Class D + No load = Shorted tda7492?

riotpack · 11th February 2012, 06:27 AM

So today I am working on a JVC mini system which uses a SMPS and class D amplifier based on TDA7492 amp IC and a TDA7468 digital preamp bass/treble etc. The fault was the CD player not reading, I fixed what I though was a bad cable to the laser and tested it without the speakers connected.

I then disassembled the stereo again and touched up some solder joints around the focus/tracking for the CD player. I put it back together and get nothing at all. I measure voltages and see the SMPS is in hiccup mode so I inspect all my work and tried again but still nothing.

I thought I may have damaged one of the thin ribbon cables but as a hunch I removed the Vamp rail connector on the SMPS and the amp powered up instantly.

I still didn't know what was shorted as a few things run from this rail, so I connected the board to a variable power supply set to 2 volts at 3 amps to smoke out the component causing the SC.

It was the Class D amp IC getting hot, so in a blaze of fury I set the current to 10 Amps and let the damn IC desolder itself!

Is this common? Will I have to put a dummy load across outputs of class D amps while working on them? I didn't even go near the amp board and there was no music going to the amp as the CD player hadn't been repaired.

Saturnus · 11th February 2012, 07:20 AM

Quote:

Originally Posted by riotpack

Is this common? Will I have to put a dummy load across outputs of class D amps while working on them?

Yes, and yes.

Pafi · 08:55 AM

No, and not neccessarily. A good amp can not die because of no load, especially without driving.

riotpack · 11th February 2012, 11:54 AM

Yes I actually wondered where the stress (there was no load) came from which actually caused the IC to fail. Is it just runaway oscillations? The IC is supposed to have over current (short circuit) and over temp. This is not the first time I have had class D amps fail, they seem very fragile.

nigelwright7557 · 11th February 2012, 11:37 PM

My 2092 based class d amps seem to run ok with no load.

However the 2092 is touchy about scope probes put on while it is running and is easily blown up.

Another thing I found with the 2092 was I blew one up by having a low energy lamp radiating near it. It is prone to interference.

11th February 2012, 06:27 AM	#1
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	Class D + No load = Shorted tda7492? So today I am working on a JVC mini system which uses a SMPS and class D amplifier based on TDA7492 amp IC and a TDA7468 digital preamp bass/treble etc. The fault was the CD player not reading, I fixed what I though was a bad cable to the laser and tested it without the speakers connected. I then disassembled the stereo again and touched up some solder joints around the focus/tracking for the CD player. I put it back together and get nothing at all. I measure voltages and see the SMPS is in hiccup mode so I inspect all my work and tried again but still nothing. I thought I may have damaged one of the thin ribbon cables but as a hunch I removed the Vamp rail connector on the SMPS and the amp powered up instantly. I still didn't know what was shorted as a few things run from this rail, so I connected the board to a variable power supply set to 2 volts at 3 amps to smoke out the component causing the SC. It was the Class D amp IC getting hot, so in a blaze of fury I set the current to 10 Amps and let the damn IC desolder itself! Is this common? Will I have to put a dummy load across outputs of class D amps while working on them? I didn't even go near the amp board and there was no music going to the amp as the CD player hadn't been repaired.

11th February 2012, 08:53 AM	#3
Pafi diyAudio Member Join Date: May 2004 Location: Budapest	No, and not neccessarily. A good amp can not die because of no load, especially without driving. Last edited by Pafi; 11th February 2012 at 08:55 AM.

11th February 2012, 11:37 PM	#5
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	My 2092 based class d amps seem to run ok with no load. However the 2092 is touchy about scope probes put on while it is running and is easily blown up. Another thing I found with the 2092 was I blew one up by having a low energy lamp radiating near it. It is prone to interference. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

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11th February 2012, 11:54 AM	#4
riotpack diyAudio Member Join Date: Sep 2011 Location: Brisbane	Yes I actually wondered where the stress (there was no load) came from which actually caused the IC to fail. Is it just runaway oscillations? The IC is supposed to have over current (short circuit) and over temp. This is not the first time I have had class D amps fail, they seem very fragile.

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