[sdiy] Akai AX80 weird PWM behaviour on Voice 8 when Saw wave enabled

james meagher w.james.meagher at gmail.com
Tue Jul 10 20:24:04 CEST 2012


Okay, since my last post I've eliminated that this issue might be
caused by IC704 and IC802 and also that there might be something amiss
with the Voice 8 saw wave trim pot level setting or soldering.

So I did comparative readings between Voice 8 and Voice 4 across the
board and noted many anomolies . . . unfortunately I've not been able
to translate this information into a solution because I'm just not
there yet with my understanding of schematics and electronics in
general. I'll think I'm on the right path and then realize I've got it
all backwards. So I've decided to share my readings and ask for
guidance.

Again, for reference here are some links to the AX80 SM, the Voice
Board Schematics and Voice Board PCB alone, and my complete
comparative readings between Voice 8 and Voice 4 and when necessary
all the other voices:

AX80 Voice 8 complete readings 57KB PDF:
https://www.dropbox.com/s/o5ayzenvavpmzha/AKAI%20AX80%20VOICE%208%20readings.pdf
AX80 Voice Schematic 11.3MB JPG:
https://www.dropbox.com/s/bhi1314hv6qxi0v/AX80%20Voice%20Schematics.jpg
AX80 Voice PCB 16.9MB JPG:
https://www.dropbox.com/s/v73wr6ql0sr0506/AX80%20Voice%20PCB.jpg
AX80 Service Manual 34MB PDF:
https://www.dropbox.com/s/88xnbxv9a44iqx7/Akai%20AX80%20SM%20compressed.pdf

I will list here only the readings with anomalies. To see all the
Voice 8 and Voice 4 component readings download the PDF I made from
the link provided above.

Wherever anomalies occurred in the readings all other voices were
consulted to see which voice was indeed aberrant. On top of
documenting the issue with Voice 8 (with which there are many aberrant
readings), it appears I’ve also unwittingly discovered an incongruity
between Voice 4 and the other voices . . . though whatever this latter
issue is, I’ve not noticed any audible difference between Voice 4 and
the other voices.

I took all readings from left to right as I looked at the board in the
AX80 (component side up). If comparing my readings to the Voice PCB
image from page 61 of the Service Manual PDF they will be backward
(mirrored) as though I read them from right to left. In all instances
vertically mounted components were read from top to bottom.

Any two readings with the word “to” in-between means that the reading
fluctuated and I did my best to record the upper and lower value of
the fluctuation. Readings with N-off and N-on were used when playing a
note with the voice in question caused the reading to change. Where
there is no N-off and N-on values playing a note with the given voice
had no visible effect on the reading for that component.

I’m sure that there is a standard way of presenting all of this, and I
apologize for not knowing it . . . but I’ve done my best to be clear
and thorough and will be more than happy to answer any questions. I
apologize for the length of this email, but I imagine if I want to ask
for help I at least need to clearly supply as much information as
possible.


Voice 8 anomalies:

Resistors

R805  -0.003  /  -1.224
R405   0.000  /   0.124
Only voices 2 and 4 have a positive value on the 2nd reading for this
resistor. The average of the other voices (1, 3, 5, 6, 7, 8) is -0.800
. . . so Voice 8 is below average, but is at least a negative figure
like 5 of the other voices.

R819  2.158  /  -4.71
R419  0.043  /  -4.84
All other voices resemble the values of voice 4

R820  2.109  /  4.93
R420  0.043  /  4.93
All other voices resemble the values of voice 4

R821  2.793  /  2.115
R421  0.018  /  0.042
All other voices resemble the values of voice 4

R824  -10.00 to -12.00  /  -11.5 to -12.5 (fluctuating)
R424          O.L           /  -1.8   to   -3.0 (fluctuating)
All other voices resemble the values of voice 4

R825  0.60 to 0.90  /  0 (fluctuating)
R425  2.54 to 2.86  /  0 (fluctuating)
All other voices resemble the values of voice 4

R831  4.10 to 4.45  /  0.59 to 0.64 (fluctuating)
R431  1.70 to 2.00  /  0.24 to 0.29 (fluctuating)
All other voices resemble the values of voice 4

