Yamaha A-S500 Amplifier DC protection

[tiggerlator]

Hi
I have this amplifier. The problem is as follows. When plugged in it powers up, as soon as the volume control is turned, with or without nothing connected the amp clicks off and the power LED flashes. I have tried the fault diagnosis mode according to the manual and it seems to be saying it is DC protection.

I have checked the voltages from the main transformer which is about 55v, and from the secondary which is about 9v. I have also checked all 4 main transistors collector to ground and they are all 47.5v DC.

I am only a amateur, and have a digital MM. Can anyone give me any pointers to check please, i would love to get this amp working again. The person i got it off said it was working and his wife unplugged it, now it has this fault.

I have linked the service manual.

Thanks.

https://www.manualslib.com/manual/1074982/Yamaha-A-S500.html

[Jeff eelcr]

Without looking at the manual, did you check bias and DC offset adjustments?
These would be the first things to check.
Jeff

[tiggerlator]

Sorry, i am no expert. Would these checks be in the service manual?

thx

[andy3055]

Don't start tweaking any of the bias adjustments! They don't go off on their own. Most likely a transistor in the power amplifier is breaking down and you get DC on the speaker line/s. Hook up a meter on DC volts scale to one of the speaker terminals and try to make it do the same thing. If you don't see anything on the meter, move the meter to to the next channel and do the same. You should see a problem in one or more of the channels. That is the channel you should start to work on.

[Jeff eelcr]

Yes the service manual should show how to CHECK these.
If there is DC offset enough on any channel output it may not
show at the speaker terminals if it is in protect.
Jeff

[David_AVD]

Exactly what was the diagnostic code /message?

It could be an output DC fault, output over current fault or a power supply voltage fault.

[tiggerlator]

Thank you for your replies. I will try what has been suggested and report back.

[tiggerlator]

Exactly what was the diagnostic code /message?

It could be an output DC fault, output over current fault or a power supply voltage fault.

The fault code is 4 flashes on the power led reapting.

[tiggerlator]

Don't start tweaking any of the bias adjustments! They don't go off on their own. Most likely a transistor in the power amplifier is breaking down and you get DC on the speaker line/s. Hook up a meter on DC volts scale to one of the speaker terminals and try to make it do the same thing. If you don't see anything on the meter, move the meter to to the next channel and do the same. You should see a problem in one or more of the channels. That is the channel you should start to work on.

Tried what you suggested.

Got a DC voltage apike on the R channel. Connected meter up to the R channel speaker terminals, power up amp, as soon as i turned the volume, got a DC spike on the out put and the amp clicked off like it always does. Voltage was about 7-12volts i think.

L ch was fine, 0 volts at terminals

I am so sorry about the multi posts.

[Martin72]

Ah, DC offset fault on channel right..
Main cause is one or more faulty (shorted) transistors.
Check the transistors with a multimeter for short or low resistance ( of course with the amp in OFF state and mains unplugged).
If they seem to be OK, check resistors.
If they are OK, check the DC supplies (when amp is in ON conditon).

[elecdonia]

Got a DC voltage apike on the R channel. Connected meter up to the R channel speaker terminals, power up amp, as soon as i turned the volume, got a DC spike on the out put and the amp clicked off like it always does. Voltage was about 7-12volts i think.

I suspect the fault is located in the line-level circuits prior to the volume control. I doubt there is anything wrong with the power amplifier section.

Try these things:

1) This unit has a front panel button labeled “Pure Direct.” Does switching this button between “normal” and “pure direct” result in any change to the fault?

2)Disconnect the 4-wire cable that goes into connector CB101. This cable connects the L and R line-level audio signals to the inputs of the power amplifier section. Does “protection mode” still happen when you turn up the volume control with this cable disconnected?

3) There should be no DC voltages (not even +/- 50mV), at any pin of CB101. 2 pins are ground. The other 2 pins carry the L and R audio signals.

[elecdonia]

Ah, DC offset fault on channel right.. Main cause is one or more faulty (shorted) transistors. Check the transistors with a multimeter for short or low resistance ( of course with the amp in OFF state and mains unplugged.

Yes, this is good advice for most DC offset faults in solid-state audio amplifiers. Such faults are usually caused by one or more failed output transistors in the power amplifier section. These failures are often accompanied by blown fuses, burned up resistors, and cracked/melted small transistors on the power amplifier PC board.

However, this case is different. There isn’t any DC offset at the speaker terminals until after the volume control is advanced from zero. Therefore I doubt there is any fault inside the power amplifier section.

Yes, there is DC offset, but this must be originating from somewhere inside the “line-level” stages.

I suspect a DC offset fault in the line-level input switching chip: IC401. This is an unusual and complicated IC chip. It has 100 pins. It appears to contain all of the switching needed for a 5.1 channel home-theatre amplifier. But only 2 of its channels are used in this stereo integrated amplifier (Yamaha must have had a huge surplus of these IC chips on hand).

