Faulty amp?

Mario Pankov · 7th September 2011, 08:35 AM

Hello all,

I`m not very much into amps, perhaps this is the best place to ask

I do own an old Denon PMA-860. Since an year or so I started to feel as it has lost its bass, one day I put a NAD 3020 into its place and it just confirmed the Denon is out of bass. Then, starting in this year it developed a crackling noise in both channels, I initially thought this might be because of the volume know ( I always run it source direct ) but contact spray didn`t help. Then, when I was doing some measurements on my speakers, I noticed that low volume sound comes from the other speaker, not the one I measure and its channel was disconnected from the amp. Today the amp produced a bump like the DC protection has engaged but it was not off. It kept working but no sound was coming out of the speakers. I connected a different speaker - no sound at all.
I guess the amp is dead but just wanted to confirm this?

Mario Pankov · 7th September 2011, 10:22 AM

Just to add, I opened the amp today and noticed that when on standby ( no connected speaker or interconnects ) one of the two radiators was burning while the other one was cold. Each has four output transistors, I suspect there is a short in one of them?

jaycee · 7th September 2011, 12:38 PM

I believe the input FETs are vulnerable on this amp - that is TR501-504 on the main board.

The hot channel points to a more serious problem - possibly a consequence of the damaged FETs

Mario Pankov · 9th September 2011, 05:41 PM

Thanks for the input. I left it for servicing today.

7th September 2011, 08:35 AM	#1
Mario Pankov diyAudio Member Join Date: Aug 2007	Faulty amp? Hello all, I`m not very much into amps, perhaps this is the best place to ask I do own an old Denon PMA-860. Since an year or so I started to feel as it has lost its bass, one day I put a NAD 3020 into its place and it just confirmed the Denon is out of bass. Then, starting in this year it developed a crackling noise in both channels, I initially thought this might be because of the volume know ( I always run it source direct ) but contact spray didn`t help. Then, when I was doing some measurements on my speakers, I noticed that low volume sound comes from the other speaker, not the one I measure and its channel was disconnected from the amp. Today the amp produced a bump like the DC protection has engaged but it was not off. It kept working but no sound was coming out of the speakers. I connected a different speaker - no sound at all. I guess the amp is dead but just wanted to confirm this? __________________ www.mariopankov.com

7th September 2011, 10:22 AM	#2
Mario Pankov diyAudio Member Join Date: Aug 2007	Just to add, I opened the amp today and noticed that when on standby ( no connected speaker or interconnects ) one of the two radiators was burning while the other one was cold. Each has four output transistors, I suspect there is a short in one of them? __________________ www.mariopankov.com

9th September 2011, 05:41 PM	#4
Mario Pankov diyAudio Member Join Date: Aug 2007	Thanks for the input. I left it for servicing today. __________________ www.mariopankov.com

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7th September 2011, 12:38 PM	#3
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	I believe the input FETs are vulnerable on this amp - that is TR501-504 on the main board. The hot channel points to a more serious problem - possibly a consequence of the damaged FETs

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