Fender 112SE BIAS setting

[Feufeu]

Hello at all.

I repair a Fender stage 112SE . The output transistors have blow.
I replaced them , (amp run on limter lampe for the moment).

when amp run with no load output signal is correct , but when i connect my 8R load ( no speaker, 8R300W resistor)
with our oscillo plug on the load, i have crossover distortion.

Amp run on automatic BIAS. no trimmer potentiometer for adjust...

How to ajust BIAS in this amp type ?

i joint the schematic

tanks in advance for your help, and sorry for my bad English i'm French .

[Jazz P Bass]

Try the test again into the 8 ohm load without the limiter.

[Feufeu]

Hello;
thanks for your reply;

yes i have try and it's same problem ...

[g1]

Have you checked diodes CR13,14,15,16 ?

[Dave H]

Bonjour. How does it sound in normal operation? I wouldn't bother about the crossover distortion if you can't hear it. Check to see if there is any bias current by measuring the voltage across the emitter resistors R106-R111 with the load disconnected.

[Feufeu]

Have you checked diodes CR13,14,15,16 ?

Hello,
yes . I have check all diode in BIAS circuit, and all other in amp. It'OK no problem ...
On the schematic there are no voltages specified for the power amp

it's +16V and -16V on R92 & R93 Which sets Bias setting ?

Thanks

[Dave H]

+/-16V is the power supply. The bias voltage is the voltage across the diode string CR13-CR16. Measure from the cathode (bottom) of CR16 to the anode (top) of CR13 (top and bottom as viewed on schematic). It should measure 4 x diode Vf or about 2.8V. 2.8V is about the same as 4 x Vbe of the output transistors so I would only expect a small bias current. Measure the voltages across R106 to R111 with the load disconnected to check the bias current through all the output transistors. The bias current is the (measured voltage)/0.47 (the value of the emitter resistors)

[Feufeu]

thank you very much. I check this soon and i take values.

Do you think is really BIAS problem ? or i'm wrong is other ?

Thanks

[Enzo]

If all the bias diodes are good, then no, I don;t think bias is your problem.

[Feufeu]

[QUOTE=Dave H;442537]+/

I have around 4.3mV on R106 to R111...

[Dave H]

I have around 4.3mV on R106 to R111...

Can you measure the voltage across the diode string as in post #7 to check the diodes are OK?

I've noticed from the data sheets that not all TIP142/147 are created equal. Depending on manufacturer the internal base/emitter resistor could be 40,100 or 150ohms. If you have transistors with 40ohm Rbe then I doubt there's any bias current through the transistor. With Rbe at 100 or 150ohms the transistors should have a few mA bias. It will only be a few mA as 4.3mV/0.47 is only 9mA in total and some of that is through Rbe.

[Feufeu]

Hello ;

Measure from the cathode of CR16 to the anode of CR13: is 2.3V

[Dave H]

Measure from the cathode of CR16 to the anode of CR13: is 2.3V

It looks a little low for 4 diodes but the data sheet doesn't show the Vf at 5.5mA it only lists 1A where Vf is 2.5V! I'd check the diodes individually to to see if they have the same voltage drop and there isn't a bad one. If the crossover distortion really is an audible problem you can't live with and not something only visible on the scope you could try increasing the bias by adding a 2.2ohm (2R2) resistor in series with one of the bias diodes. This will increase the bias voltage by 12mV which should be enough to get the output transistors conducting. Measure the voltage across the 0.47R emitter resistors to check the new bias current.

[Feufeu]

It looks a little low for 4 diodes but the data sheet doesn't show the Vf at 5.5mA it only lists 1A where Vf is 2.5V! I'd check the diodes individually to to see if they have the same voltage drop and there isn't a bad one. If the crossover distortion really is an audible problem you can't live with and not something only visible on the scope you could try increasing the bias by adding a 2.2ohm (2R2) resistor in series with one of the bias diodes. This will increase the bias voltage by 12mV which should be enough to get the output transistors conducting. Measure the voltage across the 0.47R emitter resistors to check the new bias current.

voltages values to ground ref :
-> CR13 Anode: 1.11V
-> CR14 Anode: 0.55V
-> CR15 Anode: -19.3mV
-> CR16 Anode -0.59V
-> CR16 Cathode : -1.16V

Again Thanks for your help !!! and HAPPY NEW YEAR Frome France

[Dave H]

HAPPY NEW YEAR From France

And Happy New Year from the UK! It's 2017 in France now but we still have an hour to go.