Broken Yamaha RX-A700

[CoopedUp]

I just received two broken receivers and on the first one when I plug it in, it just has a red stand by light. I have taken it apart and when the front panel with the display and controls is disconnect and I plug it in the relays go through their power on sequence. When I hook part of the front panel back up I can use the power button to power it up but then it turns right back off. I have checked and I have not seen any visible bad cap. If you need any more info let me know cause I would love to get these working.

[CoopedUp]

Another thing to add is when I hook the front panel back up I can only make three attempts to turn it on and then it does nothing but the stand by light. I kinda find this weird, I don't know if this is some sort of protection circuit or if a power rail is messed up and can only power it the three times. If I find any more useful information I'll try to post it.

[CoopedUp]

when what I believe to be the display power is disconnected the unit appears to power up with the relay clicks and all and the stand by light goes out for about 30 seconds then it powers off again still very confusing but I'm crossing my fingers it's fixable.

[bob91343]

I have been through several similar situations.  My first idea would be to check the audio output lines from the amplifiers.  If any of them is not close to zero volts, the unit won't turn on.  Get the diagram and find those output lines.  If all the outputs are good, see if they respond to input signals.  If you find a substantial voltage on one or more of the amplifiers, the amplifiers are blown and need some attention.

[CoopedUp]

 Thanks, I'll check that when I get home and let you know what's up. I was looking at the schematics for a bit but I haven't checked that.

[slbender]

Those symptoms usually indicate a shorted output transistor.  If you have a DMM with lots of ranges and functions, try diode test across the various output transistor leads ( usually three on each ) and see if one pair seems shorted, or maybe all are shorted, good pairs will typically read .667 or so in one direction and infinite when reversed, unless they are Darlington transistors, I have a 5.1 Onkyo HT-R550, it might be similar (or not).

Steven

[CoopedUp]

do you recommend that I remove the transistors to test them??? I dont think its that big of a deal to take them out I would rather not have to tho.

[CoopedUp]

I removed all of the transistors and all of them read .584 in one direction and nothing in the reverse which i guess is nicer that I dont have to worry about thoes but now thats not the problem soo

[CoopedUp]

Just an update, I have checked the resistor packages for the output mosfets and they are also all good, which makes sense cause the mosfets are also good.

[bob91343]

Measure the voltages on the outputs of each amplifier.  They should all be less than one Volt with no signal.  If so, the problem is in the protection circuit.  If not, you have an amplifier section that needs work.

[CoopedUp]

Alright Ill check that thanks

[CoopedUp]

Measure the voltages on the outputs of each amplifier.  They should all be less than one Volt with no signal.  If so, the problem is in the protection circuit.  If not, you have an amplifier section that needs work.

I checked the output by grounding one lead and putting it on the external banana plug on the outside. I got 7.5 volts which don't sound to good. what do you suggest/thing the solution to the high voltage problem is. Im pretty sure the output mosfets are all good because they are all the same and a good value. Thanks

[CoopedUp]

I just discovered something rather interesting. It appears that the 60v supply rail is 20 ish volts... now what would cause that? I don't see any capacitors that are blown, could it be something pulling it down? I will continue to look for the fault and keep everyone posted if I find something good!

[bob91343]

First of all, you can't measure the amplifier outputs at the rear connectors because they don't go there until the unit is working.  There is a relay contact that isn't actuating.

So why?  Either the protection circuit is doing its job by preventing a failed amplifier from ruining a loudspeaker, or there is something wrong with the protection circuit.

So you will need to probe the innards of the unit to find the outputs of the amplifiers.  It's often right where you measured the franctional ohm resistors and, in fact, you can usually measure there.  The individual channels are independent so you have to measure them all.

[CoopedUp]

yes the extarnal outputs are the exact same 7 volts as on the mosfet... the center pin on the mosfet is 22 volts which is no bueno

[CoopedUp]

I should probably add that both sides of the mosfet are 7 volts

[CoopedUp]

and by removing the front panel it gets forced on I believe cause the relays do the power on sequence

[bob91343]

Again, you need to understand it and mark up a copy of the schematic with voltages that you expect and voltages that you measure.

