[sdiy] saw core VCO tuning/scaling (Roland SH-2)

Sat Aug 20 11:09:24 CEST 2011

BTW, some quick links

http://www.adambaby.com/images/tech%20pics/SH_2/sh2vco.html

http://www.adambaby.com/images/tech%20pics/SH_2/ua726.html

On 20/08/2011, at 6:07 PM, Adam Inglis wrote:

> Thanks Magnus, Harry and list
>
> I've spent a few more hours looking at this..
> After a full calibration after 30 min warm up, VCO 1 tracks and  
> tunes nicely with VCO 2. After 2 hours, the highest notes are still  
> in tune, but the lowest are sharp by a full tone (e.g. from 175 hz  
> to 198 hz). After 4 hours the highest notes are just slightly  
> sharp, but the lowest are about the same, a full tone sharp.
>
> DMM measurements at the output of the CV summing opamp show barely  
> 20mV change over this time, at various octaves, and the voltage  
> differences remain linear. The off-board trimmer in the feedback  
> loop ("VCO width") and it's contacts are sound. The "Range" voltage  
> divider and it's opamp scaler that connect to the summer show no  
> variation in voltages hot or cold.
>
> The "temp adjust" pin, no.6, of the uA726 shows -6.75 V for VCO1,  
> and -6.54 V for VCO 2. I haven't yet measured the current here -  
> it's a bit of a fiddly old board, looms just soldered straight on,  
> making access a bit tricky.
>
> Questions, given the above
> 1) Can I assume that the problem is NOT in or around the CV summing  
> circuit, given the stability of it's voltages over time/heat?
>
> 2) Does the low frequency drift point to a typical problem?
>
> Thanks for any advice or comments
>
> cheers
> Adam
>
> On 19/08/2011, at 11:22 PM, Harry Bissell wrote:
>
>
>> I would make the assumption that the uA726 is OK and the trouble  
>> lies elsewhere.
>> No need to risk changing this chip until the very end.  If  
>> possible, measure the heater
>> current drawn by the two chips and see if it is similar.
>>
>> There can be other components that can drift with temperature...
>>
>> H^) harry
>>
>> ----- Original Message -----
>> From: Adam Inglis <21pointy at tpg.com.au>
>> To: *SYNTH DIY <synth-diy at dropmix.xs4all.nl>
>> Sent: Thu, 18 Aug 2011 23:10:11 -0400 (EDT)
>> Subject: Re: [sdiy] saw core VCO tuning/scaling (Roland SH-2)
>>
>> Oh dear.
>>
>> I just plugged "uA726" in to the Synth DIY archive...
>>
>> I had no idea my little SH-2 contained such unobtainium ... (been
>> with me since almost new!). I could be in trouble!
>> However there does seem to be some info on making the change over to
>> a tempco based solution - I'd better start reading.
>>
>> Pin 6 is the temp adjust pin - it connects to a metal film 120k
>> resistor to ground.
>> The voltages on the various pins seem to roughly match between the
>> two VCOs - but I take it that this may not necessarily reflect chip
>> performance?
>>
>> Adam
>>
>> On 19/08/2011, at 9:38 AM, Adam Inglis wrote:
>>
>>
>>
>>> Magnus, many thanks
>>>
>>> The Roland SH-2 SM PDF can be found here..
>>>
>>> http://manuals.fdiskc.com/flat/
>>>
>>> The VCOs are on page 4
>>>
>>> Thanks for your time - I'll see if I can find a pin-out to find
>>> this "oven-setting resistor"
>>> cheers
>>> Adam
>>>
>>>
>>> On 19/08/2011, at 4:58 AM, Magnus Danielson wrote:
>>>
>>>
>>>
>>>
>>>> Adam,
>>>>
>>>> On 18/08/11 09:23, Adam Inglis wrote:
>>>>
>>>>
>>>>
>>>>
>>>>> I realise this is probably a timeworn query - my apologies to the
>>>>> veterans.
>>>>>
>>>>> I'm having some trouble with my SH-2 (dual VCO monosynth 1979):
>>>>> I've had it up on the bench a couple of times to calibrate VCO 1.
>>>>> VCO 2
>>>>> comes up sweet.
>>>>> I leave it on for an hour, follow the SM routine ("width", then
>>>>> "linearity" then footages) and think it sounds good, check it  
>>>>> with a
>>>>> tuner, but after I put back in the rack it goes out (of tune but
>>>>> also
>>>>> scale width) after an hour - so I guess it is temperature related.
>>>>>
>>>>> The key CVs are spot-on, and the supply voltages seem solid. VCO 2
>>>>> remains fine. Measuring the voltages after the CV summing op-amp
>>>>> gives
>>>>> equal values between octaves. From that point, I'm not sure what
>>>>> to look
>>>>> for with my ancient scope and my Fluke - it's current-dependant,
>>>>> is it not?
>>>>> The expo converters are metal can uA726 - they both run quite
>>>>> warm to
>>>>> touch.
>>>>>
>>>>> I've read Ian Fritz's and Magnus Danielson's info regarding these
>>>>> type
>>>>> of Electronotes-based VCOs, but if anyone can point me to some
>>>>> troubleshooting articles, that would be most appreciated,
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>> OK, by strange coincidence I have been discussing troubleshooting
>>>> of uA726 based oscillators today, and then my name even pops up
>>>> here, and it can't be recent I wrote much about the topic. Ah well.
>>>>
>>>> The uA726 is unique in that it contains an oven. First thing to
>>>> check is if the oven is running and if the oven setting resistor
>>>> is properly hooked up. There can be other sources of this
>>>> instability.
>>>>
>>>> I haven't checked the SH-2 schematic, but if someone toss me the
>>>> link to a good schematic I might help a little more.
>>>>
>>>> Cheers,
>>>> Magnus
>>>> _______________________________________________
>>>> Synth-diy mailing list
>>>> Synth-diy at dropmix.xs4all.nl
>>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>>
>>>
>>>
>>>
>>>
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
>> -- 
>> Harry Bissell & Nora Abdullah 4eva
>>
>>
>>
>>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>
>