Polymoog Issue

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Dr. Phibes
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Polymoog Issue

Post by Dr. Phibes » Fri Feb 01, 2013 10:44 am

Fancy such a thing happening...

Anyway, I've noticed recently that when two or more notes are played in the middle range (that is, octaves 3 and 4) a distinct crackling arises that gets successively worse with each extra note. After 4 or more notes are played the audio output degenerates to the point that only crackling can be heard. It is purely confined to the two middle octaves; playing two notes there plus one or more from the upper and lower keyboard does not produce the same result.

Interestingly, the problem seems to only affect those presets/sounds which make heavy use of the saw waveform. In addition, after an hour or so of warming up the problem improves slightly and chord can just about be played without too much distortion.

My theory is that one or more of the MM5823 chips on sawtooth rank of the divider board are failing, but I can't be sure of which. I switched the two HFO boards around and the problem persisted in its current form so I don't think the problem lies there.

As always, any help is appreciated.

_________________________________________________________________________________________________

Problem was at trim pot R108 in the drive limit circuit on the modulator/reference board. Incorrectly biased amp 24.
Last edited by Dr. Phibes on Fri Feb 08, 2013 11:23 am, edited 1 time in total.

Don T
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Re: Polymoog Issue

Post by Don T » Fri Feb 01, 2013 11:03 am

.....
Last edited by Don T on Sat Feb 02, 2013 11:57 pm, edited 1 time in total.
Don Taylor

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Dr. Phibes
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Re: Polymoog Issue

Post by Dr. Phibes » Fri Feb 01, 2013 12:14 pm

That's quite a novel diagnostic technique :). But I get what you're saying, thing is, I've just been fiddling with the keyboard and discovered that if I hold the ribbon controller 'up' across the entire range of the keyboard the problem disappears! So now I'm wondering if it's not something that the ribbon controller defeats when activated - surely if it was a divider chip the problem would just remain in the same place? All I can find in the manual is that the ribbon controller directly modulates the reference oscillators.

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bochelli
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Re: Polymoog Issue

Post by bochelli » Sat Feb 02, 2013 12:36 am

does not sound like a divider issue i always thought it is when its spread over the octaves, a polymoog gremlin to test your soul and patience.
yamoto773

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Re: Polymoog Issue

Post by Dr. Phibes » Sun Feb 03, 2013 3:03 pm

bochelli wrote:does not sound like a divider issue i always thought it is when its spread over the octaves, a polymoog gremlin to test your soul and patience.
Yes I think you're right. Upon checking the schematic of the divider board it seems that key allocation is not arranged in a per ocatave pattern.

I decided to check the sawtooth shaping circuits on the reference osc board - they're actually fairly simple in design. The associated components checked out so I don't the problem is there. However, adjusting sawtooth high and low there (with the variable resistors) has a profound affect on the problem; turn down the sawtooth level and the problem improves noticeably. As such, it does seem that something is failing vis-a-vis the sawtooth output. My attention has turned to the filter board now since I've noticed that a similar problem emerges when the moog filter is applied to the lower two octaves.

This is the Keyboard version by the way. To be honest, everything other than this last glitch now seems quite stable. I want to get it properly resolved before putting it up for sale.

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