Repairing the old Farfisa Organ

[RB26DETT]

Hello! For some time I've been trying to repair a Farfisa Organ from 70s. 2 years ago the power supply blew and I managed to repair, but this year the power supply went bad again, this time was a blown fuse, so after investigating I discovered that the Power Amp section was shortcircuited. I found that one of the two big transistors that drive the speakers was completely fried. The problem came when I was unable to find the original part as it was a BD123 from Motorola. I substituted them by 2N3055 as they are compatible, but after mounting everything back again the organ is not working.

I've measured the voltages, and all I can see is that is 40V going on to the collector of the upper 2N3055 but the voltage on all the other parts of the circuit and transistors is 0V. I put the scope and the signal from the organ is arriving to the power amp, but it only goes through the first coupling capacitor and after the first transistor, it dies. (The board it even has a Bulb) hahahhaha

The power amp I'm pretty sure that follows this kind of schematic:



The organ from the outside it's one like this:



The Power Amp Transistors and the grey wire that carries 40V



The overview:



I suspect that there is a connection missing from the big transistor to the rest of the board. Some screws that hold the transistor are soldered to the PCB but the 40V is going to the middle of nowhere.



If any one has the smallest idea of how to fix this I will be very happy because I'm getting crazy.

[anotherjim]

Hi
The insulation on the power transistors looks very bad. They should be isolated from the heatsink by the mica washers. So it all needs a thorough cleaning with solvent such as isopropyl alcohol - and of course, test for isolation with your DMM. You will probably need to apply fresh heatsink compound too.

[amptramp]

Did all that corrosion come from a leaky electrolytic?  If not, there is a lot of contamination on that board that came from somewhere.

[RB26DETT]

Here I'm again! i replaced the mica insulation that was pretty bad and replaced the old transistors with the new ones. I also replaced the big cap, as it seemed it was in bad state... then I measured and it was ok, but as I have bought the new one I replace it. I don't know from where old the dirt come, but it doesn't goes away with the finger... so it's embedded in the PCB hahahaha. I measured the insulation and it's ok, but the problem I think is that the power is not going to the rest of the board. Here are some new fresh pics that I've taken.

Here is the screw that is at 40V, but as you can see is not connected to anything. On the other side, the other screw, is soldered and has a resistance attached to it so I suspect that  maybe ther is the problem



Here is the other side, the red circle with the grey wire is the +40V power supply. The blue circle, is the transistor that has a resistor soldered and goes to the rest of the board.





What am I missing? Any idea? It has nothing to do with dirt, I think. Neither isolation. Thanks for your help!

[duck_arse]

the red circled wire is connected to the collector of that transistor. C being common to the case in the to-3. and the other transistor has C to the flying resistor under, so you can work out which is which on your circuit. the metal tab on the blue circle I would think was thermal transfer for the ?Vbe multiplier? transistor.

you'll have to meter for continuity between the grey wire and the rest of the stuff connecting to the +40V rail. and you need the right circuit diagram.

[RB26DETT]

the red circled wire is connected to the collector of that transistor. C being common to the case in the to-3. and the other transistor has C to the flying resistor under, so you can work out which is which on your circuit. the metal tab on the blue circle I would think was thermal transfer for the ?Vbe multiplier? transistor.

you'll have to meter for continuity between the grey wire and the rest of the stuff connecting to the +40V rail. and you need the right circuit diagram.

I managed to find the original schematic at Farfisa.org! You're right The metal tab is a thermal transfer for the plastic case transistor, not a very good design. What I've found is that the grey wire is attached to the collector of the transistor that doesn't have the resistor. The meter says there is no connection but now that I've found the original schematic I think I will have to follow where is the missing connection.

[duck_arse]

R23 looks very likely.

[RB26DETT]

R23 looks very likely.

YES! It was R23 that was attached to the wrong collector! Now it's working perfect! But it gets pretty hot! Is that normal? It gets so hot that it even burns. Any thoughts? Thank you for the help!

[Digital Larry]

What's the trimmer doing?  I'm not analog enough to understand exactly what it is doing but it may set the quiescent current through the transistors and hence R23.  Resistors getting really hot is not a good sign.

[duck_arse]

the trimmer, along with Mr Ohm's law, set the bias for the output stage, indeed it does. if you do a search for "transistor amplifier bias setting" or like that, you will get better advice than I can provide. (once you've cleaned that trimpot track,) it is a fairly simple procedure, meter and screwdriver.

[anotherjim]

That looks a lot better
+1 Bias trim.

Tell me, is this organ capable of the famous "combo" organ sounds such as on early Pink Floyd records?
Or, is it a nasty cheese machine?

[pinkjimiphoton]

PLEASE make sure ya replace whatever electros are in there... this thing is 50 years old, and they are most likely bad.