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Repair and disassembly information for amplifiers in home audio systems.

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Why won't my Amp power up?

I have an A500 Behringer power amp that will not power up. I replaced the power cord and fuse with known working ones, with no result. Next, I took the fuse I tested in it and plugged it back into a working amp of the same model, which ran fine (so I know the amp isn't just killing fuses).

Any ideas?


(I would have put this as a comment, but needed more words. I would have edited my original post, but couldn't upload pics on an edit.) Every once in a long while, the lights on the front of the power amp will light briefly and then die, but 98 percent of the time, nothing. I have opened it up, and here is the power path:

There is the plug/fuse combo:

Block Image

As you can see, there is a green/yellow ground. There is a black wire that goes straight into the heart (massive heavy cylinder of wires) of the amp. There is another black wire with a capacitor (I think?) directly after the plug. This wire goes to the switch in the front, which is a simple push button. There is a blue wire coming out of the switch back into the heart of the amp, completing the circuit.

The rest of the components (led board, input boards, output boards, volume knobs) seem irrelevant because it's not getting power at tall (but I could be wrong). Here's a pic of the entire setup:

Block Image

Any ideas? I am not an electrician, so please use small words :)

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It sounds like loose ground wire.


Well ur not dealing with dc here so a loose ground wouldnt normally be the problem a loose neutral would b more like it but honestly id hav to look at it to tell u for sure but i would take a multimeter and check fir voltage u hav the right idea follow the curcuit from power source untill u find the loss of power then when u find where ur loosing power u can sesrch for the issue at that point if mits a bad part or a loose wire u can find the issue wherever u loose power


Hi Ben, and thank you so for your spiritual encouragemant.

I don't have an answer, but question. I have a Sherwood amp connected to TV, PS3, etc. It continues to play but when turning up the volume the whole system shuts down/turns off. Before, I could turn the volume up to about 45 or so, now if it goes past 19 (in which makes it hard to hear many movies) it will cut completely OFF! Please Help :)


You most likely are invoking the protection circuit to prevent power from damaging the rest of the unit or your toroid is open. Good news is the partial schematic your unit is available online. I'd start with testing the output of the "Heart" (Thats a toroidial transformer) It looks like the yellow and orange pairs feed the side boards. disconnect those, put meter on ac and test for voltage. maybe 15-20 volts AC should be found there. careful.... risk of shock is real! Also, disconnect power and start checking continuity all the way up to the switch from the power cord, till you get to the toroid/transformer.


Does the amp have a charger dc input? ive got a 800w amp thats runs on AC and DC. It started to hum but not power on using the AC home plug. I had a dc charger lying about 12v 3a so i thopught id try it. Worked like a dream, powered up and i now have my sound back. Just a thought.


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I happen to live in your area. Is there a way we could meet up and I could take a look at the amp for you. You could bring it by my church either this coming new years or this saturday night. Let me know so I can come early :-)


I fixed it! Here is what I did:

1. Pray that God would be glorified through anything I do

2. Plug it in to make sure it doesnt just turn on, it didn't so unplug it

3. Removed and checked the front ON switch for power continuity (with power turned unplugged, just use a multimeter on continuity check.

4. Removed and checked the two fuses on each of the amplifier circuit boards

5. Removed and checked continuity of the power connector through that fuse

6. Used a multimeter to check every easily accessible diode

7. Checked to see if any of the electrolytic capacitors had "popped tops" (if they had blown)

8. Now the scary part: unplug the power cables (yellow and orange) going from the huge inductor/transformer to each of the amplifier boards, and then plug power into the amp and turn it on. Use extreme caution, there is live 120V leads in areas that you can touch with your hand! I recommend using only 1 hand while anywhere near a large voltage, incase of any shock, the current wont go through your heart.

9. Use a multimeter to check the voltages coming out of those two power cables, they should be somewhere around 70Vrms.

10. Everything checked out ok, so I turned power off. Plugged everything back in where is was supposed to be, and turned the power back on. And it worked!! There was probably a loose connection or an oxidized lead.

11. Praised the Lord for His love. :-)

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@burt0996 if you tell us what make and model your amp is we may just come up with a solution without any divine intervention.


I could use the Lord's help in fixing my Bugera tube amp. It worked fine for 8 months, then today while playing it, it begins to hum with the power light flickering. I turned it off. Then a minute later I foolishly turned it back on, the speaker was really loud, then it just went off. It won't come back on. :(


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This amp has no tubes...

This is a grounded collector Amp. The two large caps form a center tap for the power supply which is connected to the speaker terminal. So This is cap coupled amp. Therefore if you drive high currents of low frequencies the large caps will is a very graceful failure. The signal will just fade away. The lower the impedance of your load (speakers) the faster this will happen. Replacing these caps should put you back in service...

Cap coupled amps sound bouncy...Pulses are accelerated and then fall off...So the choice of replacement cap will effect the tonal quality of the amp. The best choice would be splitting the required value over several small caps connected in parallel.

Note replacing the caps requires removing the amp circuit boards...which means disconnecting the heat sinks...lots of little bits, and heat sink paste. I would recommend replacing all the screws you remove because they will disintegrate on contact.

Replacing the twisted output wires with OFC similar temp rated wire or even untwisting the OEM wire can't hurt either.

This amp has huge potential, by design, but crippled by implementation.

I suggest taking good (clear) pictures of everything before you pull it apart. Do one side at a time. Have a container for all the bits. Avoid bending the power transistors leads once you remove the screws holding the circuit board from the heat sink.

Or just buy another one for $250

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try and checking the amplification circuit board, look for any anomalies in the circuit paths, or circuitry, also check any other boards in the amp as well. Is it a tube amp? if so, check the tubes for damage, or replace them (remember, change them all at once). check for any damage to the boards and see what you find, have you tried another outlet by any chance?

Good Luck,


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dont change all the tubes......only one is usually naffed


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Deoxit those connections and look for broken spring clips on those molex connectors!

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Ben Eisenman zal eeuwig dankbaar zijn.

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