Studio monitor buzzing and crackling. Bad cap?

[jp8]

This does NOT mean that all the other electrolytics in the unit are or are likely to be in a similar state as these 4.
It seems companies make poor choices, rather too often, and cheap out with things like a particular value capacitor at a particular voltage. Or maybe they get ripped off/misled by the cap manufacturer, or they produce a sub standard batch, or they get damaged during assembly. Who knows.

I totally agree with you that this does not mean the others are necessarily in a similar state. But as you said yourself, they could have failed for any reason, which could include being of an inferior brand. If all caps used are of that same brand, I personally would have replaced them all. But your mileage may vary, of course.

I'm not a repair engineer, but my son is and his company repairs industrial electronics. If there's just one bad cap, they always recap the whole PCBA. Or even when it has reached a certain use time without caps failing, they swap them all. OK, it's different class of electronics and way more expensive, so it's worth the money. But if you decide to repair your own stuff, why not spend a few bucks more and an hour of work if you can prevent future trouble?

From my own experience: I have a Mackie ProFX12 mixer, of which ALL 47uF coupling elcaps of the FX unit were broken and measured below 1uF. It was maybe 6 to 8 years old or so and all those caps were already dried out without being overstressed. I then randomly measured several others and one was close to the lower tolerance limit. I then decided to replace all elcaps in the mixer, simply because I didn't trust the junk brand elcaps Mackie chose to use.

Jan

 

[NoisyIndividual]

If all caps used are of that same brand, I personally would have replaced them all. But your mileage may vary, of course.

Well, yes it does.

This thread is primarily about an Adam Monitor that is known for the premature failure of those 4 main power supply caps, not failure of all the others on the PCB as well.

Someone messed up with those 4 caps.

 

[RoadrunnerOZ]

I put a scope on the power rails while the unit is faulting and see if there are dips or spikes. In audio circuitry there are usually additional filter caps at the opamp - if they are tantalum I’ll replace them - any overvoltage spikes will cook these.
As NoisyIndividual said, if you’re charging a customer you can’t just blindly replace all the caps as the labour can cost more than a replacement board. If the supply is on an independent board I disconnect it, dummy load it and see if it is the issue. I have test supplies I can quickly hook up to run the amp boards separately as well. + 24,15,12, and 5V but if the voltage is outside that range I have to work with the inbuilt supply.
If you’re doing it to sell you can afford the time and component cost otherwise the unit is just a brick.
Depending on the age of the unit there is always the possibility that it was manufactured using caps from Taiwan that had a faulty electrolyte 1999 - 2007 or even later - these would swell due to gassing of the electrolyte and blow. See below:
https://academic-accelerator.com/encyclopedia/capacitor-plague
The other issue is 85deg caps in an a high heat environment - I replace these with higher temperature ones - if a power rail cap fails it’s likely the others of the same kind in the same location will be on their way out. If I have a pair of monitors and only one has failed I’ll do both (if the customer agrees to the cost).

 

[Khron]

Someone messed up with those 4 caps.

Namely, the bean-counters...

 

[Diamondj421]

I messed up... I took a picture of the PCB I'm working on and labelled each cap with the value as I removed them. I wasn't incredibly thorough because I thought, worst case scenario, I could always reference the other monitor if I got lost along the way. Well, I got lost along the way. I didn't label 2 of the caps so opened up the other monitor and it turns out, they're different revisions of the board. The one I'm working on is Rev C2 and the other is Rev D1. Is there any way someone could tell me the values of the electrolytic caps for C19 and C20? I wrote to ADAM 4 days ago and haven't heard anything.

 

[JohnRoberts]

as I have advised numerous times if you identify old/tired capacitors during a repair, replace all of the same value, same voltage, same brand capacitors as they are likely from the same production lot and equally suspect.

JR

 

[Diamondj421]

That’s what I did/am doing. I messed up when removing the old ones and somehow forgot to note the values of 2 caps. I’m just unsure of the values for C19 and C20. Didn’t know if anyone had this information.

 

[NoisyIndividual]

It's quite possible Rev D1 addressed the known in hindsight Cap problem of Rev C2 with a different value/rating.
Or just a different manufacturer who don't supply shit caps.

So perhaps it might be wise to replace these caps with a high quality/high temp version of whatever is in D1.


Consider it an upgrade maybe.

 

[JohnRoberts]

I have told this story before but I knew an amplifier designer who cheated on rated capacitor voltage. There are dueling ASSumptions. Capacitor makers ASSume that design engineers will not use caps all the way up to their maximum voltage rating, and amplifier engineers assume that capacitor makers are conservative about their specifications.

This engineer ASSumed he had more voltage headroom than he did, the result was a series of amplifiers that resembled hand grenades.

JR

 

[RoadrunnerOZ]

That’s what I did/am doing. I messed up when removing the old ones and somehow forgot to note the values of 2 caps. I’m just unsure of the values for C19 and C20. Didn’t know if anyone had this information.

Assuming you haven’t thrown out the old capacitors you removed, eliminate the ones from the removed caps that you did note the values for, the two you have left over will be the value you are looking for

 

[RoadrunnerOZ]

I messed up... I took a picture of the PCB I'm working on and labelled each cap with the value as I removed them. I wasn't incredibly thorough because I thought, worst case scenario, I could always reference the other monitor if I got lost along the way. Well, I got lost along the way. I didn't label 2 of the caps so opened up the other monitor and it turns out, they're different revisions of the board. The one I'm working on is Rev C2 and the other is Rev D1. Is there any way someone could tell me the values of the electrolytic caps for C19 and C20? I wrote to ADAM 4 days ago and haven't heard anything.

Can you not tell the values from your photos?
I photograph everything when I am doing a service - the front panel, inside: all the boards and/or sections of larger boards to get a complete picture plus all wiring connections to every board in case a lead is frayed and comes off, then any swollen caps including their orientation and case labelling and write their cap number on top with a superfine Sharpie, burnt or blown components etc. I dot mark or label and photograph any cabling connectors before removal from inter board connections. All case and board screws go into compartmented boxes, labelled with location on paper labels stuck inside each compartment.
When doing Mac laptops I make a chart sheet to coincide with the disassembly instructions from iFixit for all the screws as they are often the same thread but differing lengths etc.
Makes life a whole lot easier.

 

[Diamondj421]

That’s the thing. They aren’t in the picture so I think I figured out what’s going on. The old board is for the A7X or the A8X. The newer board doesn’t have C19 or C20 and is specific to the A7X so I’m assuming the C19 and C20 are only needed for the A8X. I thought that I had possibly removed them before taking the picture but now I’m pretty sure they were never there in the first place which makes more sense. I had looked at pictures of an A8X repair and saw the caps there which got me thinking I was missing them but now it’s all coming together.

 

[RoadrunnerOZ]

That’s the thing. They aren’t in the picture so I think I figured out what’s going on. The old board is for the A7X or the A8X. The newer board doesn’t have C19 or C20 and is specific to the A7X so I’m assuming the C19 and C20 are only needed for the A8X. I thought that I had possibly removed them before taking the picture but now I’m pretty sure they were never there in the first place which makes more sense. I had looked at pictures of an A8X repair and saw the caps there which got me thinking I was missing them but now it’s all coming together.

Cool. You must be able to count off the old caps you removed anyway?