Active monitor - power up but no sound during a few minutes

[richard_maven]

Hello all,

I have a problem with one of my active studio monitors. I have them for many years, and it has this problem for a long time, but it's getting worse now. So, specially during winter and colder times, when I power it up, after being off for at least some hours, the power light lights up but there is no sound. Sometimes there is a very soft occasional rattle/noise. After 1-5 minutes a loud pop/bang happens and everything is fine afterwards, it starts working normally without any artifact. After that loud bang it plays perfectly for as long as it is on or warm.
This behaviour does not happen always, maybe 30% of the times I power it up (which is daily).

It's not an external thing, no external signal or power problem - I have the other monitor to test out everything.

When I power them up and this thing happens, if I increase the volume to max I can hear a very soft "pff" sound on the good one and nothing on this one (tweeter or woofer), so I assume the problem is probably on the power amp or the power supply - there is no power arriving at the tweeter/woofer. If it was in the pre amp, the power amp alone at max level would emit some noise, right? Also, by looking at the the schematics, the problem seems to be at the power supply part - this is a bi-amp system, so I suppose if there was some problem in the power amp phase, at least one of the drivers would play something, and they would not recover and return to normality exactly at the same time, right?
The problematic monitor has another weird behaviour - most times, after turning it off, there is a weird discharge noise. I assume this is some capacitor discharging. Might be a clue, I do not know.

It's tricky to test this because it doesn't happen everytime nor for a specific period, and I do not know it it is indeed happening if I remove the board from the case and disconnect the tweeter/woofer.

No capacitors are bulged. No visual clues.

I have already changed the HOT transistor and the power capacitor in the power supply, but I still got the problem.
What should I substitute next? I was going to start with the power supply capacitors/resistors.

The schematics for the power and pre amp and power supply are attached.

Cheers

 

[danadak]

Do you have a magnifying glass or better yet low power inspection microscope
to examine solder joints on the board for cracking, thermal cycling issues,
especially around the hotter running parts ?


Regards, Dana.

 

[richard_maven]

I have thoroughly looked for solder cracks and found nothing. Will do that with a magnifying glass next. Thank you, Dana.

 

[rjenkinsgb]

That type of fault can occur with aged electrolytic capacitors on a switched-mode PSU; I've seen it a few times on industrial gear.

Which specific caps have you changed?

 

[richard_maven]

Only the big 150uF/450V in the PSU (C1 in the schematics). Which ones would you substitute next?

 

[Nigel Goodwin]

Only the big 150uF/450V in the PSU (C1 in the schematics). Which ones would you substitute next?

C1 is the least likely to be faulty, as it runs at low frequency, so doesn't suffer the usual high ESR issues. The switching IC is even less likely, if it was faulty it wouldn't be intermittent, and it would have probably exploded.

Much more likely candidates are C3, C11, C13, C18, C19, C20, C23, C12, C14 and C9. Basically all electrolytics in the PSU, other than C1.

You can test them using an ESR meter (not a capacitance meter), or if you have a scope, check for ripple on the four supply rails, and on the outputs of the rectifiers. Failing either of those, measure the output voltages of the four supply rails - from when it turn it on from cold.

Historically, C11 would be my first choice - as it's used on the regulated rail, and if/when it fails all the other supply voltages will increase (this often causes considerable more damage in many items of equipment, but this is all basic robust stuff).

 

[richard_maven]

Much appreciated, Nigel. Meanwhile I'll try what you said and post the results here.

 

[drtechno]

I would look at the mute standby control circuit. (Q4 on the amp) even though it sounds more like a diode failure in that circuit since 1N4148 diodes flake out and act up with temperature. In the old days we would use a hir dryer and a can of freeze spray to troubleshoot this.

Of course, I think you could remove Q4 to disable the standby/mute since the output chips needs a low to set them in mute or standby.

 

[richard_maven]

I am pleased to say that the problem is gone. I substituted all the 1000uf/50v capacitors on the PSU and the noises/rattles are gone for a week now.
Much appreciated folks!

 

[Nigel Goodwin]

Well done, I'm pleased you've got it sorted out