Vintage Synth Repair

On Tuesday 13 June 2006 08:29 pm, Josh wrote:
> Hi all,
>
> I recently acquired a Yamaha CS5.  It sounds great, except there's a
> random crackling sound in the output signal.  I thought it might be a
> problem with the keys at first, but I even hear it if I have "initial
> level" turned up so that the oscillator sounds without a keypress, and
> also if I have nothing but the input signal (drum machine, etc...)
> turned on.  There are a few crackly pots, particularly in the amp
> section, but they've been improving with use.
>
> I've checked the output jack for bad solder joints -- it looks fine.
> Cleaned all ins/outs with contact cleaner.  Had a bit of a clean
> inside the case and removed a ball of lint that was right behind the
> output socket, but even this wasn't the problem.
>
> If anyone has an idea what might be the root cause this will help me a
> lot.  I'm a long way from anyone that will repair these things
> (central B.C., Canada), so if it's something I can fix myself I'd like
> to sort it as it's doing my head in.
>
> P.S. the boards inside are soldered directly to the pots, so all the
> components are tricky to check without unscrewing the pots from the
> main panel.  There's no sign of bad solder or corrosion on the back of
> the boards, however.  All in all it looks pretty clean in there, and
> there are no other sound defects such as wobbles in pitch or anything.
>  Also, I've tried several speakers, cords & headphones, all with the
> same problem.  The power-cord is plugged into a surge protector.

I don't recall the details of this model,  and my pile of their service data 
isn't currently accessible to me,  but look at what they're driving the 
outputs with.  I ran across the same problem in another instrument,  and it 
turned out to be bad FETs at the outputs (the instrument I'm thinking of had 
nothing but FETs in it,  nine of them altogether if I'm remembering right).  
The part in question in that unit was supposed to be a 2SK30AY,  and in some 
of the cases there had been substitutions made -- an ECG part with a 25V 
rating had been put into a circuit that was running off a 35V supply,  so 
they didn't last too long.  Another subsitute listed was another ECG part 
that turned out to be a different package,  and was a bad cross,  metal can 
with 4 leads instead of TO92,  and a 50-volt part.  The Yamaha tech support 
at that time told me that their original parts were selected for 90V 
ratings...

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
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