Vintage Synth Repair

--- 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