Farfisa F20 Amp: Amplifier malfunction

Hi!

I need some help, please! 🙂

Last night I'm trying to repair an old Farfisa F20: but I don't know which valves it mounts: EL34 OR 6L6?

Not finding diagrams on the web, I looked at the photos of the amplifier on the net: practically in all the photos the amp mounts the 6L6 I did some tests:

If I mount the EL34 the amp crackles, rustle and after a while the resistors break (I think safety, there is no fuse);

If I mount the 6L6, it crackles much less but after a while I noticed that in correspondence with the crackles I see some sparks and smoke from the output transformer. Of course I turned it off right away, I didn't do any damage.

This morning, I've found a wiring diagram; in which the 6L6 is confirmed. However, the problem of crack, bop, etc. remains ... and the problem of the transformer that makes sparks and smoke also remains. What can be the cause? Can you give me some advice?

 

Thank you!

Best regards, Alessio

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Hi, Alessio,

most likely there is an Offset in expected Current trough the Tube(s).

This causes the Anode Current to go above  given Limit. Smoke from the Output Transformer is a bad Sign. This almost ever is a clear Sign that the Transformer is already bad.

Can You please upload the Circiut Diagram? Thanks.

Best Regards,

Henning

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Hi Henning,

Thanks for your kind reply.  

Here is the wiring diagram (attached), what do you think?

Alessio

Attachments:

• Schematic Farfisa F20 (430 KB)

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Measure current through the output tubes by measuring the voltage drop across their cathode resistor R712. First, make sure that it has the correct value and that C706 isnt shorted.

Regarding the output transformer, i agree with mr. Oelkers that it is probably already bad and will need to be replaced or rewound. As this amplifier is lacking RC networks or varistors across the transformer's primary, there's always a risk that transients, for example due to a sudden disconnection of the speaker while the amplifier is delivering high output or due to a short circuit or flashover in one of the output tubes, could cause a flashover and permanent damage to the transformer.

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Hi Torbjörn,

Thanks for your responses. I have verified C706, is ok.

Regarding the voltage drop across their cathode resistor R712, do I have to put the tester in the point indicated in the attached photo with the amp on? It's must measure 27v, right?

Best regards,

Alessio

Attachments:

• Voltage drop (204 KB)

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