R832  0.011  /  0.58 to 0.63 (fluctuating)
R432  0.005  /  0.24 to 0.29 (fluctuating)
All other voices resemble the values of voice 4

R833  0.58 to 0.63  /  0
R433  0.24 to 0.29  /  0
All other voices resemble the values of voice 4

R836  0.49  /  1.91 to 1.88 (fluctuating)
R436  0.00  /  0.043
All other voices resemble the values of voice 4

R839  N-off -0.057  N-on 0.050  /  1.79
R439  N-off -0.053  N-on 0.058  /  7.32
In this case only Voice 6 was like Voice 8 and all other voices were
like Voice 4


Caps

C810  0.5 to 0.9  /  0
C410  2.5 to 2.9  /  0
All other voices resemble the values of voice 4

C812  0.52 to 0.58  /  -0.02 to 0.03
C412  0.24 to 0.29  /  -0.02 to 0.01
All other voices resemble the values of voice 4

C817  N-off 2.4 to 2.5  N-on 2.4 to 2.9  /  -0.02 to 0.02
C417  N-off 1.9 to 2.0  N-on 2.0 to 2.1  /  -0.02 to 0.02
All other voices resemble the values of voice 4

C818  0  /  0
C418  N-off 1.2  N-on 2.0  /  -0.82 to -0.85
All other voices resemble the values of voice 4


Transistors (I might very well have the legs labeled incorrectly. If
they are incorrect they will at least be incorrect consistently). As
they are mounted in the board I read them from top to bottom.

T804  D) 1.92  G) 2.6     S) 1.92
T404  D) 0.04  G) 0.04   S) 0.04
All other voices resemble the values of voice 4


Diodes

D805  2.55  /  9.81
D405  0.05  /  9.81
All other voices resemble the values of voice 4

D807  0.4 to 0.9  /  -9.0 to -11.0
D407  2.6 to 2.9  /  -1.8 to -3.0
All other voices resemble the values of voice 4


That's it for Voice 8.

As for the rarer aberrations I found between Voice 4 and the other
voices - they are as follows:

Resistors

I've listed this one in both places:
R805  -0.003  /  -1.224
R405   0.000  /    0.124
Only voices 2 and 4 have a positive value on the 2nd reading for this
resistor. The average of the other voices (1, 3, 5, 6, 7, 8) is -0.800


Caps

C809  -0.064  /  -04.82
C409  -0.064  /  -13.72
All other voices resemble the values of voice 8 in this instance


Transistors

T801  B) -0.009  C) -04.82  E) -0.003
T401  B) -0.002  C) -13.72  E)  0.092
All other voices resemble the values of voice 8 in this instance


Diodes

D806  2.29  /  -4.17
D406  4.60  /   5.06
All other voices resemble the values of voice 8 in this instance


That's all for now folks. Thanks so much for your time and interest!
This once dead AX80 is now almost 100% back on its feet and I can’t
wait to solve this last issue and get playing it!

Cheers,
James

On Sun, Jul 8, 2012 at 2:48 AM, Bob Weigel <sounddoctorin at imt.net> wrote:
>
> On 7/8/2012 1:38 AM, Veronica Merryfield wrote:
>>
>> Welcome to the list James!
>>
>> On 2012-07-08, at 12:23 AM, james meagher wrote:
>>
>>
>>>
>>> looking most suspiciously at IC704 (a 14 pin TC4011BP) and somewhat
>>> less suspiciously at IC802 (an 8 pin NJM4558D). I'm suspicious of
>>>
>>
>> 4011 is a quad two input nand chip. TC is the manufacturers code and BP is the type family.
>> The NJM4558D is a high current output op-amp.
>> Mouser should have both but from memory DigiKey don't carry the NJM4558 part.
>>
>>
>>>
>>> Again, if anyone wants a PDF copy of the AX80 Service Manual feel free
>>> to give me a shout. It's not the best PDF of an old service manual,
>>> but it's quite legible.
>>>
>>
>> Send it over.
>>
>> Veronica
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
>>
>>
>
> If anything is unclear I have a good hard copy also
>
> Haven't heard the problem you're talking there and too tired to figure it out til I finish some projects here.
>
> --
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>
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