If a voltage drifts with time it may be that there is an open circuit somewhere and you are watching a capacitor charge via a high impedance.  Or it may be a thermal effect.  You will never know unless you can trace the paths.

Perhaps all you have is an open solder joint somewhere.  It does appear that something is intermittent, and that's worth digging to find it.

[CoopedUp]

That could be possible that its intermittent, I checked it this morning and it is no longer higher voltages on the output transistor. so there could be a lose connection somewhere.

[mzacharias]

Those symptoms usually indicate a shorted output transistor.  If you have a DMM with lots of ranges and functions, try diode test across the various output transistor leads ( usually three on each ) and see if one pair seems shorted, or maybe all are shorted, good pairs will typically read .667 or so in one direction and infinite when reversed, unless they are Darlington transistors, I have a 5.1 Onkyo HT-R550, it might be similar (or not).

Steven

In the case of Yamaha's, this almost certainly does NOT indicate shorted output transistors. This is because the over-current sensing is so fast, the receiver will power off virtually as soon as you have pressed the power button, vs several seconds shut-off for a DC balance issue at one or more amp channel output, or a power supply or thermal sensing issue.

I did warranty work for Yamaha for over 20 years, so I have a "little" experience with them.

[mzacharias]

In the service manual, there are instructions for a"protection cancel" function which will allow a technician to troubleshoot at his leisure. This function is not available when the protection issue relates to high current, like shorted outputs or a shorted main bridge rectifier of main filter cap, but your issue is none of these.
To start with, while in Protection Cancel mode, you should be able to measure DC offset at any of the individual amp channels, measuring from the bias test points, or the emitters of the output transistors  to chassis ground. If only one channel has a DC offset, obviously that channel needs repair. If all channels have an offset - it likely points to a power supply issue - probably a bad voltage regulator or some bias voltage source. There are several test points in every Yamaha which relate to the various protection functions. They are labelled DCPRT, IPRT, PS1PRT, etc. Most of these are connected to a resistor summing network, the output of which (the associated test points) will give definitive indication of which one has been triggering the protection function. Generally the output should be zero volts, or close to it. When there is a failure, the voltage sum is high or low, triggering the protect. In any case the service manual shows the specified voltages in the schematics.
In the diagnostics area of the service manual these readings get trickier because the protection history value shown on the display must be multiplied by X/255 where X is either 3.3 or 5 volts (usually 3.3 on newer units) to get the approximate voltage that has triggered the Protect. The manual gives a range of acceptable voltage, anything outside this range triggers the protect.

Why they don't just have the microprocessor just report the actual voltage directly, I don't know. Maybe a little extra code required, who knows? Yamaha just does it this way.

By the way - they aren't "mosfets" - it uses bipolar transistors. (sheesh)

[CoopedUp]

Wow, Thanks so much!!! I'll check that out right now, sound like you know what your talking about

[CoopedUp]

so the NPN output transistors have 22 volts on the collector and 7 volts on the base and the emitter. the PNP side has .5 volts on the collector and still the 7 volts on the others. So is this the offset you were referring to? cause it's an offset but i'm not sure what would cause this and yes it is all of the NPNs that have the 22 on the collector. Thanks for your help!

[CoopedUp]

and about the mosfets, I never knew exactly what they were just assumed cause that's what i figured most amps used. The 55 volt power rail starts around 42v and quickly drops to 22v if that helps. Edit: the change in voltage was a poor ground it is 55 Volts

[bob91343]

MOSFETs have been used in amplifiers for a few years but before that, bipolar transistors were the common ones.  Still they are used quite a bit.

Depending on the age of your unit, you might find either one.

MOSFETs have been a long time in development.  Surmounting the technical issues that they present has taken a long time but now they have some advantages over bipolar transistors.  They have better high frequency response and lower saturation voltage, for example.  And require no drive current.

I am more comfortable using bipolar transistors, mainly because I have been using them so long that I feel they are a part of my storehouse.  For years, when repairing an amplifier, almost the first thing I would look at would be if the output transistors are shorted.  If so, replacing them (and maybe their driver transistors) and resetting the bias was most of what needed doing to get the units working again.

My experience with FETs has been much less and so my comfort level isn't the same.

I have bounced between designing, and repairing electronics.  Both have fun and rewarding for all these 50 or more years.