Vintage Synth Repair

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!

--- 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

Do you have a multimeter or a scope?

--- In vintagesynthrepair@yahoogroups.com, "GB" <grantbt@...> wrote:
>
> Do you have a multimeter or a scope?
>

I actually have both although I don't really know how to use the 
scope.  My Dad had a bunch of old electronics stuff left over from 
when he went to school and he gave me a few old multimeters, power 
supplies, oscilloscope, soldering guns, breadboards, etc..

--- 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!

Measure the supply voltages when you first turn it on
(say after a minute) and after the thing konks out.

GB


----- Original Message ----- 
From: "benmeza" <benmeza@...>
To: <vintagesynthrepair@yahoogroups.com>
Sent: Wednesday, March 22, 2006 7:26 PM
Subject: [vintagesynthrepair] Re: got it working but now the sound dies
after a few minutes...

> --- In vintagesynthrepair@yahoogroups.com, "GB" <grantbt@...> wrote:
> >
> > Do you have a multimeter or a scope?
> >
>
> I actually have both although I don't really know how to use the
> scope.  My Dad had a bunch of old electronics stuff left over from
> when he went to school and he gave me a few old multimeters, power
> supplies, oscilloscope, soldering guns, breadboards, etc..
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>