohm=25ma or so going somewhere and I don't know where.  The pin input current is specified around 1ma on the chip datasheet.  So something is rather strange here.
> > 1a.  How many millivolts?  Was the reference pin 13 or pin 15? (see #3) 
> > 2. I assume the other voices' circuits are the same and they don't share any part of this circuitry.
> > 3. Where does pin 15 go?  What is its voltage?
> > 4. What is the voltage on pin 18, for the good and bad voices?
> > 5. What is the 6V supply reading on the right side of the 220ohm resistor (R121)?
> > 6. In general, please verify that all the other voltages are the same, give or take (on the right side of the chip)?
> > 7. Are any of the resistors in the circuit, like R102 or R103, special temperature dependent parts?
> > 8. Any chance that capacitor C142 is shorted?
> > 
> > OK, we'll get to the bottom of this!
> > Hugh
> > 
> > --- In vintagesynthrepair@yahoogroups.com, "Quazimodo" <noddyspuncture@> wrote:
> > >
> > > Thanks Hugh,
> > > 
> > > OK I've uploaded a JPeg of the relevant part of the circuit.
> > > I've called it S900_DAC... it's in our photo's section here.
> > > 
> > > So I am getting around 3volts on pins 14 & 19 of the BA9221 (IC107)
> > > on faulty voice channels.
> > > 
> > > On good voices there is just millivolts on those pins.
> > > 
> > > Would you suspect the BA9221....?
> > > 
> > > Cheers,
> > > TOM
> > > 
> > > 
> > > 
> > > 
> > > --- In vintagesynthrepair@yahoogroups.com, Hugh Vartanian <bouncev@> wrote:
> > > >
> > > > What pins have the abnormal voltage readings on the 'good' and 'bad' DACs?
> > > >  Schematic of that area?
> > > > 
> > > > Hopefully this will lead us to figuring out if it is the DACs or some other
> > > > part of the circuit.
> > > > 
> > > > OK, Hugh
> > > >
> > >
> >
>

>
> Hi TOM,
> 
> Sorry for not getting back to you.  I've been busy and I noticed you had answered some of my questions in this thread.  Probably my bad for starting that.  Anyway, I haven't forgotten about your synth!
> One easy thing occurred to me: you could pull the chips, put sockets in and swap them between a good and a bad channel.
> 
> Do you have the voltages on pins 18 and 16.  
> 
> Sorry to say but it is looking more and more like the chips are bad.
> 
> Hugh
> 
> --- In vintagesynthrepair@yahoogroups.com, "Quazimodo" <noddyspuncture@> wrote:
> >
> > Hi again Hugh,
> > 
> > Sorry my fault. I should have made more notes and investigated further...
> > 
> > *The 6volts at the junction of R102 & 103 is present on all voices
> > 
> > *Pin 14 is at the full rail volts of 6v on bad voices - it is 1 or 2 millivolts on good voices
> > 
> > *Pin 19 on bad voices is at around -3volts - but on good voices is around +3volts
> > 
> > *The capacitor C112 (22uf/16v) is good on all voices
> > 
> > *Pin 15 is labelled elsewhere as "A.GND"... so that's a grounded pin
> > 
> > Again thanks for your help...
> > 
> > Cheers,
> > TOM
> > 
> > 
> > 
> > 
> > --- In vintagesynthrepair@yahoogroups.com, "hughvartanian" <bouncev@> wrote:
> > >
> > > Hi Tom,
> > > A few things:
> > > 1. Please verify that the voltages on the good chips is the millivolt reading and the bad chips is 3V. (sorry, just checking!)  If indeed the good chip has millivolts on these pins, then there must be 6V/220ohm=25ma or so going somewhere and I don't know where.  The pin input current is specified around 1ma on the chip datasheet.  So something is rather strange here.
> > > 1a.  How many millivolts?  Was the reference pin 13 or pin 15? (see #3) 
> > > 2. I assume the other voices' circuits are the same and they don't share any part of this circuitry.
> > > 3. Where does pin 15 go?  What is its voltage?
> > > 4. What is the voltage on pin 18, for the good and bad voices?
> > > 5. What is the 6V supply reading on the right side of the 220ohm resistor (R121)?
> > > 6. In general, please verify that all the other voltages are the same, give or take (on the right side of the chip)?
> > > 7. Are any of the resistors in the circuit, like R102 or R103, special temperature dependent parts?
> > > 8. Any chance that capacitor C142 is shorted?
> > > 
> > > OK, we'll get to the bottom of this!
> > > Hugh
> > > 
> > > --- In vintagesynthrepair@yahoogroups.com, "Quazimodo" <noddyspuncture@> wrote:
> > > >
> > > > Thanks Hugh,
> > > > 
> > > > OK I've uploaded a JPeg of the relevant part of the circuit.
> > > > I've called it S900_DAC... it's in our photo's section here.
> > > > 
> > > > So I am getting around 3volts on pins 14 & 19 of the BA9221 (IC107)
> > > > on faulty voice channels.
> > > > 
> > > > On good voices there is just millivolts on those pins.
> > > > 
> > > > Would you suspect the BA9221....?
