Complete first draft

content/posts/2017-09-11-denon-avr-4520-repair.md

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