Denon PMA 720a Repair Log

10 April 2016

Denon PMA 720a Front

I found this Denon PMA 720a on Marktplaats (a popular Dutch alternative to Craigslist) advertised with “made a loud popping noise and won’t turn on”. Being a huge Denon fan, I picked this one up and got to work.

Diagnostics

Well, it won’t turn on, eh? This could be for a number of reasons, so let’s remove the cover and see what we’ve got. Everything looked just the way it’s supposed to look. Little to no dust or dirt. Except one thing.

Scorch marks on the power button PCB

The blue/brown are 230VAC mains, and it looks like the two got shorted. But that should not happen. Not without an external force causing it.

The only logical explanation I can think of is lightning.

The problem here is that the PCB between the brown and blue wires is carbonized and is now conductive and flammable. Yikes!

Curious as I am, I turned on the amp. I thought this to be safe, since if there is a short that could cause damage, the fuse will blow.

After the first bits of carbon went up in smoke, not without some fireworks, this is how it looks:

You can clearly see the occasional spark creeping across the burnt area. If you want to burn your house down, this is a good way to do it.

Repair

Luckily, this PCB is pretty simple. It connects the power transformer to mains AC through a switch and a fuse. There’s also this big blue capacitor.

Power PCB Traces

The pins at the bottom are not all used. Besides the brown/blue from the AC cord, there’s a white+yellow pair going to the power transformer.

The blue/white wires are connected directly and are not used in the PCB itself.

Step 1 in this fix was to get rid of the wires and connector pins, simply cutting them off did the trick.

Next I marked the area that was burnt and cut it out. My Dremel made short work of that.

Last step: solder the wires directly onto the PCB and connect the blue/white wires off-board. I soldered the wires together and added some shrink wrap to keep them safe.

Reworked PCB

The wires will not be under stress, as they power cord is tied securely to the inside of the chassis about 5cm away from this board.

Results

After mounting the PCB back, let’s give it a try! No sparking or magic smoke, just the power-on light and after a few seconds the satisfactory click of a relay engaging.

Fixed it!

I quickly cleaned the pot-meters and switches while I was at it. This machine has been going strong since 1989 and it looks good to keep going for at least a few more years.

Now I have a good looking, perfect sounding 2x 85W Denon PMA 720a Integrated Stereo Amplifier. The question is: keep it or sell it?