Spring Reverb For Farfisa Organ

[sarakisof]

Was the organ fully working before it got damaged?

Yes after had it repaired back then (full recapping, changed tubes, key contacts cleaning, new power cord stuff like that, electronics were ok) it was working fine.
I would do the "either direction swapping" test when go home.

UPDATE:

I swapped - reversed rvrb input/output sides (no effect produced as well), BUT by tapping on springs splash sound  was louder and stronger at the "good" cart side (wich now was the receiver/output) than at the "bad" cart side (which now was the sender/input)   

So:

1. White cart as input  -  Black cart as output (original wiring) ---> normal / medium splash sound at both sides.
2. White cart as output  -  Black cart as input  (reversed wiring) ---->  loud / strong splash sound at White  and  normal/medium at Black side.

Smthng else i noticed: i don't get any voltage at either carts pins / rvrb in and out zero voltage 
Ohm resistance between pins:  White: this " 1" at multimeter (like it cannot read/infinity?), Black: 3.8 ohms

[PRR]

A ceramic element should read infinite ohms. 4r sounds like a poor short.

[sarakisof]

PRR i did the resistance measurement while carts were connected back to the circuit via turret connector pins on tank as it should. You wrote this above and made wonder. So i disconnected carts and they both measure infinite ohms as it should.
So then did a continuity test between pin turrets that cart's pins are connected with amp circuit.
White cart no beep , Black cart beeps!!  Signal wire is shorted with gnd wire. So now i have to look backwards tube chassis to find out.
BUT: before doing so, could it be normal that one side in/send or out/receiver (cannot remember now) of reverb unit is shorted?? I am asking this because i know some tanks input signal and gnd are connected. I recently bought a Accutronics 4AB3C1B  and its input rca signal-gnd beeps too. This "C" has smthng to do with this i think but cannot remember now (input insulated or smthng).
So maybe it is normal for Farfisa's tank too.
I have to solve this before get my hands on lethal 

UPDATE: Now i found out about C type insulated input tank, i guess this has not to do with my case, right?

[sarakisof]

Here are some pics with the path. Wire is the black shielded one, with white colour signal and gnd shield wire.
Red colour by me =short
Bad thing is that there is no scem of Foyer anywhere on net (plz dont search its common known that doesnt exist other guys try to do adaptations from Compacts  and other models)

This is the brown connectors/"turret pins" which connect circuit with the two pins of Black cart.


From there, the black wire  goes to brown connectors near speakers ("central/heart" connectors) as the red arrows show.
And from there, one black wire goes down to circuit into the chassis and another black one to the upper organ next to oscillator/vibrato cards, as shown.


I havent got on to the chassis yet so i didnt follow the "down" wire (there must be the problem though), just followed the one that goes upper, you can see it here


And it ends connected to those brown connectors next to vibrato card under faceplate, alongside with other basic connections there, such as input signal, power etc.



I tried to get into simple things first that i can easily see and isolate one by one and the prob is not there (no short inside wires or smthng)
I like dig into things steo by step.

So, as supposed, the problem might be under the chassis which i ll dive in by tomorrow i guess. Hope to find the short easily. ufffffff

[sarakisof]

Well, spent the whole night trying to figure out where that short comes from, isolating all possible paths one by one, to find out the obvious that the short was simply there when Rvrb tab was off and by switching to on we have the normal circuit so the rvrb to be functioned  , obviously ...

Feel tired and really disapointed as i start from point zero again. Problem is in the carts 

[Rob Strand]

1. White cart as input  -  Black cart as output (original wiring) ---> normal / medium splash sound at both sides.
2. White cart as output  -  Black cart as input  (reversed wiring) ---->  loud / strong splash sound at White  and  normal/medium at Black side.

So what that is saying is both cartridges are working (to some degree) but they are not the same.   The question that is difficult to answer is "Is that normal?".   On eway of thinking is why did they use different cartridges?  If they wanted them to behave the same then they would have use the same cartridges at each end.  So one reasoning is:  the white drives the spring harder and the black is more sensitive *but* we don't know that is true.   Also that's not what you are seeing.   You are seeing the black cartridge give a weaker signal.

So the issue of the shorts:

o find out the obvious that the short was simply there when Rvrb tab was off and by switching to on we have the normal circuit so the rvrb to be functioned  , obviously ...

I don't quite follow you here "Rvrb tab was off"  and here "by switching to on we have the normal circuit so the rvrb".

The pins on the cartridge should be not connect to ground.    Normally cartridge pins float.   You might see the Left ground pin and and Right ground pin on the cartridge connected together but normally they are separate.

This "C" has smthng

Don't worry about those connection.  They has to do with the way the spring reverbs are made.  They have deliberate connections to ground, normally on the output side.   In your case how the each side is ground is entirely upto the cartridges and I would expect all pins on each side to float.

[anotherjim]

Farfisa often used ground mute switching for sources that are switched out. They also prefered to use potentiometer volume controls as variable resistors, grounding out against a fixed series resistor. Whatever the reason for doing it that way, it means all the audio paths have a high source impedance and carbon resistors of at least 47k adding noise. I suppose it does mean that any shorts or wiring errors are less likely to damage the electronics.