> > > > 
> > > > Cheers,
> > > > TOM
> > > > 
> > > > 
> > > > 
> > > > 
> > > > --- In vintagesynthrepair@yahoogroups.com, Hugh Vartanian <bouncev@> wrote:
> > > > >
> > > > > What pins have the abnormal voltage readings on the 'good' and 'bad' DACs?
> > > > >  Schematic of that area?
> > > > > 
> > > > > Hopefully this will lead us to figuring out if it is the DACs or some other
> > > > > part of the circuit.
> > > > > 
> > > > > OK, Hugh
> > > > >
> > > >
> > >
> >
>

>
> Hi Hugh and everyone, hope you all had a great Christmas and wishing you all a very Happy New Year...
> 
> ...so, I've just returned to my Akai S900 with missing voices. I had sourced three new BA9221 IC's - they weren't cheap either. Having originally scoped them and seen inputs, but no outputs, I was pretty convinced that these IC's were faulty. After swapping out the first one (and destroying the old one in the process) - I luckily reassembled the panel and checked it out. I suppose it was to be expected... still no voice on the supposedly repaired channel. So I've ground to a halt. No point swapping the others I feel. It looks like I was completely on the wrong track here... anyone have any ideas please?
> 
> Cheers,
> TOM
> 
> 
> 
> --- In vintagesynthrepair@yahoogroups.com, "hughvartanian" <bouncev@> wrote:
> >
> > Hi TOM,
> > 
> > Sorry for not getting back to you.  I've been busy and I noticed you had answered some of my questions in this thread.  Probably my bad for starting that.  Anyway, I haven't forgotten about your synth!
> > One easy thing occurred to me: you could pull the chips, put sockets in and swap them between a good and a bad channel.
> > 
> > Do you have the voltages on pins 18 and 16.  
> > 
> > Sorry to say but it is looking more and more like the chips are bad.
> > 
> > Hugh
> > 
> > --- In vintagesynthrepair@yahoogroups.com, "Quazimodo" <noddyspuncture@> wrote:
> > >
> > > Hi again Hugh,
> > > 
> > > Sorry my fault. I should have made more notes and investigated further...
> > > 
> > > *The 6volts at the junction of R102 & 103 is present on all voices
> > > 
> > > *Pin 14 is at the full rail volts of 6v on bad voices - it is 1 or 2 millivolts on good voices
> > > 
> > > *Pin 19 on bad voices is at around -3volts - but on good voices is around +3volts
> > > 
> > > *The capacitor C112 (22uf/16v) is good on all voices
> > > 
> > > *Pin 15 is labelled elsewhere as "A.GND"... so that's a grounded pin
> > > 
> > > Again thanks for your help...
> > > 
> > > Cheers,
> > > TOM
> > > 
> > > 
> > > 
> > > 
> > > --- In vintagesynthrepair@yahoogroups.com, "hughvartanian" <bouncev@> wrote:
> > > >
> > > > Hi Tom,
> > > > A few things:
> > > > 1. Please verify that the voltages on the good chips is the millivolt reading and the bad chips is 3V. (sorry, just checking!)  If indeed the good chip has millivolts on these pins, then there must be 6V/220ohm=25ma or so going somewhere and I don't know where.  The pin input current is specified around 1ma on the chip datasheet.  So something is rather strange here.
> > > > 1a.  How many millivolts?  Was the reference pin 13 or pin 15? (see #3) 
> > > > 2. I assume the other voices' circuits are the same and they don't share any part of this circuitry.
> > > > 3. Where does pin 15 go?  What is its voltage?
> > > > 4. What is the voltage on pin 18, for the good and bad voices?
> > > > 5. What is the 6V supply reading on the right side of the 220ohm resistor (R121)?
> > > > 6. In general, please verify that all the other voltages are the same, give or take (on the right side of the chip)?
> > > > 7. Are any of the resistors in the circuit, like R102 or R103, special temperature dependent parts?
> > > > 8. Any chance that capacitor C142 is shorted?
> > > > 
> > > > OK, we'll get to the bottom of this!
> > > > Hugh
> > > > 
> > > > --- In vintagesynthrepair@yahoogroups.com, "Quazimodo" <noddyspuncture@> wrote:
> > > > >
> > > > > Thanks Hugh,
> > > > > 
> > > > > OK I've uploaded a JPeg of the relevant part of the circuit.
> > > > > I've called it S900_DAC... it's in our photo's section here.
> > > > > 
> > > > > So I am getting around 3volts on pins 14 & 19 of the BA9221 (IC107)
> > > > > on faulty voice channels.
> > > > > 
> > > > > On good voices there is just millivolts on those pins.
> > > > > 
> > > > > Would you suspect the BA9221....?
> > > > > 
> > > > > Cheers,
> > > > > TOM
> > > > > 
> > > > > 
> > > > > 
> > > > > 
> > > > > --- In vintagesynthrepair@yahoogroups.com, Hugh Vartanian <bouncev@> wrote:
> > > > > >
> > > > > > What pins have the abnormal voltage readings on the 'good' and 'bad' DACs?
> > > > > >  Schematic of that area?
> > > > > > 
> > > > > > Hopefully this will lead us to figuring out if it is the DACs or some other
> > > > > > part of the circuit.
> > > > > > 
> > > > > > OK, Hugh
> > > > > >
> > > > >
> > > >
> > >
> >
>