Help needed in amp troubleshooting.

Tiim

Disabled Account
2016-02-03 1:29 pm
Stockholm
Hello you!

I´m in need of some assistance trying to troubleshoot my Class D amp and i hope someone kind could help me out here!

This is regarding the subwoofer amp on my FBT Amico 1000 PA system.
(Some info of this system in this link: http://www.audiotechnology.com.au/PDF/REVIEWS/AT40_FBT_Amico_1000.pdf)


A description of what happened, and whats up with it now:


I had been playing music for a couple of hours, testing the system and had a feeling something was a little bit unstable (Small pops and hiss noises) but still it worked ok.
Cranked the volume up and bang! The sub started popping and banging, with a loud hum, at this moment there was no more music playing trough the sub, but the satellites kept playing uninterrupted.. (I played louder than this before, and the system have an protection circuit)

After that there is only hum and small pops, (sounds a bit like thunder) but no music signal is getting through. Today there is only hum, like 50Hz ground hum. (I live in Sweden)

I have made some measurements, but i´ll leave that aside for now.




Would someone be kind enough to assist me on troubleshooting this thing?

(I have a multimeter, and just basic knowledge about electronics and schematics.)

The schematics was to big to upload here so this is a link to my dropbox:
https://www.dropbox.com/s/wr0ui0j9ymfqvis/AMICO1000.pdf?dl=0


Thank you for your time!
Regards
Tim
 
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Tiim

Disabled Account
2016-02-03 1:29 pm
Stockholm
Thank you for your reply JonSnell!
That was what i thought, i´ve been trying to borrow one but haven't got my hand on one yet.
What exactly do you mean with no input in lower amp?
I can tell you this: without the jumper cable from mixer there is still hum. I have tried disconnecting the transformer power to LF power supply, and there's still hum.
 
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Tiim

Disabled Account
2016-02-03 1:29 pm
Stockholm
Update!

I forgot to mention that my multimeter can measure frequency too, if that is of any help. (It's a UNI-T UT61B)

I made an audio-probe to trace the signal through the amp, it picks up a lot of interference from the air though.. Im finding it hard to tell if the signal to LF amp is getting LP-filtered right after the mixer Sub/LF output so i haven't used it yet.


Question!
I have to unsolder the MOSFET's to check them, right?

Thank you in advance!
Regards
Tim
 
I meant with no signal source to the IR2110. Here is a data sheet that may help to give you an idea of what to look for, http://www.irf.com/technical-info/appnotes/an-978.pdf
Mos Fets can be measured in circuit. They fail to short circuit, so pretty easy to check. Beware, the gate capacitance can switch the device on so Gate to Source link momentarily to remove any gate voltage.
Hope that helps.
 

Tiim

Disabled Account
2016-02-03 1:29 pm
Stockholm
Thank you! :) I'll can not emphasize how much i appreciate your help here.

I'll try that and see what comes up. One problem is that the preamp PCB are covering the IR2110's, and i'm not sure how they behave if i remove the preamp, if I'm not mistaken there are PTC-connections involved there too..

Here you can see one of the IR2110's:
https://www.dropbox.com/s/bb8ajtuisw5loip/IMG_0044.JPG?dl=0

Anyways, I'll start with checking for shorts in the LF PU MOSFET's.
 

Tiim

Disabled Account
2016-02-03 1:29 pm
Stockholm
Update!

I can't get any proper reading of the output MOSFET's. I don't know if it's because they're in circuit or they are broken. Sometime it starts around a volt (in diode check) and climbs slowly to OL. The multimeter can't charge the gate because, if i understand this right, the gate needs a voltage between 3V to 5,5V and my multimeter only gives 2,8V. So i tried charging it with a 3,7V battery but this doesn't change anything..
 

29285

Disabled Account
2010-01-17 9:20 pm
No need to fiddle with the gate - simply check a short circuit between drain and source. If the diode check shows 0.6V or more, obviously this MOSFET is NOT shorted. Drifting diode check voltage are caused by charging supply caps due to active measuring.
 

Tiim

Disabled Account
2016-02-03 1:29 pm
Stockholm
Update.

Yesterday the amp blew a fuse (first time), but i got some measurements done on the LF power supply before that and got some weird readings i guess, can someone confirm this?

The supply rail showed 90V DC. A bit high isn't it?
Then tested for AC and the meter went up and down like crazy. (there should not be any ac here right?)
 

Tiim

Disabled Account
2016-02-03 1:29 pm
Stockholm
Hello again, haven't done much more on the amp.

Never mind the last post about 90V DC, i made some calculations and that seems about right.

The AC part is still a bit weird to me, but i don't have enough experience to understand if this is good or bad. The only thing that differs the first stage LF PS from the HF PS are a couple disc type EMI Filters ("DST310", L1/L2), i don't know how to test them or if they are likely to fail in some way, i have never come across these before..


I made a current limiter, the bulb type, just to be a little more safe. (105W halogen bulb.)

I suppose the amp would benefit of a recap job, the caps are from 2004/2005 and possibly exposed to some stress in their life. They are 85C types, and what i understand 105C would be better for this application.

I find it hard to find new suitable caps for this though. Any tips of brands and types for the power supply section would be much appreciated! :)

Thanks in advance!