[sdiy] Prophet 5 scale problem

Sun Oct 16 08:53:17 CEST 2016

I wouldn't think the trimpot would normally be expected to set near the end 
of travel.  I'd look closer there.  Maybe someone else here knows if that's 
normal.
Could be a bad 4051 downstream from the DAC output (also one in Tune 
circuit) but a single turn crap trimmer here seems a bad idea.  Does it look 
original? Maybe someone couldn’t find a 20T and put a 1T in?  Maybe even 
wrong value...?  Also it says VCO bias voltages are designed to be around 5V 
when in tune - you could check to see if they all are too far off one way or 
another.

Barry

-----Original Message----- 
From: Adam Inglis
Sent: Saturday, October 15, 2016 9:40 PM
To: synth-diy DIY
Subject: Re: [sdiy] Prophet 5 scale problem

> On 14 Oct 2016, at 8:13 AM, Barry Klein <barryklein at cox.net> wrote:
>
> Do you have the service manual for the P5?
> How did the DAC gain adjustment/test go?   (20T trimmer for 723 ic37)
>
> Barry

Thanks Barry
The DAC figures are in the ballpark, but I can only get one decimal place 
accuracy, not three as suggested in the service notes, before I hit the end 
of the trimmer. The  trimmer on this board (a rev 3.2 later upgraded to 3.3) 
is a flat single turn thing.

> I am having trouble following your problem. Are you saying the that this 
> 'random' voltage is the computuned bias voltage for each VCO? Is there 
> anyway you can temporarily disable this 'correction' voltage say for one 
> voice and see if the problem goes away?
>

Thanks Tony, good idea. I *think* computune is disabled in the Scale Trim 
test mode, but I’m getting very strange results that I’m trying to quantify 
(it seems at least 3 of the 10 VCOs are way off tune in this mode, the 
others are all close, not sure whether that is just telling me what is 
already happening). There does not seem to be a way of disabling computune 
on a per voice basis.

The 5 volt supply on the CPU/DAC board is less than stellar, around 4.88v, 
but the DAC, muxes and related circuits use the +/- 15 v supplies anyway.

> Which method are you using to tune, if you don't mind. lissajous, strobe 
> tuner, freq. counter, guitar tuner, beating and 'stretch tuning' ?
>
> TK

I’m using a Korg instrument tuner.

Investigations are continuing

cheers
AI

>> On Oct 13, 2016, at 4:01 AM, Adam Inglis <21pointy at tpg.com.au> wrote:
>>
>> Thanks Michael
>> I had a look at the output of the buffers for the SH circuits after the 
>> 4051. I’m looking for stable voltages here, yes? They all seem OK.
>> As I understand it there is no exact right voltage for reference here, as 
>> the CPU calculates a bias that will be different for each of the 10 VCOs.
>>
>> I should have been a bit more precise - the problem exists only at 
>> oscillator frequencies below high C - about 520 Hz. Wherever I pitch the 
>> oscilaltors across the keys, the problem follows the frequency. It is 
>> irrespective of the key CV.
>>
>> 2 things occur to me:
>>
>> 1. The P5 uses the upper 7 bits of the DAC for the voltage controllable 
>> voice functions, only the individual VCO pitch CVs use all 14 bits.
>>
>> 2. If the tuning mux (4051) or it’s comparator were faulty, the CPU may 
>> read the frequencies incorrectly during computune, thus generating 
>> incorrect biases, hence throwing out the scale accuracy.
>>
>> ?
>>
>> AI
>>
>>
>>> On 13 Oct 2016, at 10:18 AM, Michael E Caloroso 
>>> <mec.forumreader at gmail.com> wrote:
>>>
>>> You do not mention if you probed the key CV path fanning out from the
>>> 4051 with a 'scope (DMM isn't enough).  That should tell you a lot.
>>>
>>> MC
>>>
>>>> On 10/12/16, Adam Inglis <21pointy at tpg.com.au> wrote:
>>>> I have a P5 here that seems to be working OK except that the lower 
>>>> ranges of
>>>> the 2 VCOs for each of the 5 voices are randomly off scale/out of tune
>>>> across the keys.
>>>>
>>>> It seems to correspond roughly to a key CV lower than 3 volts with the 
>>>> VCO
>>>> pitch pots at their mid-range (as evidenced by the CV out jack). Above 
>>>> this
>>>> both VCOs track well in tune and scale. If I pitch both VCOs to their
>>>> highest range, the top 4 octaves of the keys are in tune, just the 
>>>> lowest is
>>>> out. Doing the inverse, most of the keys are out of scale except the 
>>>> top
>>>> octave.
>>>> It does appear to be all 5 voices affected, so I’m having trouble 
>>>> narrowing
>>>> it down to one chip, say e.g. a CV demuxer 4051, which are a common 
>>>> problem.
>>>>
>>>> All 14 bits of the DAC are pulsing.
>>>>
>>>> Further tests are obviously needed, but any clues greatly appreciated.
>>>>
>>>> AI
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