Connected back up with both L/R channels of CB101 connected to the power main board and when turned on it still clicks off with a flashing power led. It seems the IC401 need isolating more somehow. If i connect it with the R channel disconnected from the power main board, it works as follows.
The volume works up to about number 1(if there were numbers) then the loudness takes over. The input selector still lights up and does need to be on line 1.
QuoteConnected back up with both L/R channels of CB101 connected to the power main board and when turned on it still clicks off with a flashing power led. It seems the IC401 need isolating more somehow. If i connect it with the R channel disconnected from the power main board, it works as follows.
The volume works up to about number 1(if there were numbers) then the loudness takes over. The input selector still lights up and does need to be on line 1.There is something odd going on here but I'm still certain it can be resolved.
By "volume" which knob are you rotating?
Your photo showed you rewired the Function(2) PC board correctly.
After the rewiring, then the large "Volume" knob (right end of front panel) should have no effect at all.
The smaller vertical "bar" knob labeled "LOUDNESS" should now be controlling the volume.
Please tell me which knob is now controlling the volume: is it "VOLUME" (large knob) or "LOUDNESS" (smaller vertical bar knob) ?
As for the input selector switch, it is normal for the LED indicators to continue to change when you rotate the knob. That portion isn't changed by the rewiring. However, the position of the selector switch should not have any effect on what you hear. The signal you are applying to the Line 1 input jacks should be coming through all the way to the speakers regardless of what position the selector switch is in.
I'm hoping that the service manual actually matches your amplifier...
The images of the Function(2) PC board in the service manual look exactly like your photo of the board.
And your rewiring looks exactly the way it should be to send Line 1 through the 4 middle knobs (BASS, TREBLE, BALANCE, LOUDNESS) and then feed it into the power amplifier section.
There is a chance of having an issue located on the board named OPERATION(2). This board is right behind the front panel and down near the bottom of the unit. Gaining access to it requires removing all knobs and then both the outer and inner front panel assemblies. But don't start taking this apart until you can confirm which knob is controlling the volume:
is it VOLUME knob or LOUDNESS knob ?
The big "VOLUME" control works up to number 1 if there were numbers, then nothing until the "LOUDNESS" control up to full volume. i guess I need to check if leaving the main "VOLUME" control on zero still produces sound when turning the "LOUDNESS" control up.
big "VOLUME" control works up to number 1
Both channels of the power section work, the problem is the R output from the pre section.
Have you actually checked any of these 10uF caps to at least rule them out as the suspect?
Have you actually checked any of these 10uF caps to at least rule them out as the suspect?This unit isn't terribly old ( I'd guess 10 years). Audio coupling caps don't get hot (no ripple current) and can last much longer than 10 years. Although I always use my ESR meter to check every electrolytic, the vast majority of high-ESR capacitors which I find are located in switching power-supply circuits (where there is high ripple current). Also the failure mode of small electrolytics (10uF, etc.) is almost always "high ESR." I can't recall a single case of one shorting out or becoming leaky.
Where I'm feeling a challenge helping the OP diagnose this fault is from the extreme complexity of the circuit design employed in this integrated amplifier. I'm beginning to suspect my pervious diagnosis of IC401 being at fault. After reading the schematic several times I discovered a 2-transistor "muting circuit" (1 transistor per channel) is located on a PC board located just behind the front panel. These transistors shunt the line-level audio signals to ground when turned on by receiving a logic-level control signal from the MCU. After this muting circuit the 2 line-level audio signals go straight into the power amplifier section. The OP determined that disconnecting the right channel at the power-amp input prevents the "protection" mode from getting activated. Everything works perfectly when the line-level signal from the left channel preamp section is connected to either the left or right channel of the power amplifier. But the R channel preamp signal always causes "protection" whether connected to L or R of power stage.
Perhaps one of these muting transistors has a fault which puts the logic-level voltage onto the audio line?
So what we know now is that the R channel signal coming from the preamp-level circuitry behind the front panel carries a large DC transient which occurs while rotating the main volume control. It is important to note that audio signals do not pass through this main volume control. Rather, its resistance is measured by an A-D converter inside the MCU, and then the MCU takes various actions depending on the detected resistance of the volume control variable resistor. I really haven't encountered this in many many years of repairing/restoring audio gear!
