Farfisa Mini Compact Note problem

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Rufer
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Farfisa Mini Compact Note problem

Post by Rufer » Sat Mar 19, 2011 5:26 pm

Hi there,
I picked up a farfisa mini compact today and it is physically in great condition. It sounds great too--with the exception of all of the D notes. A sound is produced when I hit the note but very dischordant and rapidly pulsating.

I am pretty sure I shouldn't do this but I pulled the D oscillator card and after doing some initial examinations, I fired up the organ. If I run my fingers along the back of the pcb card while playing a D, I can get the note to sound appropriately if I touch certain spots. I have to be making contact in more than one spot. Sometimes the note will be reproducable after stop making contact with the card. If I continually run my fingers along the back of the card, it will cycle through a whacky series of tones.

Hopefully someone can help me figure out how to proceed.

Thanks.

crow
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Post by crow » Sat Mar 19, 2011 7:37 pm

When I acquired a somewhat functional Mini Compact a few years ago, I received a lot of help in this thread:

http://messageboard.tapeop.com/viewtopi ... ht=farfisa

If you're causing the note to sound by touching the circuit, pay attention to whether or not the "D" is sounding in the correct octave. As you may know, those cards work by oscillating at the frequency of the highest octave and then dividing the frequency for each lower octave. you may notice that the top two "D"s sound OK, but those below sound bad, or some such similar symptom.

My "C" card wasn't working (but hey, who uses "C"?) and I eventually got it going by replacing transistors in the main oscillator circuit. I bought NOS germaniums off of ebay, but recently a friend told me that you can use silicon without any problem. I have not tried this, and don't know if it's true.

Also, I was able to drastically reduce the "All keys at once" whine endemic to all transistor organs I've heard by recapping the power supply. I tested it by alligator clipping a cap in parallel to the main filter cap and I was ecstatic to hear the whine almost completely disappear.

Good luck; I think you'll find a lot of good info from the experts in the aforementioned thread.

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Post by Rufer » Mon Mar 21, 2011 9:11 am

Thanks, crow. All good info--what you provided and through the link. All of my D's are bad, so I think my initial problem is in the main oscillator too. I found a schematic online--and am trying to figure it out.

I may be back with some follow-up questions. The only electronics work I have done (successfully) was a Build You Own Clone fuzz box--so yeah, chances are good I'll be back.

The vibrato has also stopped working too which is kind of a bummer because it worked great the first few hours I had it. I may want to get that sorted out too before I button everything shut.

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Post by SureShot » Tue Mar 22, 2011 12:59 pm

the farfisa's are pretty easy to fix, i've fixed a few in the past. Like any combo organ the sound is produced by a fixed oscillator for every note (12 in total) and all the harmonics are produced by octave dividers. That means that only the highest note is made and all lower notes are derived from that.

On all of the farfsa's i've fixed that had note problems, it was one of the voltage divder transitors that went bad. As far as i can recall i could just replace the pnp germanium transistors (sftXXX) with standard silicon transistors.

if you want all original then i still have a junked farfisa compact with all oscillator cards. So i can easily supply you with a bunch of germanium transistors.

+1 for replacing all lytics.

greetings,

Thomas
when the power of love overcomes the love of power,
the world will know peace.

- Jimi Hendrix -

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Post by crow » Tue Mar 22, 2011 5:23 pm

I can't remember if it's in the old thread, but in case you haven't been there yet, http://www.combo-organ.com is a great resource for this sort of thing, especially since the service manual for the Farfisa Compact is to on their free schematics page:

http://www.combo-organ.com/Schematics/index.htm

Enjoy!

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Post by Rufer » Thu Mar 24, 2011 8:33 am

Thanks to the both of you for your help. SureShot, I do plan on replacing all of the electrolytic caps. In the thread that crow linked to, someone with a similar problem to mine found that replacing a non-electrolytic did the trick. But yeah, I also plan on replacing some of the transistors too--likely with silicon versions. I've read in multiple places that those work just as well.

crow thanks for linking to that schematic. That one is for the Compact. I have a Mini Compact. Might be very similar. Either way I was able to find a schematic for the Mini. Which brings me to my questions....

As I mentioned before, I am very new to doing this type of stuff. In fact, I am brand new to reading schematics but this one seems pretty easy for me to grasp. There are a few mysteries for me though.

The components I have circled in orange--are these capacitors? Are they the C#B's I see in the schematics? I can figure out the value of each C#B using the table, but if what I have circled on the picture indeed are capacitors the type is new to me. They are each labeled 'mial 1000 5%'.

Also, is that cluster of capacitors circled in green, what I see in the columns 'A to B'?

Image

Image

Image

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Post by Rufer » Fri Mar 25, 2011 8:13 am

After further, inconclusive research, I think those parts circled in orange are polystyrene capacitors. Does this seem right?

I am still confused as to why they are labeled 'mial 1000 5%' when the schematic indicates differing values. Maybe those labels are not values?

Thanks for any help.

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Post by Scodiddly » Fri Mar 25, 2011 10:58 am

Those should be capacitors, yes.

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Post by Rufer » Fri Mar 25, 2011 12:09 pm

Thanks. Would it make sense for caps of different values to be labeled 'mial 1000 5%'?

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Post by Scodiddly » Fri Mar 25, 2011 6:32 pm

Maybe there's a color code or something going on. I don't remember ever looking at those caps super-close. If they're some variety of poly then it's unlikely they've gone bad.

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Post by winky dinglehoffer » Sun Mar 27, 2011 7:08 am

Those are polystyrene caps. Mouser carries them. I don't know that they'd be your problem (if I were looking for suspect caps, I'd look at electrolytics and/or tantalums first), and be aware that you want to be careful soldering them--they can melt.

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Post by Rufer » Thu Apr 07, 2011 6:46 am

Thanks everyone. After some research, the first thing I did was swap out the transistor in the lower-left corner of the picture above. Magically it did the trick.

I have a couple of lesser issues I want to (now more confidently) deal with before buttoning this back up. I am tempted to do so without replacing the electrolytic caps that don't seem to be problematic.

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Post by crow » Sat Apr 09, 2011 8:21 am

Awesome! I don't remember if this is in the older thread I mentioned, but when I fixed up the power supply in mine, I evaluated the big filter cap by alligator clipping another cap in parallel with it. The hum & whine went away, so I went ahead and replaced it. That way, you can perhaps get a sense of which caps would yield the results you want if you replace them.

Scodiddly, experts one and all, is this an unwise method for troubleshooting caps? It sure worked well in this case, so it's tempting to try this again in my maintenance adventures.

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Post by Scodiddly » Sat Apr 09, 2011 4:40 pm

crow wrote:Awesome! I don't remember if this is in the older thread I mentioned, but when I fixed up the power supply in mine, I evaluated the big filter cap by alligator clipping another cap in parallel with it. The hum & whine went away, so I went ahead and replaced it. That way, you can perhaps get a sense of which caps would yield the results you want if you replace them.

Scodiddly, experts one and all, is this an unwise method for troubleshooting caps? It sure worked well in this case, so it's tempting to try this again in my maintenance adventures.
It's a valid technique. Just make sure that you remove the old cap when do you the actual fix.

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Post by crow » Sat Apr 09, 2011 9:34 pm

Thanks!

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