Behringer LPA1180 Power supply in short

[spunko]

Hello friends, I am new in this forum.
I have read some info about this power supply, there are a couple of threads here that were very usefull, but I can't figure out what is the problem with this unit.
It came to me working, but the owner said it sounded weird (is a bass amp BXL1800A), We plugged it into a Behringer Bass cab, and it sounded good, but the jack of the cab was rusty, and it didn't make good contact.
Then the client took out the cable speaker cable of the amp when it was still on and the fuse blew up. We replaced the fuse and it blew again.
I opened the unit and changed these components based on past threads:
- 7805
- 7815
- 7915
- 9140
- 140n

Then I tested the unit without the cab and it power on, changed channels with nop problems, then I turned it off, plugged the speaker cable to the cab, powered it on and the fuse blew again.
Then I changed all the parts above again, and this time I used a 200watt bulb Current Limiter, tested it again and the bulb turned on, so there is still a problem, even when I already changed all these components twice.

I read another thread and I took all the components changed above to test the Power supply with the Current Limiter, but without those components, and the bulb turned on again.
The last thread I read said the drivers were bad, so I took out these components too:
- T4 340
- T5 350
The bulb again turnned on.

I suppose that if all these components are out of the PCB, the power supply shouldn't be in short and it will be open, but even with all these parts out, the bulb still goes.
Sorry for my bad english.

[Dave H]

The last thread I read said the drivers were bad, so I took out these components too:
- T4 340
- T5 350
The bulb again turnned on.

I suppose that if all these components are out of the PCB, the power supply shouldn't be in short and it will be open, but even with all these parts out, the bulb still goes.
Sorry for my bad english.

Welcome, T4,T5 can't short out the power supply as they have 220R resistors in series. It will be T2,T3 or possibly D1-D4 that are shorted. Remove T2,T3 and check the diodes for shorts. The regulators you replaced were probably OK. Check their output voltages with T2,T3 removed then replace T2,T3 and any shorted diodes. Power it up with the light bulb limiter in circuit but without the speaker connected. Check there is no DC offset on the output before connecting the speaker. Connect the speaker with the power off.

behringer_lpa1180_rev_f.pdf

[Enzo]

Stop throwing parts at it, we need to find the problem.

Since it only blows fuses when you connect a speaker, DON;T CONNECT A SPEAKER until you know the amp is stable and does not make DC. Plug a cord into the output, but leave the other end free - do not plug it into a speaker or load. Nw at that free end measure for DC voltage. Is there a substantial voltage there?

[spunko]

Hello and thank you for your answers.
I took out of the board D1, D2, D3 and D4 to test them with my fluke multimeter. They all tested good.

[Dave H]

I took out of the board D1, D2, D3 and D4 to test them with my fluke multimeter. They all tested good.

It must be T2 or T3 then. I'd just replace both of them and test it as I said in post #2. If it tests OK set the bias (with the speaker still disconnected) as stated on the schematic (5mV across R1 in cold state).

[spunko]

Hello and thank you for your help.

So I put back all together except for T2 and T3, tested it with the current limiter and it turned on the bulb again.
That is why I took those components out in the first post (7805, 7815, 7915, 9140, 140n) and the bulb still turns on. Is this a normal behaivior? that without these components the power supply stays in short? Could it be the PCB?
I don't understand why the power supply is still in short without all those components.

[Enzo]

Did you test the rectifiers for shorts? D5-12.