Vintage Synth Repair

easterleggs wrote:
> I have repaired my Arp Solus.  To review, the Solus had a weak output
> with very apparent popping sound at key press and release.  Also, when
> the LFO was enabled to control both FM and PWM, the synth would start
> some mystery oscillation, the rate of which could be controlled by the
> Attack and Release sliders!  After ruling out other suspects (key
> contacts, trigger circuit, envelope circuit), I traced the problem
> back to the VCA, specifically a 4558 8-pin IC labeled Z20 in the
> schematic. 
>
> In addition, an "excellent, working Octave Cat" I recently purchased
> arrived with a broken switch, a malfunctioning VCO, and CV tracking
> problems (don't buy from donesha2007 on ebay).  The fix for the CV
> problem was also replacement of a 4558 IC (socketed components so it's
> pretty easy).
>
> So be suspicious of those pesky 4558's!  Is there something comparable
> of better quality?  Anyway, I hope this is useful to someone.
>
> --Stephen
>
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If I remember correctly the 4558 consists of 2 simple op amps, can't 
remember the number of the single unit but it is nothing special.  I too 
had oodles of trouble with the 4558 in the early days and stopped using 
them.  For performance the 72 series bi-fets are first class and I think 
I'm right in saying that the pin out is the same.  Specifically the 
TL072 or TL082 will do the same job, only many times better and they are 
super stable, but please check out the pin-outs as it is several years 
since I used either.

I believe the 4558 is 2 off 451!  'Orrible little beasts they were.

Regards

Brian G3OYU