Sony STR-DE485



The Sony STR-DE485E is a surround sound home theatre amplifier, with five 80-watt channels, and an un-amplified sub-woofer channel. It has a built-in radio receiver, for stereo FM, and AM reception, Dolby Pro Logic, and DTS decoder circuitry.

All the channels are electronically identical and the power output is 80-watts per channel into an 8-ohm load. It has a frequency response of 40 Hz to 20 kHz, with a THD of 0.09 %.

Digital Audio / Video Control Centre

A Digital Audio / Video Control Centre is the rage these days. Everyone wants to have their personal Digital Theatre System in their living room. Being a poor Brit living in Croydon it is a little too rich for my tastes though.

Knob Heads

The master volume control, also known as the 'knob head' in UK, is very easy to remove. There is often a screw to the base or side of the unit, which will need removing. Unfortunately, people often remove the knob to prevent other people tampering with the volume setting. Replacement knobs are very difficult to find and expensive to purchase from Sony.

There are many 'knob experts' on the Internet capable of repairing simple cosmetic damage. However, there are very few people capable of component level repair. This is not a repair guide, but it might prove useful to a qualified engineer.

If you are not a qualified and experienced engineer, then you could sell your amp on ebay as spares. The parts might be useful to someone fixing theirs.

Cracking Sound

This amplifier came for repair, and I was told that it had begun to make a very loud 'cracking' noise from both channels. This noise occurred twice during the initial five minutes of being switched-on. It sounded like a discharge of current through the speakers. At first, I suspected that it might be the smoothing capacitors within the power supply. Capacitors C804, and C803 were 4700uf / 63V, of a Nichicon brand. Checking on Google regarding Nichicon capacitors indicated the production of faulty ones for a brief time. However, there was no visual indication that these capacitors may be faulty. There was no bulging, no electrolyte leakage, and they tested fine on the digital meter.

On full volume, there was a slight hum on both channels, indicating that the capacitors may have a slightly diminished performance; however, nothing that could account for such a large noise.

I decided to disconnect the front speakers and connect them to the rear channels to see if the same noise was produced there. The rear channels were fine. There was no noise heard. Had the capacitors been faulty, the noise would have occurred on all four channels since they share the same power rails, and each channel is an identical 80W power amplifier circuit. They use identical power transistors, and each circuit is the same all the way to the sound processor stage.

This indicated to me that the fault was isolated to the front channels only. This left only one other possibility, which was a transient short of the front channel power transistors.

The Equations

I have never come across transient semiconductor faults before, however after some mathematics involving Fermi-Dirac equations, involving quantum reverse breaking voltages of semiconductor devices, a cup of coffee, and some headache pills, I concluded that it might be possible because I was getting a -1/2 spin!

However, it is always good to consult someone older who might have come across this type of fault, so I decided to consult 'The Doc'.

The Doc

I call him Doc because he reminds me of the character in those Back to the Future films. He has a similar full head of curly white hair that always seems to be in a constant state of flux. Moreover, he also talks like the character in the film. I suppose eccentricity is universal. He used to work for the MOD years ago designing submarine electronics. He would like to think it was all top-secret stuff. The kind of things he could tell you, but would have to kill you afterwards. Except he would choose a slower route, to bore you to death by talking about being posted somewhere in the high seas and how once he fixed a radar system just in time before the 'Commies' arrived.

Commies, Klingons, it is all the same to me. However, it is all out dated electronics of the 80s TTL revolution. He is very knowledgeable though; he has a doctorate in electronic engineering. This was something that I always wanted to achieve, but instead came across much prejudice and discrimination.

Just as I was about to knock on his door, he opened it. It was almost as if he knew I was coming. He is one of those gadget freaks, and probably saw me on his security camera, approaching the house.

'Come in, come in.' He said as if in a hurry, grabbing my coat and pulling me inside. He had dozens of wires taped to his clothes. There was a camera taped to the side of his head, with double-sided tape. A long ribbon cable from the camera on his head connected to a small circuit board taped to his jumper.

'Hey, Doc, I hope we haven't been invaded by the Borg.' I said with a cheeky grin.

'I am working on a Raspberry Pi Project. Astronauts could use this camera module in space! Attached to the side of their heads, the command base can see everything the astronauts look at! Isn't that exciting!' he replied.

'Hey, it all sounds good it me Doc. However, this motorcycle battery for powering the PI, and all these wires might pose a problem in space' I replied.

'Oh, that's nothing! I am working on a new power supply that will revolutionize the world!' He replied excitedly.

'Imagine a power supply that takes minutes to charge, but lasts 5 years!' He continued.

'It sounds good to me Doc. I hope it doesn't require 2.21 giga watts of electricity to charge.' I joked, but he did not get it.

I figured I had better ask him quickly before he gets any ideas to use me as his guinea pig for one of his experiments. The last time I visited, he decided to test drive his ultrasound scanner on me. He often acquires huge scientific instruments from closed down hospital auctions.

'I was just wondering Doc, have you ever come across power transistors that short momentarily.' I asked.

'Yes! Yes! I have come across those! Back when I was fixing valve instrument panels! Why?' He replied very quickly.

'It is nothing really. I am just repairing an amplifier that appears to have that fault.' I answered.

'Well, bring it to me! I'll fix it!' He replied, as his eyes bulged, nearly popping out of their sockets.

He is a very kind soul. I once brought him a simple AM pocket radio to fix, and by the time he was finished with it, it could detect signals from the next galaxy! The fact that he had managed to interface it to an Amstrad satellite dish, scares me to think what he is capable of.

The Repair

Well, if the doc says that power transistors can fail transiently then, hey, who am I to argue? Therefore, I decided to replace them. However, the replacement power transistors were a little pricy for my pocket, so I decided to do the next best thing. I decided to do the old switch-a-rooney trick.

Since the rear channels used the same type of transistors, and I was never going to use the rear channels, I decided to take them out of there and use them for the front channels!

I have never liked the idea of surround sound. I have always thought it was for those over-grown brats with too much money and not enough sense. Besides, I do not own a television, and since I only ever listen to Classic FM, I can live without surround sound. Nice clear stereo is good enough.

After fixing the amplifier, the awful loud noise was gone! It seems to have cured it. There was a slight hum when the volume was fully up. I decided to see what would happen if I replaced the capacitors. I found some cheap replacements on eBay, and that cured it!

Sony of course makes some of the best consumer electronics products, which is why I always try to buy Sony. I love their circuit diagrams, which are so clear and useful to engineers such as myself.

This Article Continues...

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