Complete first draft
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@ -108,17 +108,60 @@ of the scope output.
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{{< youtube rtnBzLxHoA0 >}}
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This means one (or two) of two things:
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This is good. I now have a provable and reproducable fault condition. The output of the `SBR` channel will get
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a load of DC voltage during start up and seems to have trouble stabilizing when hot. Cooling down resolves the issue.
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The `SBR` channel is not in use, so any heat it takes is from the heatsink.
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1. There's a bad capacitor on the `SBR` amp board
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2. The output transistor(s) are faulty when warm
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Next step: take out the board and check the capacitors.
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Now as to what is causing this and how to fix it!
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![](/img/denon-avr4520-sbr-amp-board.jpg)
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I always mark the capacitors with a permanent marker by ticking one side (e.g. )
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Above is the `SBR` amp board taken out of the amp. I have three things I'd like to check:
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1. The 1/4W and more resistors (basicall all non SMD resistors on top). I'd like to check for any shorts, open loops, or a deviation in resistance from the spec?
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2. All electrolytic capacitors: how does their capacitance and ESR compare to the spec?
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3. Power darlington transistors: any obvious shorts? How do they react to heat?
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I always measure resistors in circuit first. This works fine most of the time and when I get a weird
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read out I can desolder them when I'm working on the capacitors. All of them seem fine.
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Next I desolder one (or two) legs of each electrolytic capacitor. I always mark their orientation in
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relation to the board before taking them off. Some PCBs have polarity indicators, some do not. All
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electrolytics measure within their specifications for capacitance and ESR. So no issues there.
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That leaves the darlington transistor pair. My gues is that they start to misbehave when hot. Keep in
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mind that the `SBR` channel is not used in my stereo configuration, so they do not produce any heat
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themselves. They get heat from `FR` that's mounted on the same heatsink. The plan to test these
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darlingtons is simple:
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1. Run them amp until it gets hot
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2. Try to get the amp hot enough to get into the DC Protection mode.
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3. Freeze spray each of the darlingtons to see if it resolves the problem
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Freezing the `DHCT-A3` has no effect on power protection. Freezing the `DHCT-C3` however makes
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the problem go away.
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![](/img/denon-avr4520-darlingtons.jpg)
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With that, I've narrowed the problem down to `Q808`, a `DHCT-C3` darlington transistor.
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## Repair
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As with almost every project, diagnosign the issue is much harder and time consuming than the
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actual repair. The process is quite simple:
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1. Order a new pair DHCT-A3/C3 (they are sold in pairs)
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2. Replace both `Q808` and `Q809` with the new pair
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Done.
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## Conclusion
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Replacing the DHCT-A3/C3 darlington transistor pair has resolved the power protection mode
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issues. I've been running the amp in my living room for a few days now and not seen any
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issue at all.
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So, for the cost of a used entry-level AV receiver and €40 in parts and shipping I've got
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an almost brand new Denon AVR-4520 top-of-the-line AV Receiver. :-)
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[hifiengine]: https://www.hifiengine.com/manual_library/denon/avr-4520.shtml
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