Faulty supply problem

[somejohndoe]

I posted in the ss section but since I narrowed the problem down to the ps, thought I'd post here. The issue is the ps in my 3rd old pro amp is cutting in and out to the output stages intermittently. When I tap the supply caps I can replicate the behavior. I reflowed all the connections and the caps but it still behaves the same.

[Mooly]

Intermitent faults can be misleading. Often it's a transistor or reg or even a resistor that runs very hot and the solder cracks on the lead outs. It's impossible to diagnose from the info available and if it is a dry joint somewhere the best solution can often be to resolder the whole PCB completely.

Any wiring harnesses/plug/socket interfaces ? that could be a problem.

[somejohndoe]

Originally Posted by Mooly

Intermitent faults can be misleading. Often it's a transistor or reg or even a resistor that runs very hot and the solder cracks on the lead outs. It's impossible to diagnose from the info available and if it is a dry joint somewhere the best solution can often be to resolder the whole PCB completely.

Any wiring harnesses/plug/socket interfaces ? that could be a problem.

Loads.

It's only the output boards that are affected. The rest of the amp, input boards, meters, protection circuits, fans, work fine. This leads me to believe that it's the connections to the output boards or the main caps as the supply to the rest of the circuits has their own caps. The connectors on the output boards themselves look fine. I managed to get my old multimeter working to the point where I can measure voltages (yes, time for a new one) and the problem appears to be coming from the supply.

The only mechanical support for the wires to the output boards on the ps is solder. There is no glue or strain relief of any kind. This is the same for all three rails, +ve, -ve and ground. The joints look ok but perhaps they have stress cracks?

I'll also add that it is intermittent from cold, usually taking a good boost of volume to get it going.

[Mooly]

I'm not familiar with the amp. Is it one or two channel. Sometimes speaker protection relays can be troublesome (if it has them)... just with you saying the volume needs to be turned up to get it going.

If it's a 2 channel amp and both go off then probably PSU related , or, again, relay drive if it's common and shared.

[somejohndoe]

It's two channel, bridgeable. It's not the relay because that has a particularly mechanical sound when connecting and that doesn't happen. Also, the protection indicator LED does not light. Both channels fail but the left more so than the right. At some points the output sounds crackly, very much like a loose connection.

Thanks for your help, btw.

[tatus]

Originally Posted by somejohndoe

I'll also add that it is intermittent from cold, usually taking a good boost of volume to get it going.

That smells a lot like a microscopic crack in the pcb tracks, my first bet would be just at the border of a solder pad. I suggest to remove the solder resist close to the pads in question and to extend the soldere to the first few millimeters of the pcb track. This way you would bridge possible cracks.

[somejohndoe]

Originally Posted by tatus

That smells a lot like a microscopic crack in the pcb tracks, my first bet would be just at the border of a solder pad. I suggest to remove the solder resist close to the pads in question and to extend the soldere to the first few millimeters of the pcb track. This way you would bridge possible cracks.

Excellent idea. I'll look into extending the solder around these joints. Might provide a little more mechanical support too.

I noticed a mistake in my op. It's a 3 year old amp, not 3rd old amp. Damn iPad!

[Mooly]

Originally Posted by somejohndoe

It's two channel, bridgeable. It's not the relay because that has a particularly mechanical sound when connecting and that doesn't happen. Also, the protection indicator LED does not light. Both channels fail but the left more so than the right. At some points the output sounds crackly, very much like a loose connection.

Thanks for your help, btw.

If the contacts are oxidised it can give the fault you describe, and turning the volume up breaks down the insulating layer and it works.
Don't rule it out just because it gives a positive click Give the relay a tap.

[somejohndoe]

Originally Posted by Mooly

If the contacts are oxidised it can give the fault you describe, and turning the volume up breaks down the insulating layer and it works.
Don't rule it out just because it gives a positive click Give the relay a tap.

I'll give it a go when I have reassembled the amp today. I would be surprised if the contacts have oxidized after only three years but I suppose if there is a manufacturing fault in the relay, it might be possible.

[gmphadte]

Cracks in the solder can best be viewed by using eye loupe, the watchmaker's lens.
If you inspect the areas around hot components(mostly near heatsinks), you will notice a circle formed on the solder, which is a crack. Look for any such circles and solder them and other nearby ones.

gajanan phadte