barbetta 3105 schematic

[lowell]

Got 2 that need repair. One is blowing the fuse. Can't find anything online and barbetta support # is disconnected.

[Enzo]

Unknown amp of unknown technology (you didn't say tube or solid state) blows fuses. First look for shorted output devices. If they are transistors, check with a meter. If they are tubes, just pull them from the socket and power up. Beyond that, always check the power supply rectifiers for shorts. Much less common are filter caps shorted, but easy enough to check. Tube amp have flyback diodes? Are they shorted?

[lowell]

Yah checked those things. Schematic is nice for me as I'm not as good as solid state as I am with tubes. High v rails seem good and outputs are not shorted. Btw this is the unit that just has no sound but powers on. The other one blowing fuses I haven't got to yet.

[lowell]

Here are a few pics. I am getting -52vdc on all opamp pins in the preamp. So seems maybe the low v supply is shot and maybe all opamps as well.

[lowell]

ignore the opamp issue i measured wrong. so I believe this is a complementary output. NPN stacked on a PNP with the speaker output in between. Correct? So I'm getting -5vdc on the speaker out with no load. Curious as to why my speaker isn't making that DC hum sound. I even tried another speaker. Also the MJ15003 and MJ15004 output devices both have -5vdc on their emitters and their bases both sit at -4.65vdc. Any clues? There is offset but without a schematic I'm weary.

[lowell]

Also there are two LM391's driving TIP transistors which I believe to be the drivers. If this helps you help me Enzo.

[J M Fahey]

It has the infamous LM391N power amp driver.
Google its datasheet and post the suggested schematic here, it can't be far from that.

Good luck finding one beyond shady EBay deals.

FWIW that LM391 was also much used by Polytone, so post that schematic too for reference and comparison.

Barbettas should be easy to fix, being basically a simple powered box intended for Keyboard players.

Most popular one was not much more than a "by the book" power amp, driven by a simple mixer (4 line inputs, *no* controls, not even a master volume) because "everything is supposed to be controlled from the keyboards themselves" .

Maybe this one grew a few volume pots, don't know.

[Jazz P Bass]

LM391N datasheet
http://www.ti.com/lit/ds/snvs753a/snvs753a.pdf

[rf7]

Got a bad Barbetta on the bench that measures 100 ohms between the HV rails. Fun --- not. Different model though. Wish I had a schematic.

[J M Fahey]

Even without schematic you already have some important measurements do do.

Such as rectifier diodes, filter caps, power transistors, unloaded transformer voltages, case to heatsink insulation, etc.

[lowell]

So I talked to Tony @ Barbetta. He is very nice. He said 5v offset isn't a big real, but I think later in the conversation he said 5mv... So...not sure if he misheard me. He is tough to get ahold of and I'm trying to clear this up. I think 5v is way too much but pls advise. Also, I've injected signal at various points with a speaker load on output and nothing... Dead. So I don't think its a connection issue. Finally he said the schematic was lost years ago.

[lowell]

So I talked to Tony @ Barbetta. He is very nice. He said 5v offset isn't a big real, but I think later in the conversation he said 5mv... So...not sure if he misheard me. He is tough to get ahold of and I'm trying to clear this up. I think 5v is way too much but pls advise. Also, I've injected signal at various points with a speaker load on output and nothing... Dead. So I don't think its a connection issue. Finally he said the schematic was lost years ago.

[J M Fahey]

Ok, then download the LM391N suggested application schematic, print it *large* (as in 2 sheets taped together) and start matching it to what you have.

Then repost the edited schematic here, with parts labelled as in the PCB and we'll see *if* something can be suggested.

[lowell]

Thx JM will do

[Jazz P Bass]

Like this.