Behringer BXL 3000 A , hot resistor.

[fjt]

I have a Behringer BXL 3000 A bass combo. It was dropped and broke leads to two ceramic power resistors on the LPA 1300 amp board. after repairing leads i only get a loud hum from the speaker. searching for the problem i found a hot resistor, a 1.5k ceramic radial. I have the schematic its resistor R4. No fuses blew and pre amp works but distorted, attached is the schematic. Section is marked. Thanksbehringer_lpa1300_rev_d_and_f_power_amplifier_sch (1) (1).pdf

[DrGonz78]

I have a Behringer BXL 3000 A bass combo. It was dropped and broke leads to two ceramic power resistors on the LPA 1300 amp board. after repairing leads i only get a loud hum from the speaker. searching for the problem i found a hot resistor, a 1.5k ceramic radial. I have the schematic its resistor R4. No fuses blew and pre amp works but distorted, attached is the schematic. Section is marked. Thanks

Please check that you posted the schematic correctly.

[52 Bill]

When you turn on the amp, does the speaker push out or pull in and then stay there? If it does, that is a sign of dc voltage on the output and you should disconnect the speaker from the amp until you have it fixed. You could damage the speaker by running a dc voltage through it.

[fjt]

52 Bill thanks for your response. No it dosent push or pull. It just hums.

[DrGonz78]

searching for the problem i found a hot resistor, a 1.5k ceramic radial. I have the schematic its resistor R4.

I am a little confused by what you have written above... R4 is not 1.5k but rather on the schematic it says R4 is 47k.

[52 Bill]

I am a little confused by what you have written above... R4 is not 1.5k but rather on the schematic it says R4 is 47k.

R2 and R3 are 1K5 resistors. They are part of the +15 & -15 volt power amp dropping circuit, so I'd guess that they will get hot.

fjt: Do you have the equipment and experience to measure voltages?

Is the hum changed by physical stimulation of the chassis? If you look for the main filter caps C8 and C9 (3300uF@100vdc), do they look normal or are the ends domed, or do they feel loose from the board?

[fjt]

hot resistor

I am a little confused by what you have written above... R4 is not 1.5k but rather on the schematic it says R4 is 47k.

sorry the resistor is R2. the other resistor that broken lead was R13. Thanks

[Enzo]

OK, just to be sure, disconnect the speaker, and measure for DC voltage between the wires to it. SHould be none.

A loud hum usually means either DC on the speaker or a loss of filtration on one or both main power supplies.

R2 and R3 normally get warm/hot. Measure DC voltage to ground at each end of both. They should have the same voltages on them, except one is opposite polarity from the other. One end of each resistor ought to have 15v, the other, I have no idea, something in the 50-80v range I bet. In aany case, whatever it is, the two resistors should be close to the same, other than the polarity. If one is substantially lower, it probably is not being filtered. The two large capacitors C8, C9 are the main filters. Since this happened after teh amp was dropped? I'd suspect broken solder on one or both of them.

[Enzo]

Ive checked the voltages on the regulators there good . I cant find a short on the speaker jack or the power transformer. what are the green and red leds for. the green stays on, the red flashes for a split second then off. also I'm able to power up the power amp board to take voltage measurements. and took out power transistors they tested all good, resistors good.could thereh be something in the pre-amp.

I asked about the voltages on R2,3. The actual voltage regulators are for the preamp. The red LED is a peak light, and seems normal to blink at power up. Green looks like a power indicator.