The audio output pins on this IC401 switching chip should always be within approximately +/-10mV DC with respect to chassis ground (earth). IC401 operates on balanced +/-7V power supply rails. Therefore if IC401 has failed it is likely the failed audio output pin on IC401 will have either +7V or -7V.

Another unusual feature of this Yamaha A-S500 is the “Pure Direct” switch. When in “normal” mode the audio signal passes through traditional rotary analog variable resistors (potentiometers) for volume control, balance, bass, and treble. But in “Pure Direct” mode the volume is controlled digitally by IC401 itself.

I asked the OP to test whether the on/off state of this “Pure Direct” button causes any change to the observed symptoms.

[Martin72]

You´re right, miss that "after turning up the volume" thing.
Is it a complete dc-coupling design, from the inputs to the output ? Will look at the manual later, haven´t done it.
Only then a defect in the input section could cause a offset in the output.

[elecdonia]

You´re right, miss that "after turning up the volume" thing.
Is it a complete dc-coupling design, from the inputs to the output ? Will look at the manual later, haven´t done it.
Only then a defect in the input section could cause a offset in the output.

There are 10uF coupling capacitors throughout the line-level audio circuits. But these are polarized capacitors and the audio circuitry operates from balanced +/-15V and +/-7V supply rails. So depending on the polarity of the DC offset it could pass right on through the 10uF coupling capacitors.
Also it is likely for the speaker relay to click off and “protection” mode to get triggered while rotating the volume control because some of this changing DC voltage will get through the coupling capacitors. Perhaps rotating the volume control very very slowly would prevent the DC transient from being large enough to trigger the protection circuitry.

[tiggerlator]

You´re right, miss that "after turning up the volume" thing.
Is it a complete dc-coupling design, from the inputs to the output ? Will look at the manual later, haven´t done it.
Only then a defect in the input section could cause a offset in the output.

There are 10uF coupling capacitors throughout the line-level audio circuits. But these are polarized capacitors and the audio circuitry operates from balanced +/-15V and +/-7V supply rails. So depending on the polarity of the DC offset it could pass right on through the 10uF coupling capacitors.
Also it is likely for the speaker relay to click off and “protection” mode to get triggered while rotating the volume control because some of this changing DC voltage will get through the coupling capacitors. Perhaps rotating the volume control very very slowly would prevent the DC transient from being large enough to trigger the protection circuitry.

Turned the volume control really slow and it went all the way to full without the amp clicking off, though it seems hard to recreate this now, though it will do it if the volume control is tured reeeeally slowly. Also pure direct switch has no effect on the fault, it does the same thing with the switch off or on.

With CB101 disconnected, the amp turns on but just flashes the power led.

There is a small 3.3mv voltage on the E/R pins of CB101

Thanks for the help with this guys.

[elecdonia]

Question: If you connect speakers and an audio source are you able to hear sound from both left and right channels?

You will need to rotate the volume control slow enough to avoid having it go into protection mode.
Also I recommend rotating the “balance” control all the way counterclockwise which will play left channel only, followed by turning it all the way clockwise to hear right channel only.

I’m curious to learn what you observe because this is such an unusual fault.

[tiggerlator]

Question: If you connect speakers and an audio source are you able to hear sound from both left and right channels?

You will need to rotate the volume control slow enough to avoid having it go into protection mode.
Also I recommend rotating the “balance” control all the way counterclockwise which will play left channel only, followed by turning it all the way clockwise to hear right channel only.

I’m curious to learn what you observe because this is such an unusual fault.

I will connect some stuff to it and try it now, gimme 10 mins. thx

Edit.

Well tried with both speakers connected, no matter how slow i turn the volume i get a loud pop from right speaker and amp goes off, power led flashing.

With just the left, no sound from left channel no matter how slow i turn the volume, it gets to a certain lvl and amp goes off, power led flashing.

If i set the volume to half with just the left speaker connected, i get sound for an instant before the amp goes off, power led flashing.

Interesting one. I disconnected the R channel input from the CB101 connector and amp works just from left channel.

Edit 2. Switched the left channel input on CB101 to the right channel and it works, so both channels on the poer amp section work fine.

[Martin72]

I’m curious to learn what you observe because this is such an unusual fault.

+1, I hadn´t seen such fault in the last 25yrs of repairing.

[tiggerlator]

See edits guys

[Audiorepair]

I would be measuring the 10uF 16v caps, this looks to be a DC issue the caps are supposed to prevent.

If you find any that are dodgy, you should replace them all.

[tiggerlator]

I would be measuring the 10uF 16v caps, this looks to be a DC issue the caps are supposed to prevent.

If you find any that are dodgy, you should replace them all.

