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

[Adam Inglis]

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
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>>
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