Old Farfisa Transistor Amp

[sarakisof]

Hello everyone. I've locally found an old hand wired transistor Farfisa combo 2x12 keys & guitar amp for a little bucks.
Seller says indicator light gets on but it doesn't get any signal at all and that one speaker needs to be replaced or a reconing job.
Cannot test the thing (PT secondary windings, out trafo etc.) as he's a bit far from my area. Kind of "sold as is" case.

Would love to fully repair & restore it but I'm wondering if it's smthng worse.
Could it be smthng unfixable/unobtainable parts? If so what it could be & what's the possibilities of such a case?

Schematic:


https://www.synthxl.com/wp-content/uploads/2020/04/Farfisa-TR-60-Schematics.pdf


I already own its bigger brother of same era (FR-40 full tube amp) so even if it's smthng unfixable I can always use it for  donor spare parts (plenty of'em are same). But it'd be nice to get it running though, so I can use both.

Any thoughts?

[Rob Strand]

It's more a matter of how much to fix as opposed to fixable.

Big ticket items would be:
- Transformer - light comes on so there's hope there.
- Speaker - It would be nice if the owner can report pop at turn-on
  or any hiss when powered up.
- (An extreme issue would be it has no speaker or the box is empty.)

Annoying things would be:
- Faulty switches which are funky and hard to replace.
  You can clean them and with With care pull them apart; depends on you confidence.

The rest would be easily fixable.  Just debugging time:
- bad caps
- bad connectors
- bad parts

[sarakisof]

How are you doing Rob? Thanks for the reply. You're always around willing to help.

Speakers are both there.
If that helps, from the only pic I can tell, grill cloth appears to have a tear in one speaker's side only big enough to put me think it could be related to that speaker's damage.

Just to be more specific, what things can terribly go wrong in such ss amps stating the unit unfixable when we only know the light comes on but there's no sound at all and that one sp is dead?
And where should I first start troubleshooting from when (if finally) get it in my hands?

[Clint Eastwood]

If there is something wrong with the output transformer, it could be very hard to find a replacement. But it should not be too difficult to rewind it.

I would buy it!

[amptramp]

The schematic shows a driver transformer rather than an output transformer.  This gives isolated drive to the totem-pole output stage with the top and bottom output transistors being driven in the opposite phase to get push-pull output operation.  The same type of transformer is used for the reverb driver.  This is the only critical part of the amp.  If these two transformers are OK, then it should be easy to restore this unit.

The power transformer shows two windings but if you know the voltages, you can replace it with two separate transformers to supply each of the voltages.

[sarakisof]

The schematic shows a driver transformer rather than an output transformer

Thomas Vox amps kind of right?
http://www.geofex.com/article_folders/voxamp/voxprot.htm
I have a feeling one or both of them are dead. Haven't met a driver transformer before, is it impossible to rewind like you'd do with an output one?

[Rob Strand]

How are you doing Rob? Thanks for the reply. You're always around willing to help.

I'm OK but the last 6 months have been an eye opener for me.  You can't predict what happens in life - maybe for the last year.

Speakers are both there.
If that helps, from the only pic I can tell, grill cloth appears to have a tear in one speaker's side only big enough to put me think it could be related to that speaker's damage.

Torn speaker can often be fixed.   Burnt coils are a different matter.   The good thing about that amp is there is an output capacitor so if the amplifier had a catastrophic failing there not much chance of it blowing/fusing the speaker coils.   The output cap could short but that's not a common fault.   If the speakers are underrated for the amp it's still possible to fry the coils in normal use.

If there's no hiss or pop on power up it would make the wonder what is wrong.

Just to be more specific, what things can terribly go wrong in such ss amps stating the unit unfixable when we only know the light comes on but there's no sound at all and that one sp is dead?
And where should I first start troubleshooting from when (if finally) get it in my hands?

Blown output transistors (T13, T14) can occur.   The driver (T12) can blow up.  Pretty easy to diagnose and fix.  In fact by checking the DC voltages you can find blown transistors anywhere in the amp quite easily.   It's not common to blow the preamp transistors.

The next set of faults is bad electrolyic caps and bad switches.   With time you can track these down.

I have a feeling one or both of them are dead. Haven't met a driver transformer before, is it impossible to rewind like you'd do with an output one?

The good thing about drive transformer is when a transistor blows it doesn't take out the transistors around it.   A blown drive transformer is pretty rare.  On the drive side the transformer is fed by a resistor.  That's likely to fry before the transformer.

[Clint Eastwood]

In this book https://worldradiohistory.com/UK/Bernards-And-Babani/Bernards/176-Transistor-Audio.Amplifier-Manual.pdf you will find, from page 38 onwards, a discription of a 75 watt amplifier with similar topology. It gives a detailed description of the driver transformer, and it seems not too complicated to wind. Should that be necessary.

[sarakisof]

Things are getting more clear to me now. Thank you for all valuable infos here Rob, appreciate.

Nice input Clint, excellent source for studying.

Thank you all for contributing here guys, this is a great community to be part of.

Cheers.