[sdiy] Prophet 5 scale problem

Thu Nov 17 01:29:28 CET 2016

The DAC from Taiwan turned out to be a dud. Grrr..

Before I take Peter up on his gracious offer, can I just ask the list, has anyone here dropped a different DAC in to a Prophet, and done the attendant mods? Is it straightforward? This is out of my sphere of experience.

I note in the data sheet the COB version has a +/-10 volt output, whereas the stock CSB is 0 - +10 volts, is that a problem?

many thanks 

Adam

> On 17 Oct 2016, at 9:29 PM, Ullrich Peter <Peter.Ullrich at kapsch.net> wrote:
> 
> Hi!
> 
> I have 8 unused (NOS) DAC71-COB-V that I don't need.
> They have never been used - so they should be OK.
> Stored on antistatic foam for years.
> 
> 10€ per piece + postage 4.90€
> 
> Datasheet: https://www.usbid.com/datasheets/dsid/166157.pdf
> 
> This is the version with the voltage output where the service manual says that
> You should bypass the external opamp.
> 
> COB is the version with the 10k feedback resistor instead of the 5k resistor.
> According to the diagram on page 3 it should be possible to connect another 10k resistor in parallel 
> to the internal 10k resistor (mount this resistor between pins 17 and 21)  so that you get the same 
> output level as with the CSB-version.
> 
> Ciao
> Peter
> 
> -----Ursprüngliche Nachricht-----
> Von: Synth-diy [mailto:synth-diy-bounces at dropmix.xs4all.nl] Im Auftrag von Roman Sowa
> Gesendet: Montag, 17. Oktober 2016 11:13
> An: synth-diy at dropmix.xs4all.nl
> Betreff: Re: [sdiy] Prophet 5 scale problem
> 
> At that price I would go for DAC8820 and adapter PCB.
> 
> Roman
> 
> W dniu 2016-10-17 o 01:07, Adam Inglis pisze:
>> This guys page would suggest that a faulty DAC is a possibility:
>> 
>> http://www.mindburner.co.uk/synth/prophet5_DAC%20Information.html
>> 
>> Looks like these sell for around 60 - 100 $
>> 
>> I’m starting to think this could be the problem. Going through the VCO scale trim procedure as per the manual shows each VCO can be tuned manually, but as soon as the computer re-calculates the bias, they go out again.
>> 
>> AI
>> 
>>> On 16 Oct 2016, at 4:53 PM, Barry Klein <barryklein at cox.net> wrote:
>>> 
>>> 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
>>>>>>> _______________________________________________
>>>>>>> Synth-diy mailing list
>>>>>>> Synth-diy at dropmix.xs4all.nl
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