On the input section boards?

I guess the main board is all good?

[Audiorepair]

Those in the audio path around the tone/volume /balance circuit.

C951 C952 C915 C916 etc.

I regularly come across PCBs where all the caps of the same type start failing all at the same time.  Particularly SMD electrolytics.

My simple ESR meter across a few of these caps in circuit will soon show an example or two of high ESR or low capacitance, where all of the other caps of different uF and voltage ratings are still OK.

So you just replace every one of these you can find, because if they aren't dead now, they will be next week.



Edit: you can't always reliably measure caps in circuit, but a high ESR reading is proof positive of a dodgy cap, so that's what you should be looking for.

[elecdonia]

Your observations prove that IC401 is faulty.

-   It is likely that replacement of IC401 will fully repair this amplifier.
-   However this is a very unusual IC chip.It may be a challenge to find a replacement.
-   Also because IC401 has 100 pins it will require considerable effort to desolder the old part without damaging the PC board. I’ve done this many times but it was a “learning process” for me. In fact I’m still perfecting it.
-   Another option is to find someone who is “parting out” a similar Yamaha model which happens to contain the same IC401 chip.
-   If someone is selling the PC boards from a similar Yamaha amplifier then you might be able to purchase the entire “Function(2)” PC board and simply replace your faulty PC board with it. No soldering required.
-   This “Function(2)” PC board is the board with IC401 on it. So if you can get this board it will fix your amplifier.
-   Note that this “Function(2)” PC board is also the same board with the Line 1, 2, 3 RCA jacks, the tape-out RCA jacks, and the Tuner RCA jacks.
-   eBay is a good place to find people who take apart non-working audio gear and resell the individual PC boards and other parts.
-   Because the most common failure of these models is “blown-up power amplifier sections” you might actually be able to find a working “Function(2)” PC board cheap.

After another review of the service manual I discovered the following unusual characteristics of this Yamaha A-S500 model:

1) Although the volume control with the large knob is a variable resistor, the analog audio signals themselves do not pass through it.
2) This volume control is driven by a motor which is controlled by the remote control so that the volume knob will rotate when the remote control is used.
3) The actual method of controlling the volume is with digital attenuators embedded inside IC401.
4) When the volume control is rotated its resistance is measured by an A-D converter embedded inside the system control MCU, IC502.
5) The MCU (IC502) then transmits digital commands to IC401.
6) IC401 interprets these commands and then uses its internal digital attenuators to control the left and right channel volume.

This system is so complicated that I must refer to it as “Byzantine.” I have never encountered anything like this in many years of working with all sorts of audio gear.

That said, I am certain that replacement of IC401 (or replacement of  “Function(2)” PC board) will fix it.

[elecdonia]

On the input section boards?

Although those small 10uF electrolytic capacitors are known to fail, this isn’t what’s wrong with your amplifier.
You have a faulty IC401. It is a digitally controlled audio chip which handles all the audio source switching (CD, Line 1,2,3 , Tuner, Phono, tape recording outputs).
IC401 also controls the volume. It is the source of the loud pops heard when changing the volume.

I guess the main board is all good

Yes, I think every part in your amplifier is OK except for IC401 which is located on the “Function(2)” PC board. This is the vertical PC board at the rear of the amplifier which has the RCA input jacks for Line 1, 2, 3, Tuner, and the “tape out” RCA jacks. The large rectangular IC chip on this board with pins on all four sides is IC401.

[elecdonia]

If i set the volume to half with just the left speaker connected, i get sound for an instant before the amp goes off, power led flashing. Interesting one. I disconnected the R channel input from the CB101 connector and amp works just from left channel. Edit 2. Switched the left channel input on CB101 to the right channel and it works, so both channels on the poer amp section work fine.

Here’s another thing to try:

1) While amplifier power is switched off: Rotate large “volume” control knob to about 1/2 of its rotation. Also make sure “Pure Direct” button is released (off). Then rotate “Loudness” control knob fully counterclockwise.
2) Switch on power.
3) Slowly advance the “Loudness” knob to raise the volume. If this is kept at a fairly quiet level any “pop” noises which occur may be low enough to avoid triggering the “protection” mode.

I’m expecting there won’t be any loud pops or “protection” mode activation as long as you do not touch the main volume control knob,

If you do hear any loud pops from the right speaker then rotate the “balance” control fully (or most of the way) counterclockwise This will effectively turn off the right speaker or keep the pops quiet enough to avoid triggering “protection.” This way you won’t need to disconnect the right channel speaker wires.

If this works then I suggest checking whether all of the line-level input jacks are functional. These are Line 1, 2, 3, CD, and Tuner.
It is likely that everything will work on the left channel. However you may find that the audio signal doesn’t get through on the right channel, or that moving the selector knob causes pop noises to be heard from the right speaker.