[sdiy] Memorymoog VCO trim

[Barry Klein]

Anyone have unusual mods to the unit?
Bizarre modulation paths, external inputs...
Anyone experimented with replacing the CEM3360's with something else?
Noise circuit...
Looks like from what my son is fond of listening to I need to kludge in
an 8 bit digi-masher somewhere....  :-)
My keypad select for the C7/osc.? tuning process sometimes fails.  Is this
typical or a sign that a cap or two needs replacing?  I think I saw this on
a previous MM as well.

Barry


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Luís Marka
Sent: Monday, June 16, 2008 4:13 PM
To: dragon.servicing
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Memorymoog VCO trim

Hi folks,
When servicing my Memorymoog a few years ago I observed the instability 
in pitch was not always originated on the VCOs themselves, but from 
fluctuations in the CV feeding the VCOs. New connectors, IC sockets and 
trimpots across *all* boards sure helped stabilizing it. I would say it 
is a good idea to get the schematics and track the CV path from, say, 
the AM6012 IC and its associated S&H circuits all the way to the VCO 
ICs, replacing or cleaning connectors and such. It is also a good idea 
to replace +15V/-15V/GND connectors on the boards and associated wiring: 
at times, *touching* these connectors was enough to introduce pitch 
fluctuations!

dragon.servicing wrote:
> Hi,
> I think I read a while ago that the trimpots in the Memorymoog ( and 
> some other older synths ) can wear out causing unpredictable 
> performance. I did find this with a Minimoog I worked on a couple of 
> years ago. ( but the trim pots are probably a different type ).
> I think it was on the Memorymoog that the power supply trims were not 
> ten turn pots but had to be set very accurately ?
>  hopefully someone else can confirm this....
> regards Peter
>
>
> Barry Klein wrote:
>> Hi all,
>> I have been occasionally working on the MM+ I just purchased.
>> Trying to get 6 tuned oscillators but only get 5.  When I go back to 
>> check
>> usually I find one that is off by >2 digits from 7F.  Is that what would
>> cause the tune failure or something else?  The oscillator that is off 
>> is not
>> consistantly the same.  I also see a problem where every 6th note 
>> does not
>> play in this test state. 
>> This is the 3rd MM I have worked on and (as before) the trimpot 
>> adjustments
>> are damn touchy.  So much so that I wouldn't want to even bump one 
>> that was
>> tuned....
>> Is this due to old trimpots - do new ones make life much easier here?
>> Anyone tried installing digital pot IC's?
>> What improvements have you made to your MM's to either make calibration
>> easier and more stable or even just mods you have made to increase
>> capabilities?
>>
>> Barry
>>
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