83 lines
3.0 KiB
Markdown
83 lines
3.0 KiB
Markdown
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date = "2016-04-10"
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title = "Denon PMA 720a Repair Log"
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tags = ["ohmexpress", "repair", "audiorepair"]
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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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slug = "denon-pma-720a-repair-log"
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![Denon PMA 720a Front](/img/denon-pma-720a-front.jpg)
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I found this Denon PMA 720a on Marktplaats (a popular Dutch alternative to Craigslist)
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advertised with "made a loud popping noise and won't turn on". Being a huge Denon fan,
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I picked this one up and got to work.
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## Diagnostics
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Well, it won't turn on, eh? This could be for a number of reasons, so let's remove the cover
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and see what we've got. Everything looked just the way it's supposed to look. Little to
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no dust or dirt. Except one thing.
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![Scorch marks on the power button PCB](/img/denon-pma-720-scorched.jpg)
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The blue/brown are 230VAC mains, and it looks like the two got shorted. But that
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should not happen. Not without an external force causing it.
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The only logical explanation I can think of is lightning.
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The problem here is that the PCB between the brown and blue wires is
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carbonized and is now conductive and flammable. Yikes!
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Curious as I am, I turned on the amp. I thought this to be safe, since if there
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is a short that could cause damage, the fuse will blow.
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After the first bits of carbon went up in smoke, not without some fireworks, this is how
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it looks:
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{{< youtube 0tRGVj7n3vc >}}
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You can clearly see the occasional spark creeping across the burnt area.
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If you want to burn your house down, this is a good way to do it.
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## Repair
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Luckily, this PCB is pretty simple. It connects the power transformer to
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mains AC through a switch and a fuse. There's also this big blue capacitor.
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![Power PCB Traces](/img/denon-pma-720-pcb-back.jpg)
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The pins at the bottom are not all used. Besides the brown/blue from the
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AC cord, there's a white+yellow pair going to the power transformer.
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The blue/white wires are connected directly and are not used in the PCB itself.
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Step 1 in this fix was to get rid of the wires and connector pins, simply cutting
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them off did the trick.
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Next I marked the area that was burnt and cut it out. My Dremel made short
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work of that.
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Last step: solder the wires directly onto the PCB and connect the blue/white
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wires off-board. I soldered the wires together and added some shrink wrap
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to keep them safe.
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![Reworked PCB](/img/denon-pma-720-pcb-rework.jpg)
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The wires will not be under stress, as they power cord is tied securely
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to the inside of the chassis about 5cm away from this board.
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## Results
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After mounting the PCB back, let's give it a try! No sparking or magic smoke,
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just the power-on light and after a few seconds the satisfactory click of a
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relay engaging.
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![Fixed it!](/img/denon-pma-720-fixed.jpg)
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I quickly cleaned the pot-meters and switches while I was at it. This machine
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has been going strong since 1989 and it looks good to keep going for at least
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a few more years.
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Now I have a good looking, perfect sounding 2x 85W Denon PMA 720a Integrated
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Stereo Amplifier. The question is: keep it or sell it?
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