Vintage Synth Repair

It could also be that one of the top divide down oscillators is failing.
   Paul T


________________________________
From: Lorne Hammond <lhammond@...>
To: vintagesynthrepair@yahoogroups.com 
Sent: Sunday, May 19, 2013 11:43 AM
Subject: RE: [vintagesynthrepair] Hohner string melody logan

  
Sounds like one of your top octave oscillator boards is out of tune.  As they then go to divide down for each octave the lower octaves will be more like a chorus detune and less obviously “wrong” to some ears, mine included.
If it was an older Italian organ it would have a tuning pot about an inch tall for each of the 12 notes, each on its own circuit board.  By that time this was designed it might be happening at a single IC level with an overall external tuning on one board.  
 
If it has one per note tuning pots be very very gentle with them!  They break and are not easy to find for Farfisa’s.  
 
The very first thing I would do is get a frequency counter or a precise chromatic tuner.  Check each of the top twelve notes for pitch.  I suspect one is off.  Do you find 12 circuit sections and twelve tuning pots linked to those top twelve keys? Trace back from the key to that tuning pot.  Likely they will use colour wires.  Someone told me some might use the same colour pattern as resistor values.
 
If the logan/hohner does not have adjustments for all twelve, I would carefully look for mechanical issues.  Check the contacts of the offending note from the key.   Continuity trace it back to the board, eyeing solder joints and key contacts for corrosion, dirt, cold and cracked solder joints and mechanical difference with good notes.  Compare that with a “good” key.  
 
I would distrust any typical fault parts along that path, capacitors for one of course.  New tuning pots would help, but I try and avoid mismatches.  If there is only one tuning pot for everything then it might be that the oscillator generation of each of 12 top octave notes is done inside a single master chip feeding the divde down per octave chips.  Test the pins with the frequency counter to see if the problem is coming out of one pin.  If so, replace the IC.  Organ divide chips are easier to find than the sometimes complex tuning pots which in the Farfisa/Vox/Briscoe/GEM line are quite tall wire wrap things whose internal lubrication could age badly.  The names of divider chips are here.  It was a Logan design, Italian made, and licensed to Hohner.
http://en.wikipedia.org/wiki/Logan_String_Melody
I suggest searching for Logan schematics, they may be easier to find than the Hohner, and likely the same design.  The unit also uses Bucket Delay chips but those would be messing everything.
 
Lorne in Canada
 
From:vintagesynthrepair@yahoogroups.com [mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of letcista
Sent: Sunday, May 19, 2013 2:22 AM
To: vintagesynthrepair@yahoogroups.com
Subject: [vintagesynthrepair] Hohner string melody logan
 
  
Hello:
I got a Hohner String Melody I that has a problem.
When two different keys are pressed theres a kind of beating/detuning effect that in some case seems almost distortion.
This problem is most evident at the top octave.
When two equal notes are pressed the sound is normal.
Can someone point me a way to solve this problem?
Thanks and best regards.
José
Portugal