Farfisa FAST 2 piccolo problema

[garcho]

Buon giorno signore e signori,

I realize this is about a keyboard and not an effects pedal, BUT it involves an amp, 1/4" jacks, and a perennial favorite, transistors housed in little shiny metal top hats, so I figure that's relevant enough for this forum...

I'm repairing a lovely little orange Farfisa FAST 2 compact combo and I'm running into a problem. Well, two problems. First, anyone with experience repairing this particular keyboard know how to take the amp off the chassis? The first rule of electronics repair is "do no harm" and they way they've attached it looks like an accident waiting to happen. I can take pics and post them if I've piqued anyone's interest. Secondly:

The tone and pitch are normal, and when plugged in to an external amp, it sounds it's as loud as it should be (and adjustable by the internal trimmer). The speakers however, are very faint. When turned all the way up, they're barely audible. Both the volume knob and the internal trimmer attenuate the speaker output the way it would while working normally, as in, no scratchiness, no jumps or skips along the turn, etc. But again, the volume at max, is super quiet.

So, I assume it's something in the power amp circuitry, between the pre (and line out) and the speakers. The Farfisa "FAST" series is all transistor, no tube. Any help, suggestions, service manuals or schematics? Thanks for your time y'all!

[anotherjim]

I think Farfisa tended to use the same amp circuit in many models, so you may not have to find that particular models manual.
A photo would be useful.

Ones I have seen have dedicated trimmer on the board for the amp volume that only affects the amp.

The amps often also drive headphones. Then the speakers are wired via a normally closed contact in the headphone jack socket which disconnects the speakers when headphones are plugged in. That contact might be dirty.

This 40W amp is very common, but I don't know if that's what you have.

[ElectricDruid]

Sounds like the good news is that all the clever bits are working!!

I'd second the remark about the headphone jack socket if there is one. I've seen things where a dummy load is switched in/out by the headphone socket contacts to prevent the headphones from blowing up. If that stayed in circuit when it wasn't supposed to, you might see something like you've got.

Good luck!

Regards,
Tom

[garcho]

EDIT: I fixed the pics, sorry about that

thanks for your thoughts gents! i'll look at this more today and get back to yous.

here are a few pics:

[anotherjim]

A 100R ceramic resistor can be found mounted on the headphone jack. Surprisingly, it isn't always to attenuate the headphone feed, but always in circuit to 0v to make sure the amps output cap negative plate is always charged to 0v if no speaker load is connected. Looks like Farfisa sold mono headphones at one time that were high enough impedance so that the amp gave much less power into them than the speakers and no series resistors needed.

That amp "could" be the same circuit I linked, but a different assembly than the one I know. That's the board towards the right with the simple folded "U" heatsink plate.

[garcho]

I seem to have made some observational mistakes:

The trimmer ONLY adjusts the speaker volume (and it still works even the though the output volume is so low).

The external volume knob doesn't control output for the headphone jack. Is this normal? It does adjust output for the line out jack.

When looking at the two TS jacks, the one I'm assuming is the headphone jack has an old clunky resistor (it's 5.6KΩ) and is also a switching jack. When the jack has a connector in it, the switch is open, when the jack is empty, the switch is closed. However, regardless of whether or not anything is plugged in, the speakers still sound (albeit super quietly). I've tried putting an alligator clip on the jack to make sure the switch is closed, but with no different results. The jack I assume is the line out, which has adjustable output via the external volume knob, is a simple TS jack.

Guess I need to finesse that damn amp off the chassis...

[anotherjim]

The external volume knob doesn't control output for the headphone jack. Is this normal? It does adjust output for the line out jack.

That may be right, since the headphone is usually only a replacement of the internal speakers.
Normal organ volume might depend on a special swell pedal. Do you have one? With combo organs, the pedal often gets lost.

Some Farfisa's use a dual control pedal (optical with 2 LDR's and 2 filament lamps). One for the internal speaker/headphone and one for line out and/or Leslie. Do you have such a pedal with one lamp blown?

[garcho]

Well, I'm a fool, sorry to waste everyone's time. 99% of the gear i repair is either user-interface damage, as in, someone hamfisting a cable jack or stepping on a cable while it's plugged in, or cranking a knob too far, etc.,

OR

a big ol' faulty capacitor.

Which this was. Sorry i didn't check that first y'all, but it was on that upside down amp that i finally finessed off the chassis without breaking any ancient wires or solder joints. Replaced it and blammo, speakers are loud and annoying as anything. Cheers y'all!

[ElectricDruid]

Good find and well done! 

Tom