Vintage Synth Repair

Hi Roy,

Thanks for the info.  The synth must've heard me, cos the day after I
posted here the problem disappeared.  It was apparently working
flawlessly before it was shipped across Canada, so I guess it didn't
like the journey!  I still hear the odd crackle now and then but it is
much much better.  

I've made a note of your post for future reference.  I have to track
down a service manual before I can check the FETs for substitutions,
etc...  but it's nice to know where to start for a long-term solution
if it becomes necessary.

Thanks again!
-Josh


> 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"
> -
> Information is more dangerous than cannon to a society ruled by
lies. --James 
> M Dakin
>