Capstone Electronics, LLC
This is one receiver that kicked my rear all over the bench! This unit came in stuck in protect mode when powered on. From the start most likely had bad output device(s). This receiver has a 7 channel power amplifier, so lots of devices to check. Eventually I found a channel with the expected blown output transistors. After these were removed from the board, I started finding other issues within the same channel. EVERY transistor on that particular channel was blown along with their associated cooked resistors. OK, time to make a shopping list, resistors "check" no problem. New capacitor for power supply "check", I have no idea of why this failed. It seems fairly isolated, probably the off brand part just failing. Now on to the transistors...Output transistors, "check", small TO-92 transistors and a TO-126 transistor.......this where things start going wrong. For some reason Denon picked some of the hardest to locate devices. They must have been given a great deal on them thats for sure. I eventually, and I do mean eventually, find some of the small signal devices in stock...thanks to Talon Electronics for these. The 2SD947 that is used to bias the output stage (Q103) is now end of life from what I can tell. I did find the datasheet for the 2SD947 and began to use it's parameters to try and cross a current device to fit into the circuit. I found a match that should work, we'll just call it device "number 1" for now. By the way, most of the transistors in the power amplifier channel are Darlingtons (not necessarily wrong, but a great way to cut cost.) Well, after getting all of the new parts installed I ran the receiver power up slowly with a variac while watching the amplifier's output for DC voltage as well as monitoring the current. So far so good, at this point input voltage was up to around 100 VAC a slight DC offset but not enough to be alarming. OK close enough, let's remove variac and plug in to 120V. After powering on, the power amp channel blew within 10 seconds. What?!?!
Thankfully only the output devices where blown and their resistors opened this time. Round #2, put in new output devices and resistors again and this time lets be smarter on how we do things. This time when the unit was powered up, I was monitoring the current through the emitter resistors. Ah Ha!! WAYYY too high. OK, this tells me to check the bias current area of the amp which is the now questionable device "number 1". On a whim I looked up the original 2SD947 in NTE cross reference. I can't believe it but they actually have a cross for it. Great!! These guys have been at it for years, they know what they're doing...I order a NTE253 to put in place of the original. Eventually these come in and get installed. Now we are on device "number 2". I then do the same start up process as last time. As the input voltage is increased, the bias current is much lower that before, however it is still way too high, although it can actually be adjusted now. I still can't get it close to where it needs to be. The channel works now, but is running very warm. I shutdown the unit start scratching my head...I give it a couple of days to process in my head...Then it hits me...If these are Darlington devices, there are probably some internal resistors to set the bias in the individual transistors inside the device. I went back and got their values from the NTE253 datasheet, and then I went back to the datasheet of the original 2SD947 device. There was a significant difference in the values. OK, now what...at this point I can either change resistors on the channel to get the desired bias point (REALLY don't want to hack things up like that) or send it back to the owner (don't want to do that either.) Off to the internet...again. Several searches later, I found a distributor in China that had some of the original 2SD947 devices.
I figured it was going to be my last attempt before throwing in the towel..So device "number 3" finally arrives. I install it and everything is perfect!
After trimming the bias current and a 48hr burn in, it is on it's way back home.
Above is the extent of the devices in one of the amplifier output stages..3 whole devices. For what its worth, Q104 is the only device that is protecting the entire amp stage from short circuits on the speaker outputs. This would be a good idea if shorts only occurred on the positive half-cycle!! Come on guys, spend a little extra and go the extra mile.
Take aways from this one:
1. Pay attention to details. Overlooking the smallest detail cost me a lot of time chasing problems. A more robust design would also have been a good thing as well.
2. Replacement semiconductors don't always work. Even from "the big guys".
3. Every manufacturer cuts corners, no matter their reputation.
Every once in a while a repair comes in that test the limits of sanity...I thought I would pass these experiences along to hopefully help out someone down the road.