behringer a500 amp repair

[mellowamoya]

hi all i have a behringer a500 to repair one channel appears to be going into protection,
when i switch on with a signal on the channel and just advance the volume you hear a little bit of sound but as you advance the volume you hear plop and then no sound,
On checking i find that the +15volt line goes to 0volts on lowering the volume it will restore to 15volts. this is measures at the junction of D4/C1 on channel 1 im guessing something it drawing too much current on the 15volt line when the volume is raised so the voltage is being droped on R4 but what could cause that. and it also does the same with no signal on the input but with just raising the volume. schematic attached.
Behringer-A500-power-amp.pdf

[g1]

So if you are using both channels, when the bad side cuts out, the good side keeps working?

[mellowamoya]

yes

So if you are using both channels, when the bad side cuts out, the good side keeps working?

yes the channel 2 works all the time the channels are basically completely separate, the only thing they share is the transformer

[g1]

Isn't the power supply common to both sides of the amp?
It's not really clear from the schematic, but I only see one supply for +/- 17V, it sounds like you are just losing the connection to the +15V on the one amp.

[mellowamoya]

no each amplifier is fed from an ac winding on the transformer then they have a set of diodes which produces +-50 volts the 15 volts is derived from that supply via R4 D4 C1, the 17volts is just for the front bargraph leds.
ill upload a picture of one on the amps.

[mellowamoya]

[g1]

Have you checked for excess voltage drop across R4 during fault condition? How does it compare to voltage across R4 when working?
If not a big change (or if decrease rather than increase) then it is not an excess current problem but a connection problem.

[mellowamoya]

volt drop

Have you checked for excess voltage drop across R4 during fault condition? How does it compare to voltage across R4 when working?
If not a big change (or if decrease rather than increase) then it is not an excess current problem but a connection problem.

the voltage is all dropped across R4 so am fairly sure it is excess current somewhere.

[g1]

Any IC's getting hot when in fault mode?

[mellowamoya]

ok problem solved replaced ic 1 and 2 not sure which one failed but they are on the back of the board so easy to change both while the board was off the heatsink,
amp now working
difficult to get to while the board was mounted so not easy to fault find on them, there are no obvious shorts on them

[choy]

Mellowamoya, I have the same exact problem. Could you provide which ic you replaced it with? I know that it's a mount 4580 ic but dont know what size it is. I have read on many forums that njm4580 is a good replacement but they come in different sizes. Could help?

[mellowamoya]

hi,
i replaced with a 4558 smd ic it works without any problem the 4558 can replace the 4580.
NJR NJM4580E-TE1 from mouser.com

[choy]

hi,
i replaced with a 4558 smd ic it works without any problem the 4558 can replace the 4580.
NJR NJM4580E-TE1 from mouser.com

Thank you so much for your help.

[choy]

hi,
i replaced with a 4558 smd ic it works without any problem the 4558 can replace the 4580.
NJR NJM4580E-TE1 from mouser.com

Okay. I took the a500 ic chips out and measured with a calibur which measured a little bigger then what you recommend, 5mm x 5mm x 1.6 mm. Whereas, the njr3580e-te measured 3.9mm x 5mm x 1.5mm; however, I have order it. Hope it fit. Will update later.

[choy]

hi,
i replaced with a 4558 smd ic it works without any problem the 4558 can replace the 4580.
NJR NJM4580E-TE1 from mouser.com

Got the chip yesterday but installed today and it perfect. Now, bumping it for testing. Thank you very much bro!