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

Sat Aug 20 10:07:13 CEST 2011

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
>>>
>>>
>>>
>>>
>>>
>>
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>
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> -- 
> Harry Bissell & Nora Abdullah 4eva
>
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