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Repairing Denon AVR-900 Receiver

post #1 of 21
Thread Starter 
Hey guys... This thread is being moved from the Dedicated Source forum because it's more of a DIY task.

I just acquired a Denon AVR-900 receiver that doesn't power on... The standby light flashes, so my guess is that it's in "PROTECT" mode. From what I've read, I might need to "initialize the microprocessor." I don't have the manual, but after reading other threads about this issue, I know that it involves holding a button combination while plugging in the receiver.

Does anyone know the button combination for this particular receiver?

I opened up the case and the fuse appears to be in tact; I'll test for continuity with a multimeter later. Everything else appears to be in place, though I noticed some white goop under the larger capacitors. I'm assuming that's there for isolation/shielding purposes.

For you gearheads, here are a few pictures of the insides (click for higher resolution):

Top view

Power circuitry

Side view of main board and daughterboards

All three fuses appear to be in tact. I tested them using the continuity mode on a multimeter. Any suggestions?
post #2 of 21
Does that avr-900 have a model number different on the back as opposed to the number on the front? Looked through my denon dealer stuff does not come up.
post #3 of 21
Thread Starter 
It says "Denon AVR-900" on both sides... I've been having trouble finding information about it, but I think that's mostly due to its age; it's only a Dolby Pro Logic receiver, and was probably a budget model.
post #4 of 21
Thread Starter 
Any other suggestions?
post #5 of 21
I just picked up a non working AVR 900, there are a total of 4 fuses inside the unit: 2 on the protect board, 6.3A and 8A, and 2 in the middle near the front.

I was told by Denon the following might reset the microprocessor.

1) unplug unit
2) press and hold tuner and video select on front panel
3) plug in while holding buttons

Unit display should flash until you let go of buttons.

Unit stays on you should be ok/ goes back to red- service

They also said:

There are 3 types of protection that will commonly occur with our receivers. This is designed to protect the unit from permanent damage.

1) Thermal Protect = When the unit overheats this will occur, usually if there is not enough air space above and around the unit. There cannot be anything directly on top of the unit and there must be at least 4 inches of airspace above the unit.

2) Overload Protect = Most commonly occurs when a strand or more of copper speaker wire is not securely connected to the speaker terminal and is touching the chassis of the receiver. If the volume is turned higher than 85% this may also occur. If you need to turn the volume higher than this point to get the level you want, you need a more powerful amp.

3) DC Protect = When an amplifier fails this will occur. This will protect dc current from damaging the speakers. When this happens the unit will need to be serviced.

I had a bad 6.3A fuse – I replaced it and tried to reset. The microprocessor would not reset.

A IC chip on the front panel went up in a small puff of blue smoke.

I hope you have better luck.

post #6 of 21
Thread Starter 
Thanks for the informative post, and welcome to Head-Fi!

Very, very informative. I completely missed seeing the 6.3A fuse by the power section... and it is indeed blown. I also tried resetting the microprocessor, just because I finally had the button combination (why do they keep changing it?), but of course that didn't work.

So there's problem #1, diagnosed.

This is the second piece of Denon gear I've acquired, and now it appears that both pieces suffer from possible manufacturing defects. Grr.

What do you think caused that particular fuse to blow?

Which fuse did you buy to replace the 6.3A fuse, where did you get it, and how much did it cost?

Why do you think the chip fried?

The plot thickens...

Will these slow-blow 6.3A 250V fuses from Radio Shack work?

post #7 of 21
Thread Starter 
(Edit: I should add that in all cases, nothing was plugged into the receiver.)

I decided to go ahead and buy the 6.3A 250V fuses from Radio Shack.

I put one in (with the amp/voltage specification facing the same way as the original fuse... are fuses directional?)... Here's what happened when I plugged in the receiver:

-a loud power hum
-about half a second later, a blue/green flash of light

I unplugged it and checked out the damage. It looked like the 6.3A fuse had blown again (a silver spray on the inside), but I checked the continuity and it was still good. However, the 8A 250V fuse had blown.

Everything else appears to be intact...

What's the next step to troubleshoot what appears to be a power problem?
post #8 of 21
I bought the 6.3A at RS - they are not directional. We both had almost the same results. I salvaged some knobs and connectors and the AVR900 will be tossed out.
post #9 of 21
Thread Starter 
Ouch... Shouldn't Denon know what's going on? Somehow, I doubt our experiences are isolated.
post #10 of 21
What can you do? It's not a car on the highway. No loss of innocent lives. Even if a batch were defective, it is out of warranty. The AVR 900 is about 10 years old.

Denon did not have the owner's manual or service manual when I called last week. Not even a .pdf version. You will find nothing re. the AVR 900 on the Denon website. It is unfortunate that this happened. The only good to come of it is to know NOT to purchase Denon product in the future.

I have a Kenwood amp from 1976 - flawless. Never a problem.

I asked Denon if it was worth repairing if the reset failed. They said "no."

If you look at the front board you might see the chip that "fried" on my board, it is under the ribbon connector in the photos.

post #11 of 21
hmm something could be shorting out near that fuse, and the flash is kinda eep never a good sign. For fuse selection in electronics someone correct me if I'm wrong but don't you want a quick blow? slow blow would be for a motor or some sort where starting currents will be higher. I know this doesn't help diagnose but my suggestion would be to look around that fuse for a short somewhere on the board or looking at some of the compenents and checking them for defects.
post #12 of 21
Try unsoldering the output chips, so they are dis-connected. Replace the 6.3 A fuse (using a 6amp would be ok)(only use the slo-blow for diagnostics, replace with regular fuse once its fixed) and try powering up again. If it doesn't blow the fuse, then connect one output device at a time and powering up. When one of them blows the fuse, you'll know what to replace.

The more professional approach is to check each device for short circuiting with a DVM or DMM. Find one shorted, disconnect and continue testing the rest of the chips. If you need, Google , there's alot of information out there.
post #13 of 21
Thread Starter 
A few questions...

What do you mean by "the output chips?"

How would I begin testing with a DMM?
post #14 of 21
Originally Posted by infinitesymphony View Post
A few questions...

What do you mean by "the output chips?"
Ok, can you post a picture of the devices that are mounted to the heatsink?

How to use a DMM

Google is your friend!

Buy a service manual when in doubt!

I'm happy to show you some light, but you'll need to do the heavy work
post #15 of 21
Thread Starter 
I understand how a DMM works, I was just wondering if there was a testing methodology I could use without having the service manual.

Is this what you wanted to see? (click for larger image)

Here's another angle of the power board:

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