Yamaha A-S501 DC protection gets activated when increasing volume above 1/3

[atbivanov]

Hi there,

I'm trying to fix Yamaha A-S501 from the dumpster  , and so far it's been real pain. Here is what I done so far.

Service Manual can be downloaded from here:
https://www.dlmanuals.com/manual/yamaha-a-s301/service-manual

The symptom:
The amp is working fine no problem tested for many hours as long as it's not very loud. However as soon as I increase the volume about 1/3 the amp shuts down and the power LED stars blinking. The 1/3 volume position I believe is not relevant to the problem because if I lower the input signal enough I can turn the volume the MAX without triggering the protection but the output sound is very low. The issues also seems to be frequency related because the amp can work for hours but a song with deep base can also trigger the protection.

I use 8 ohms speakers and the Impedance Selector is in position LOW


In self-diagnostic function:
The power LED makes 4 short blinks which is indicating  that the DC protection was activated the AD value shows below the normal (48 - 129) usually around 42. (F/W version: V0011)

I checked the output DC voltage sensing circuit on the AMP PCB R177, R178, R179 all good and replaced C133 just in case, then I checked R503, R515, D502 and C532 all looks good but the problem is still present.

Then I disconnected the PRD signal (pin 1 on W505 connector - red wire) from the AMP PCB and ground it through a potentiometer. By adjusting the potentiometer I was able to verify that the DC protection only triggers when the voltage on PRD is outside the normal values (0.947 V to 2.51V). In my case the allowed range is from 0.9V to 2.48V little bit off but I believe still within the tolerance.  Based on this I believe there is no problem with the MCU or F/W. With PRD  signal disconnected and the potentiometer adjusted so PRD measures 1.5V with reference to ground (MG on the circuit) I put on some music and start to increase the volume this time obviously the DC production didn't trigger and I was able to put some real just to my speakers . However connecting the PRD back the problem is still present.

Then I focus on the AMP PCB and I measured the DC voltage at the speaker terminals it measured -0.5V on both channels.
Is -0.5V of DC voltage at the output of the amp consider abnormal ?

I tried to measured the biasing voltages of the power transistors of the AMP if they match the voltages on the circuit (the pink numbers), however I have a trouble understanding with reference to which ground should I measured the transistors biasing voltages is it the chassis or the middle tap of the AMP transformer?

Any ideas what could cause this problem ?

Thanks in advance.

[coromonadalix]

How many speakers you have ?   normally 2 x 8 ohms speakers ( left and right )  are needed to be set at high

A or B are two switched channels with each (left and right in them)  ex:   one pair of speakers in a room and the other in other room

The low impedance are  ex:  4x 8 ohms speaker with A and B  switched on,  meaning  4 ohms speaker load (parallel) for left and right.

[Gyro]

Welcome to the forum 

Wow, that's quite a service manual, I tried delving through it for a while before thinking about getting back to basics...

- Tripping on deep base suggests that the time-constant of the DC offset protection filter has become too short (aging capacitor), but it sounds as if you have checked the relevant components.

However:

- Yes, a DC offset of -0.5V is too high, that represents 60-80mA of standing current into 6-8R speakers. You would normally expect <100mV, probably more like <10mV. That makes me think that it may be a genuine DC offset trip.

I would suggest monitoring the DC offset on the outputs as you increase the volume (or signal level). If you can't get a stable reading with the meter set to DC volts on a music signal then try filtering it with a low pass filter, say 10k and 1uF or higher. If it increases with volume then you've confirmed your problem.

Without printing out the schematics and spending time going through them with a highlighter I don't think I'm going to see it, but I would trace back from the power-amp stages to see where the signal becomes AC coupled and then look at where the offset is creeping in - it's curious that it is the same on both channels, maybe a clue.

Hopefully with a little effort, you will achieve a very nice dumpster amp.

[SpecialK]

I suspect your B+ power supply rail sags more than your B- rail when loaded.  This would cause a DC imbalance. I think. I suspect C134 maybe faulty.

Can you monitor the voltage at the collector (middle pin) of both the NPN and PNP finals with repect to star ground?  With the power off. connect two multimeters with grabber leads.  The manual states +56V and -55V.  Obviously this will sag quite a bit when the voulme is turned up, I think down to 40V or so. See how close the voltages stay to each other.

I'd also feel the temperature of the two main caps C134 and C135 when running for a prolonged time at the 1/3 volume.  A bad one might get hot.

[SpecialK]

Oh.  Before anything else, check that the star ground is tight and that everything that needs to be attached to it is corrosion-free.

[mzacharias]

This is possibly one of the easiest problems to diagnose if you are equipped with an audio signal generator, hopefully an oscilloscope, and a usable digital multimeter.
What is probably happening is either a driver transistor or an output transistor is open circuit emitter-base.
As you increase the volume the 1/2 waveform looks like DC to the protection circuit.
Easy to spot with an oscilloscope; and the "apparent" DC will also show on your multimeter. Look at the bases of the output transistors as the clipped signal appears and you will see the increase in measured DC voltage. What should be say 0.6 or maybe 1.2 volts will now start to be more like 2.5 or more; as the clipping worsens the DC will increase.

[coromonadalix]

And put the impedance switch at the right place ....... high