[Rob Strand]

Farfisa often used ground mute switching for sources that are switched out. They also prefered to use potentiometer volume controls as variable resistors, grounding out against a fixed series resistor.

Thanks Jim, it's *that* scheme.

[sarakisof]

I don't quite follow you here "Rvrb tab was off"  and here "by switching to on we have the normal circuit so the rvrb".

i mean rvrb tab off-->beeps, rvrb tab off-->beep stops. Yeap i ve found that in other farfisas too and truth is as i was isolating the problem by disconecting those wire's path from botomm chassis to upside, while i reached at the point with brown connectors going to the "classic" farfisas gaceplate "spaghetti"(tiny wires) connections, i immediatelly told to myself "sh###tt that wire must go to the rvrb switch and they ve shorted to gnd when is off" grabbed the multimtr and while it was beeping i pressef rvrb button to "on" ... beep stopped. I had xperienced that too in Farfisa Fast model and Elka Panther.

Still cannot understand why i cannot get rvrb fx signal out of the unit, only can hear the splashes. Having tried with both sides piezo discs, both sides other cheapo ceramic carts, nothing

[Rob Strand]

i immediatelly told to myself "sh###tt that wire must go to the rvrb switch and they ve shorted to gnd when is off" grabbed the multimtr and while it was beeping i pressef rvrb button to "on" ... beep stopped.

Thanks.  As soon as Jim mentioned the switch it was clear what was going on.

Still cannot understand why i cannot get rvrb fx signal out of the unit, only can hear the splashes. Having tried with both sides piezo discs, both sides other cheapo ceramic carts, nothing

Yes.  If I had that problem in front of me I would probably be checking the drive side amplifier ie. the circuit that drives the white "send" cartridge.

A simply way would be to get a speaker, put 100 ohm to 220 ohm in series with it,  then connect it to the drive amp and see if you can hear a signal coming out.   If you have an oscilloscope you can look at the waveform at the output of the drive amplifier.   Measure one pin at a time,  one pin will have a strong signal relative to ground and one will not.

Another check would be to measure the AC signal across the two pins on the send cartridge.  You should get 0V with no keys presses then some voltage 100mV to 3Vrms (I saw 1V on a schematic) when a key is pressed.  Use middle C.  You want test with continuous tones from the keyboard.

[Rob Strand]

One more thing that came to mind.

Reverb circuit appears to be in the tube chassis at the bottom of the unit.
Circuit diagram is probably close to the "Compact" PSU & Reverb circuit.
- White _cable_ to drive transducer
- Black _cable_ to receive transducer

How confident are you that both conductors of the white cable are connecting
all the way from the tube chassis through to the reverb spring.  If that cable
is damaged, eg open, then you will not get any drive signal to the reverb.

The receive transducer/black cable seems to be working since you can hear the reverb springs "splash".

[sarakisof]

Thanks again Rob i will do when find a little time, have my 2 yrs son with fever.

The receive transducer/black cable seems to be working since you can hear the reverb springs "splash".

yeap i have xclude that scenario as i have followed all black's wire path up way to its final end to Rvrb tab at faceplate. I even put alligators in parallel with a new one same value  to this resistor (dont know if u cans see is in the bottom) that black wire positive lead is connected to, cause i was hearing that beep (that ended up to be that switch rvrb tab thing that we were talking anyway) so i have followed all the way from there, 1ax7's pin etc to Rvrb switch on faceplate.
So now i have to do the same with white one, will be back asap with results.

[sarakisof]

How confident are you that both conductors of the white cable are connecting
all the way from the tube chassis through to the reverb spring.  If that cable
is damaged, eg open, then you will not get any drive signal to the reverb.

Did checked, no open circuit, both conductors are properly connected all the way from chassis to reverb spring.

BUT, i get no voltage at driver transducer's pins at all. So i started take measurements for this white cable from its  start point, which is the 12AX7's pin. This pin is connected (amongst others) to a mustard capacitor, then gets connected to the white cable's positive lead.

You see this mustard cap in white circle?  Measured 160 V. at tube's pin/mustard's one leg (marked green)  and  then after this cap  got 0 V.(marked white) 
Is this normal ??



Some more i just shot

[Rob Strand]

You see this mustard cap in white circle?  Measured 160 V. at tube's pin/mustard's one leg (marked green)  and  then after this cap  got 0 V.(marked white) 
Is this normal ??

Yes it it normal.  In fact even 160V looks good.   On the "Compact" power supply schematic the tube voltage is marked at 170V.

http://mezzoauto.blogspot.com/2019/05/farfisa-compact.html

So one option is the cap has gone open circuit, usually from a crack, *but* I wouldn't go down this path just yet.

What you need to do is press a sustained note on the keyboard them measure the *AC* voltage on each side of the cap.   The schematics indicate 1V AC; not the schematic above but another one.   The other thing you should see is that AC voltage is zero when you don't press a key and goes to 1V AC (or whatever) when you press the key.

If you don't see a voltage maybe the signal is not making it's way to the Send amplifier.

