Vintage Synth Repair

Hello, 

If there is anyone out there who has repaired and tuned a Micromoog
I would appreciate some advice.

Everything works fine on mine except its OUT OF TUNE.

I HAVE a copy of the ORIGINAL SERVICE MANUAL and I have been following
the tuning procedure. On page 4-2 when I get to section 4.2.1.3 I run
out of adjustment with the OSC HI trimpot. I've played around with
adjusting the OSC RANGE and OSC HI trimpots but again I run out of
adjustment.

So basically I cant get past page 4-2 of Micromoog manual when trying
to tune it.

On page 5-4 of the troubleshooting part of manual there is a
suggestion to replace certain trimpots and ICs. I've planned to
replace many of the components to fix the OSC tuning problem and
replace some as preventive maintenance. 

Has anyone else replaced their Micromoog ICs, trimpots, etc. to
correct OSC tuning problems? If so I would like to know where to start
and what typically should be replaced.

Any advice is appreciated. 

Br
Allan

>>I run out of adjustment with the OSC HI trimpot. I've played around
with adjusting the OSC RANGE and OSC HI trimpots but again I run out
of adjustment.<<

Allan,

it's a long shot- I only really know the later models (rogue, prodigy
& source, & of course my beloved LAMM memorymoog) but: 

is there a component in the vicinity of the oscillator that's supposed
to track temperature changes? 
I once had a korg mono/poly in bits, & each of the four SSM oscillator
chips had a thermistor badly glued to it's top surface. the idea was
to temperature-compensate the IC, but it fails rather spectacularly if
the thermal device & the chip aren't properly coupled. 
this can happen when the glue goes brittle & cracks, leaving you with
an air-gap & no temp-comp. good temperature compensation is essential
for accurate V>Hz tracking at the higher frequencies, though the
errors can go un-noticed below 1000Hz or so.

there may be the vaguest reference to it on the schemo- as I recall,
it's a regular old-school resistor symbol (zigzag) with a dot next to
it & a circle round the whole lot. I may be wrong on this... but in
any case, the actual object is likely to be quite obvious on the PCB.

hth-
duncan.

Hi Duncan,

Thanks for the feedback.

I also heard that replacing the trim pots is the first thing to try.
So I will try this.
I've been told that cerment track type trim pots are what to use. If
the trim pot is not the trick then also the replacement of any
surrounding compontents of the trim pots that are out of range should
help.
Then I was advised to replace the tantal and electrolytic capacitors
of the power supply as a preventative service measure.

There is a component in the Micromoog for temp comp. I will look at
replacing it a as well.  

Overall my Micromoog is stable. I does not go way out of tune or
change much. I can get one or two lower octaves in tune (and they stay
in tune). But I can't get beyond that...(upper octaves in tune).  

So I will give these tips a try. I'm confident one of the above
repairs will be the solution :)

Br
Allan



--- In vintagesynthrepair@yahoogroups.com, "duncan" <ferrograph@...>
wrote:

>
> >>I run out of adjustment with the OSC HI trimpot. I've played around
> with adjusting the OSC RANGE and OSC HI trimpots but again I run out
> of adjustment.<<
> 
> Allan,
> 
> it's a long shot- I only really know the later models (rogue, prodigy
> & source, & of course my beloved LAMM memorymoog) but: 
> 
> is there a component in the vicinity of the oscillator that's supposed
> to track temperature changes? 
> I once had a korg mono/poly in bits, & each of the four SSM oscillator
> chips had a thermistor badly glued to it's top surface. the idea was
> to temperature-compensate the IC, but it fails rather spectacularly if
> the thermal device & the chip aren't properly coupled. 
> this can happen when the glue goes brittle & cracks, leaving you with
> an air-gap & no temp-comp. good temperature compensation is essential
> for accurate V>Hz tracking at the higher frequencies, though the
> errors can go un-noticed below 1000Hz or so.
> 
> there may be the vaguest reference to it on the schemo- as I recall,
> it's a regular old-school resistor symbol (zigzag) with a dot next to
> it & a circle round the whole lot. I may be wrong on this... but in
> any case, the actual object is likely to be quite obvious on the PCB.
> 
> hth-
> duncan.
>