[sdiy] Jp-8 head scratcher

[rtarcan]

Try to change sample hold capasitors of vco. Due to leaky capasitors , 
osilators  retune to wrong frequency during the autotune process.
I had similar problems with my Chroma Polaris.

On 08.11.2013 10:54, Maarten Halmans wrote:
> Hello,
>
>> Maybe try deliberately detuning those oscillators by the opposite  
>> amount in calibration mode, and see what Compu-tune does then? Might  
>> be a workaround!
>
> Nah, tried that. Oscillators are tuned again to the same/wrong 
> frequency. Cheers,
> Maarten
>
>
>> Adam
>> On 07/11/2013, at 9:14 PM, Maarten Halmans wrote:
>>
>>> Hello All.
>>>
>>>> So you have *only* the KCV input driving the VCO pitch ?
>>>> Even after it leaves auto-tune ?
>>>> And it still has an error, even if you move the voice card to a  
>>>> different
>>>> location ?
>>>
>>> Yes I disconnected all modulation sources and the problem still  
>>> persists! Moving the card moves the problem, so this rules out  
>>> wiring, module controller board, interface board and cpu board. I  
>>> moved the board serveral times because the problem I am having  
>>> points more to these boards than to the voice board. I also 
>>> replaced  the tuning multiplexers, cv summing amp, transistor array.
>>>
>>>> Could there be some kind of HF oscillation coming from somewhere  
>>>> that's
>>>> affecting the tuning ?
>>>
>>> I looked with my scope at the cv summing amp, tuning output of the  
>>> oscillator but I don't see any oscillations nor on other places in  
>>> the tuning circuit (sample&holds multiplexers etc).
>>> When I look at the output of the comparator which goes to the 
>>> counter I see no difference between working and bad channels. I 
>>> also  compared the output of the cv summing amp of the channels but 
>>> the  slew rate is the same. I stop the repair for now because I 
>>> keep  running in circles and keep testing the same things. But 
>>> suggestions  are still more than welcome.
>>>
>>> Best,
>>> Maarten
>>>
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>>
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