[sarakisof]

What you need to do is press a sustained note on the keyboard them measure the *AC* voltage on each side of the cap.   The schematics indicate 1V AC; not the schematic above but another one.   The other thing you should see is that AC voltage is zero when you don't press a key and goes to 1V AC (or whatever) when you press the key.

Well on each side of that cap i got 1.38V DC stable but on AC multimeter mode (100AC scale) i got smthng like unstable 000 - 001 V dont know if thats ok ... 
No voltage change at all by pressing any key neither DC or AC measurements...

[Rob Strand]

Well on each side of that cap i got 1.38V DC stable but on AC multimeter mode (100AC scale) i got smthng like unstable 000 - 001 V dont know if thats ok ... 

You probably need a bit more resolution to make a reliable decision.

Do you have a 1V AC or 10V AC range?  That will give you more resolution.
The meter should handle the high DC voltage on the AC mode.  If you want to be extra safe set the meter to 100V AC before connecting then switch to 1V AC (or 10V AC) after connecting.

No voltage change at all by pressing any key neither DC or AC measurements...

It could be because the AC measurement resolution isn't good enough.

Once you are convinced there's no signal coming out, the next step would to move back up the circuit to find where the signal is lost.

[sarakisof]

Do you have a 1V AC or 10V AC range?

nope and i cannot find one in local or from friends i have asked about.

Once you are convinced there's no signal coming out, the next step would to move back up the circuit to find where the signal is lost.

i think i dont have signal so i might start finding where it gets lost, dangerous way though lethal voltages in there and as i said in my first post in here there is no schematic for this thing anywhere 
Idea of quiting and use my external diy spring reverb gets closer and closer  (i am sure it is just a small detail though and funny thing is if i solve this, this Farfisa beauty will be 100% functional and like new!!!!)

[Rob Strand]

nope and i cannot find one in local or from friends i have asked about.

That's a pain.  If you take the output of the Reverb Send amp, the wires that go to the Send transducer,
wire 1M to ground, and then connect that to an amp you should hear sound coming out of that stage.

The 1M is precautionary to give that 10nF cap a path to charge.

i think i dont have signal so i might start finding where it gets lost, dangerous way though lethal voltages in there and as i said in my first post in here there is no schematic for this thing anywhere

The best compilation of info I found was this.  There's two reverb schematics using tubes.  One says "mod", which has two stages, and one doesn't.   There aren't many parts around these circuits so it's easy to compare.
http://www-personal.umich.edu/~damont/FarfisaComboCompactCompleteUserServiceManual.pdf

You can use an audio probe idea to see if there is signal or not,
https://www.diystompboxes.com/pedals/debug.html

A few precautions:
You will need a 400V cap to do tube ckt testing.  I would add a 1M resistor to the amp side.
Be aware cap will start charged to 300V after you do a test.   
You can discharge it by shorting a resistor across the cap, say 1k.
Don't touch the cap or resistor wires as you can get a shock from the cap.
If you move from high voltage  to low voltage circuits you should discharge the cap between tests as you could blast 300V from the charge cap into the low voltage circuit.

There's only a few points to test so go slowly and discharge the cap each test to reduce any unforeseen problems.

[sarakisof]

So back again, hope Rob is around here 
Did a signal trace test according to this video https://www.youtube.com/watch?v=9GUwE6JIoJY, so i used a 50V cap instead of a 400V+, dont know if thats bad, i didnt damaged anything/one though  .
With Rverb tab/switch on and by pressing a sustain note with a tape i got those signal results. Marked in green is where i got signal(either loud or lower, there is signal) and markred in red where i didn't got any signal at all.
Please note -as we mentioned above- that this schemo is from Compact that uses preamp tubes for spring reverb, not from a Foyer (cannot be found on net nowhere) so it is not the same, but in general and for the reverb thing it follows about the same circuity - not identical though (for example in Foyer at V2 tube pin2 gets connected with pin7 with a capacitor as i have painted below in green).
Could anyone show me in the schem which is the signal path for reverb. i mean where should i get signal that now i dont?

[Rob Strand]

Could anyone show me in the schem which is the signal path for reverb. i mean where should i get signal that now i dont?

It's actually very simple.  The bottom row of parts is all for the normal organ signal and the top row of parts is for the Reverb.

So the signal progresses like this:
- Organ signal on V2 pin 7
- Organ signal passes through the cap you added on the drawing, then to V2 pin 2
- The tube V2 (pins 1, 2, 3) amplifies the organ signal then the amplified signal
  is passed to the Reverb Send transducer via C512.
- The signal passes down the reverb spring, is sensed by the receiver transducer
   and comes out at V1 pin 2
- V1 (pin 1, 2, 3) amplifies the reverb signal and the final reverb signal is on V1 pin 1.
- The reverb signal passes through C514 and is mixed with the dry organ signal at the
  circled point 4 on the right.

So what you are seeing is the organ signal getting to V2 pin 2 but not coming out on V2 pin 1.

If the voltage on V2 pin is about 145V it is highly likely the tube V2 (pins 1, 2, 3) is working.  So assuming your 50V cap isn't giving us false reading when connected to ~200V, I would say the signal on the reverb side of C512 has a short (ie. circle point 7).