Add denon PMA 720 repair log

content/posts/2016-04-10-denon-pma-720a-repair-log.md

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+date = "2016-04-10"
+title = "Denon PMA 720a Repair Log"
+tags = ["ohmexpress", "repair", "audiorepair"]
+description = "A classic 1989 Denon PMA 720a Integrated Amp that made a bang and wont't turn on. Let's solve this!"
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+
+![Denon PMA 720a Front](/img/denon-pma-720a-front.jpg)
+
+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](/img/denon-pma-720-scorched.jpg)
+
+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:
+
+{{< youtube 0tRGVj7n3vc >}}
+
+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](/img/denon-pma-720-pcb-back.jpg)
+
+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](/img/denon-pma-720-pcb-rework.jpg)
+
+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!](/img/denon-pma-720-fixed.jpg)
+
+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?