Vintage Synth Repair

--- In vintagesynthrepair@yahoogroups.com, "synth47good" <crm@...> 
wrote:
>
> --- In vintagesynthrepair@yahoogroups.com, "benmeza" <benmeza@> 
> wrote:
> >
> > I just joined a band where we had this tempramental old Farfisa 
> > Matador.  When it works it's a great light version of the reedy 
> Farf 
> > sound, but the thing would produce loud hum or just stop working 
> > most of the time.  One day it just quit all together and didn't 
> work 
> > again.
> > 
> > I took it home, opened it up and saw some pretty gnarly old caps 
> > that were starting to "explode" at the ends as well as some 
> > resistors that appeared to have leaked and disentigrated making 
> the 
> > values unreadable.  I bought the schematic for this old beast 
and 
> > went through and replaced every part that showed visible sign of 
> wear
> > (a bunch of small electrolytics in the tone section & a couple 
of 
> > resistors), and every cap(as well as a couple of resistors) in 
the 
> > power section.
> > 
> > A little more investigating finds two ground lines from other 
> parts 
> > of the keyboard that had come unsoldered from the ground wire 
that 
> > attaches to the pinout on the tone control section.  I resolder 
> > those points, close the machine up and give it a whirl.  
> > 
> > Success!  No hum, and I got a perfectly good Farfisa sound that 
> just 
> > needs to be tuned....until the volume starts to decrease after a 
> few 
> > minutes and quickly there is no more volume to it.  Hmmmm...
> > 
> > I open it up again, discharge the caps, and poke around again - 
> > everything is connected properly...I marked the postive & 
negative 
> > ends directly on the fiberboard BEFORE I removed the old 
> > electrolytics...  I turn it on again and it works again for a 
> minute 
> > or two and then the sound decreases to nothing again.  Unplug 
it, 
> > discharge the caps, plug it back in and same thing...
> > 
> > At this point I am thinking there is some cap in there that 
isn't 
> > functioning properly but I don't know how to tell or where to 
> start 
> > troubleshooting.  I have a very basic knowledge of electronics, 
a 
> > full schematic, a good multimeter, and a broken keyboard.  
> > 
> > Can anyone run me down the checklist I should use to 
troubleshoot 
> > this?  I really appreciate it.  I hate to take it to a tech but 
if 
> > you guys know of a good one in Austin/Central texas I will take 
it 
> > there if no one can guide me out of the woods.  Thanks again for 
> all 
> > your suggestions!
> >Changing the filter caps and resistors in the power supply is a 
> great first step.
> Next thing that I would check are the diodes that rectify the 
power 
> supply.I have an old Wurlitzer portable organ from the same 
general 
> period and I had a similar problem yrs.ago.I changed the old white 
> barrel-style diodes and the thing came back to life.Hard to 
believe 
> these diodes ever went bad because they were rated somewhere 
around 
> 250v. at 2-3amps!If you have a digital-meter with diode-check 
> capability,great.you should read something around .7v going one-
way 
> and infinite with reversed leads.If you read something like .542  
> testing in the opposite direction,then the diode is more than 
likly 
> bad.When turned on,these diodes heat-up for a minute or so and the 
> organ works and then they cut-out and you will get a little pitch-
> drift-up and then zip.
>  Hope this is of help to you,
> Robby Matthias
>

Good advice Robby - I'll take a look at that tonight and see if they 
are bad.  Thanks!