Are my caps leaking? (and did it toast something?)

[mansr]

1 hour ago, gmgraves said:

 

This is pretty good indication of at least ONE shorted cap and perhaps both.

I think that's more likely glue that's meant to be there. There's more of it elsewhere on the board.

[mansr]

I was recently goofing around with an old Cambridge Audio AVR when it started emitting smoke. I found this behind the speaker terminals:

Only visible after some disassembly.

[mansr]

10 minutes ago, m5sime said:

Thank you. That's my dilemma....  Is it glue gone brown or leakage..  I may well as suggested check cap resistance..  The amp is working and has been fine. 

 

The smoke was very odd 

Brown glue is not unusual. If you poke it with a screwdriver or similar, it should be obvious if that's what it is.

[mansr]

3 minutes ago, m5sime said:

None or the caps are bonded down that have the brown goo so hard to tell if it was supposed to be a bonding. They wobble a little 

If you prod the brown substance, is it rubbery or brittle. Leakage from capacitors is usually nothing like glue in texture. Also, capacitors rarely smoke gently. They either leak slowly or blow up with a bang. Leaked electrolyte can cause shorts resulting in other components overheating, however.

[mansr]

3 hours ago, m5sime said:

I did get to the bottom of my smoke though.. two teeny resistors gave their life for a noble cause.. Now just to find out from the manufacturer the values as I cannot see from the damage.. Same each side. Thank you @gmgravesand @mansr

 

This looks similar to what happened with mine. Is the CN3 connector by any chance the speaker output? You'll probably find that the resistor is connected in series with the little capacitor (C29) next to it across the output. It's likely that only the paint on the resistor was smoked and it is still close to its proper value which if I'm right is around 10 ohms.

[mansr]

43 minutes ago, m5sime said:

Hi Mansr, No, the connector is the input I believe..

In that case it's a different problem than I had.

43 minutes ago, m5sime said:

This amp board is pretty complex being all discrete components.. I think the resistor is part of the on-amp board PSU. Both smoked and both measure 19k8 so I am hopeful they survived. I have run the amp without inputs for 30 mins and checked temps and all ok. 

Strange. Those look like ⅛ W resistors. To reach smoking temperatures they'd need to be dissipating quite a bit above their rated power which at that resistance means a considerable voltage. 140 V would give about 1 W. Burnt resistors generally increase in resistance as the conductive material is destroyed, so it's possible the original value was lower.

[mansr]

19 minutes ago, m5sime said:

Plot thickens.. I have the repair/service manual. Both resistors are not the right value now.. But they seem to be some kind of protection circuit.. There is the full PCB schematic for the amplifier board. and then this on the page.

 

 

would you have any idea what this is for? Is this a PCB ground circuit to limit current?

It probably has something to do with the chassis grounding assuming that's where the screw goes.

[mansr]

2 minutes ago, m5sime said:

 Think I solved my mystery.. My new DIY starquad Dc cable managed to short +12v from my external dac PSU. To the shield of the connector when I bent it as installing it. The chassis of the external linear PSU is grounded so I assume I sent +12vdc into mains earth. This in turn created a voltage across the amp resistors that isolate the chassis earth from the PCB signal ground.

That makes sense. 12 V across 47 ohms generates 3 W of heat, far more than that resistor is designed to handle.