Couple questions while repairing an audio receiver

I had a rather high end receiver given to me. It stopped working and the repair shop just opened it, verified the fuses were good, and told him to buy a new one...

It will turn on, then shut right back down with a blinking light. After some fishing, I did find what "might" be a concern. Testing + to - on all the speaker outputs, I found that the center channel is a dead short. One might immediately assume the transistor is toast but they are all testing fine. I am thinking the issue might be right near the output for the center ch. There are a few ceramic caps and a few power resistors and a relay that I am not quite sure about yet. I have never seen a ceramic cap fail but I guess it is possible.

I do have the manuals for the unit but not real sure what to think of this low resistance on the speaker output just yet.

Presumably you've checked for leaky electrolytics?

The protection circuit usually opens the speaker terminals if it detects DC on the output.

Although I havn't seen it done, it could short the speaker output on a fault. Some of the relays can transfer after a delay on power up, others right away except if DC is sensed.

The center channel could be a "bridged" output. Is this output an IC amplifier?

You might have better luck testing the transistors in diode mode.

Ceramic caps don't usually fail, but I've seen some including in an instrument I'm working on now. Failure was visible though.

You didn't say what make and model receiver this is. The more info you give, then more likely people here would be able to help you.

OK, we are getting somewhere! AVR is a 2308 or 888. I found that at the very tail output of the center ch, a small capacitor was dead shorted. I have no doubt that is a big issue and the relays are indeed to kill the output should something happen so a speaker is not damaged. I think what is happening is the relay is N.O. When powered up, the relay tries to latch, senses the short, and shuts down.

My question is what to use as a replacement. The cap indicates a 472J. I am not sure if I need to stay with the same type for whatever reason or even if the voltage would make a difference. I am not real sure why the cap is there to be honest. It is mylar or polyester. Not sure if a ceramic disc can work here? bad idea?

That's what the protection is supposed to do.

The 47J means 47 + 2 zeros Pf Dipped Tantalum Capacitor (J). So that means 4700 e-12 farads. 1e-6 is a uF. So it's 4.7 nF or 0.0047 uf if I did the math right.

In the early days of tantalum there were major problems with leakage. Sometimes you can see it. Tantalums don't tolerate high surge currents or overvoltages. Increasing the voltage is generally a good idea.

Tantalum caps have very good frequency response and very low ESR (Equivalent Series Resistance). It could be a bypass cap especially if it's from a power supply pin to ground. Without knowing it's use, it hard to make a recommendation.
I'd at least measure the DC voltage at the pin and at least double it for a replacement.

Being a speaker output, I would not expect to see any DC there. However, I guess it would stand to reason that maybe this cap is supposed to act like an MOV? If you want to know how it is wired up, imagine putting this cap in parallel with the speaker output. It is actually the very last component before the speaker terminals. There are Mylar and Poly caps. From what I understand, they look the same. I can't make heads for tails of what needs to go in there... The 7F8 on the cap under the 472J makes no sense to me at all..

OK, kicking back over to the power up issue. it seems that there is a small 6V circuit that they call "E power" that is hot all the time that provides the switching for turn on. The small PSU creates 6VDC and sends that out somewhere that I have not found yet. When switched, that 6V will come back and turn on a relay that powers up everything else. Here is my dilemma, the PSU is making 5.69VDC when AC is applied. That voltages goes to 5.98VDC when I unplug. I have verified that if I short over that 6V to the return 6V to the relay circuit, it will turn on. So...., either there is something wrong with the voltage and the system does not like it, or whatever is supposed to switch this is not working... I know the switch mostly works because it also returns 12V power for the transformer relay and that works.

So, could the 5.69V be an issue here? Seems a bit off to me.. I do not have a scope right now to check or ripple but remember, it was hit with lightning so something probably popped open.

A small cap and a resistor in parallel with the speaker is known as a Zobel Network: https://en.wikipedia.org/wiki/Zobel_network A single capacitor might be an RF filter. I would not put a tantalum cap across a speaker terminal.

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The 7F8 is probably a lot number.

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~ 6V may not be. It might maintain presets etc.

I know the CPU voltage is 6V per the manuals. The relay on the supply board is 6V so basically it makes 6V and sends it out, then gets that 6V back on another pin that fires the relay. I am trying to learn what the "voltage protect" circuit is. It is at 5.96V but I am not sure if it should be present or not. There is also a power down voltage that is also present. I just don't think the 5.69V seems right. Coming of a switching circuit, I would expect better accuracy, but...

fastline said:

I know the CPU voltage is 6V per the manuals. The relay on the supply board is 6V so basically it makes 6V and sends it out, then gets that 6V back on another pin that fires the relay. I am trying to learn what the "voltage protect" circuit is. It is at 5.96V but I am not sure if it should be present or not. There is also a power down voltage that is also present. I just don't think the 5.69V seems right. Coming of a switching circuit, I would expect better accuracy, but...

Click to expand...

The outputs of the amplifiers shoulkd be at zero volts, within a few mV. The protection circuits are there to keep the speaker disconnected if there is any DC on the output, as this would kill the speaker.

The relay is N.O, and will only close after the amplifier has settled down, and if there's no DC on the output.

If the capacitor is across the speaker output, it's not going to be a tantalum capacitor, since they don't work very well with AC. Most likely a plastic film type. Polyester (Mylar) is the cheapest. Polypropylene is better for audio. The J means it is a 5% tolerance.

As for the 5.69 volts. Unless that voltage has it's own switching supply, it won't be tightly regulated. If the PSU is making multiple voltages, only one of them can truly be tightly regulated, The others are regulated as a percentage of the main voltage, based on the turns ratio. The output with the highest power is usually the one that is monitored to close the regulation loop.

If you need tight regulation on two or more outputs, you either need to have multiple PWM controllers and transformers, or you post regulate the others. 6 volts sounds high for a CPU, more likely they are bringing a nominal 6V from the power supply, then using a linear LDO to make 5V at the processor.