Electronics • AVR Repair
Denon AVR-3312 Protection Mode – Blinking Red LED (ASO) Repair
Step-by-step notes from repairing a Denon AVR-3312 stuck in protection mode with a blinking red LED due to a faulty ASO protection circuit.
⚠️ High Voltage Warning
DO NOT ATTEMPT THIS REPAIR UNLESS YOU ARE EXPERIENCED IN WORKING WITH 240 V CIRCUITS. Let’s be very clear: if you don’t know what you are doing, THIS CAN KILL YOU.
The Symptom
The moment you press the power switch, the AVR attempts to start for around 5 seconds, then switches off with a blinking red LED.
A look at the manual says that it has entered into ASO/DC detect protection mode. ASO is to ensure the amp is not overloaded due to shorted speaker terminals. DC Detect is to protect expensive speakers in case there is a malfunction within the amp circuit.
How to Keep the AVR Switched On for Diagnosis
The hardest thing to do in this situation is to keep the AVR switched on so that you can take some measurements.
If your unit goes into protection with a fast blinking LED (~2 blinks per second) instead of a slow blinking LED (~1 blink per second), you can try the following trick to keep it switched on. Make sure you remove all your speaker connections first as you do not want to blow your expensive speakers – this overrides part of the protection mode.
On most Denon and Marantz AV receivers, there is a 5-pin plug coming from the amp board to the HDMI board with these labels: Thermal A, Thermal B, ASO Detect, DC Detect3, DC Detect4. On this unit, the connector is called BN704.
To override the protection, disconnect the plug from the HDMI board. Use some cables to connect the Thermal A and Thermal B pins (only those 2 pins) between the disconnected plug and the socket on the HDMI board. (Hint: Arduino male and female pin cables are perfect for this.)
This should allow you to keep the unit on briefly to do your diagnosis. (Hint: keep the power-on diagnosis as short as possible.)
Is It ASO or DC Detect?
Now you need to determine whether it is ASO or DC Detect that triggers the protection. With your DMM set to voltage and the negative lead on chassis ground, measure the pin voltage of each of: ASO, DC Detect3, DC Detect4.
The line that has more than a few mV is the culprit.
On my unit, the ASO line measured 2.6 V – and that was what triggered the protection.
ASO or Protection Circuit Fault?
Sometimes ASO can be triggered by a genuine problem with the amplifier circuit. Other times, it can be due to faulty components in the ASO protection circuit itself. The schematic below is an extract showing the relevant protection components.
First step: you have to take the amp board out. Remove all the cables and ribbon connectors. Then unscrew the 7 screws (3 at the bottom and 2 on each side) that hold the heatsink to the chassis.
Then you have to find a power source to power the amp if you want to take some voltage measurements with the amp board out. You will need to connect power to 2 separate connectors on the board, as the last 2 back channels’ power is supplied separately from the first 5 channels.
Hint: if you want to do frequent amp repair, having a ± high DC voltage source is invaluable. If you don’t have one, just buy an old faulty amp on eBay (hopefully with a good transformer) and reuse its transformer / power supply. Try to ensure that the voltage is no higher than the voltage of the AVR-3312 (±65 V DC) or whatever you are repairing. 10–20 V lower should not be a big problem for diagnosis.
If you don’t have access to this, then you will have to do your diagnosis by measuring the components the traditional way – power off and use your DMM to figure out which components have faulty values.
Key Voltages to Check
To determine if it is a faulty amp or a protection circuit fault, the next step is to measure the voltage across the output resistors R7131 / R7132. You can go to the bias adjustment test points (those brownish 3-pin sockets – use the two outer pins and ignore the centre pin) on the board.
If all the measurements are no more than a few mV, then congratulations – you have confirmed that your amp outputs are good.
ASO Detection Transistors
Referring to the schematic, transistor Q7139 turns on if the current across the output resistors (R7131 / R7132) is too high. You can measure the voltage at the base and collector of Q7139. The base should be close to 0 V and the collector should be close to the positive rail voltage (e.g. +65 V).
Hint: the components on each amp channel (there are obviously 7 of them) have the same 1st, 3rd and 4th digit in their designator. Only the 2nd digit is different to denote the channel. So Q7139 is the same type of part as Q7239 on another channel.
Unfortunately, the protection circuit components are on the foil side of the PCB. You will have to unscrew the 5 screws holding the PCB to the heatsink and then gently open the PCB and heatsink like a book to about 90°. That should give you enough room to work with.
Finding the Fault – Q7639
In my case, I found that one of the transistors (Q7639) was leaky and its collector had a much lower voltage. Due to the leak, it switched on Q7001 even though there was no voltage on its base.
Through the resistor network (R7106 and R7002) on Q7001’s collector, it generated a small voltage (1/23 of the positive supply voltage – hence the 2.6 V) to the CPU on the HDMI board to indicate that there was a problem with the ASO.
Replacing Q7639 is straightforward and the unit is happily working again. Not bad for something you can fix for not much more than 10 cents.
Wrapping Up
This article hopefully gives you some ideas of how the protection circuit works on Denon and Marantz AVRs (they are very similar designs).
When the protection is really indicating something wrong, the repair is more likely to be around the output transistors and their related components – that will be another article for another day!
💬 Have thoughts? Leave a comment or question below — I read every one!