Denon AVR-X4500H amplifier repair, stuck in DC projection mode

[sovamind]

I'm trying to repair a Denon AVR-X4500H home theater receiver. The fans over the heatsink stopped working and it overheated and shutdown into protection mode.

Having repaired audio amplifiers in the past, I focused my efforts on checking the power transistors (FETs) first and found one channel had both the "push" and "pull" FETs blown with a dead short between the source and drain. I replaced both of those and now all the FETs appear to be fine (around 0.5V in diode check between the source and drain in correct polarity, no volts reverse polarity) but the amp still turns off after 6 seconds. The service manual says that the it shutting off 6 seconds after turning on and flashing the power LED red with .5s on and .5s off = "DC Protection Mode" but doesn't give any more troubleshooting information.

Complicating matters is that the power amplifier module has the FETs mounted at the bottom, so you can't really check the voltages with the thing assembled. The bottom of the chassis basically blocks access to them and it only staying turned on for 5 seconds makes trying to take measurements pretty impossible as well.

Service manual (with schematics) can be found here: https://www.manualslib.com/manual/2798277/Denon-Avr-X4500h.html#product-AVR-X4500H

 

[Nigel Goodwin]

It means there's standing DC on the output of one (or more) of the amplifiers - easily checked to find which one(s) with a multimeter on volts. The easiest place to check is often the emitter resistors.

Unless fan failure is a common problem on these models, it seems VERY unlikely that the issue was caused by the fan.

If the output devices have failed, then simply replacing just those rarely produces a cure, and often will destroy the new devices, as the fault hasn't been fixed. You must replace ALL faulty components to repair DC coupled amplifiers, and also make sure there are no bad (dry) solder joints. BTW, looking at the circuit shows it uses darlington transistors, and not FET's.

You should also be very aware that they are a lot of fake power transistors from China on the market, so make sure you get them from reputable sources.

 

[sovamind]

Sorry, just said FET from habit. I ordered the replacement transistors from Denon's site (actually encompass), so I'm confident that I got identical parts to the originals.

I don't have a bench supply that can generate positive and negative 70 volts for the amplifier power rails. The only way I can generate that power is through the power-supply in the receiver, but it only turns on for 4 seconds before turning off with the protection error. Additionally, when the system is assembled, it's impossible to reach the power transistors.

Do you have any suggestions on how to test the voltage at the transistors?

 

[Nigel Goodwin]

Solder wires to the junction of the emitter resistors, and run them out to where you can get to them - re-assemble the system, and connect a multimeter to each wire in turn while you turn the power ON. They should all be near to zero volts, the faulty one will be either positive or negative (probably more than a volt either way).

But such systems are a nightmare to work on, I'm very happy to be out of that business