I just don't think this is a case of faulty 10uF capacitors.
problem is, i can still only use the L channel. If i connect the R channel of the CB101 connector, the amp still clicks off with the power led flashing.
After reading the schematic several times I discovered a 2-transistor "muting circuit" (1 transistor per channel) is located on a PC board just behind the front panel.
Two "last chances":
-Observe the solderings on the pcbs for faulty areas.
-Check the transistor Q802 in the mute circuit.
OK, but my point is the OP apparently hasn't, or been able to test these capacitors to indicate they are the real source of the problem or not. Surely it would make sense to replace a few cheap capacitors in order to now have a tool to use to test the rest of the system. You would now know for a fact it is not a capacitor problem. As it stands, you are merely assuming that the caps are good, and are trying to find what else could possibly be wrong based on that logic.
Two "last chances":
-Observe the solderings on the pcbs for faulty areas.
-Check the transistor Q802 in the mute circuit.
Yes, a DC potential could be there if the transistor is broken, but the mute signal is for both mute transistors. Means the 5V (or whatever is the H level) could only appear on the pre-out if the circuit is in mute state(and the transistor is broken). "We" strongly need a scope for pointing the failure out...
Working with OP on another forum. Confirmed the following, DC spike at CB101 right channel, no spike at IC802 pins 5 & 7 (right channel opamp input, output)
Q802b -3.2Vdc., Q802e -2.6Vdc. Suggest replace Q802
Quoteproblem is, i can still only use the L channel. If i connect the R channel of the CB101 connector, the amp still clicks off with the power led flashing.As I mentioned earlier, let's keep the R channel wire disconnected from CB101.
But... Let's do something more with this: Connect your multimeter +probe from this R channel wire and -probe to chassis ground. We need to see if and when any DC voltage appears on this R channel wire.
These 2 wires (one for L, other for R) carry the audio signals into the power amplifier at the CB101 connector. These wires should never have any DC voltages on them. Also the audio signals on these same wires ("audio" is AC, not DC) should generally be no larger than 100 to 500mV (millivolts) unless the volume is really cranked up all the way.
A signal large enough to trigger "protection" would likely measure at least 1V on the DC scale of your voltmeter, either positive or negative in relationship to ground. We do know that rotating the large volume control causes large (and unwanted) transient signals to appear on this R wire. So watching the voltmeter while rotating the large volume knob should be informative.
Do you have any access to an oscilloscope?
After reading the schematic several times I discovered a 2-transistor "muting circuit" (1 transistor per channel) is located on a PC board just behind the front panel.
Therefore I´ve wrote :QuoteTwo "last chances":
-Observe the solderings on the pcbs for faulty areas.
-Check the transistor Q802 in the mute circuit.
https://www.eevblog.com/forum/repair/yamaha-a-s500-amplifier-dc-protection/msg4792805/#msg4792805
But no feedback.
Possibly Q802 is used to mute transients during power up, eg, those annoying thumps through speakers that's somewhat common in vintage gear. Try power up while monitoring the voltage at CB101 right channel, you may see a dc spike. In any case the item should be replaced, you might need it one day to protect your speakers.
So i took Q802 off, reconnected the R channel to the power amp input and the amp works, on both channels with no protect
So i took Q802 off, reconnected the R channel to the power amp input and the amp works, on both channels with no protectThis is good news.
It is OK to temporarily use the amplifier without Q802. But it should eventually be replaced.
Q802 is a specialized “muting” transistor. I’ll check and post a list of part numbers that will work to replace it in case it is difficult to find an exact replacement.
And if you haven’t already done so, You should reverse the modification of FUNCTION(2) PC board, restoring it back to its original wiring:
Unsolder and remove the twisted pair of jumper wires that were added
Reconnect the free ends of C460 and C461 back to the PC board as they were to begin with
Use short pieces of wire to reconnect J406 and J407 as they were originally
I have reversed the mod of function 2. The amp works fine. There is -11.2mv on right speaker terminal, but no protect and amp works and sounds fine.
Will it be ok to use it like this for a while?
So i took Q802 off, reconnected the R channel to the power amp input and the amp works, on both channels with no protectThis is good news.
It is OK to temporarily use the amplifier without Q802. But it should eventually be replaced.
Q802 is a specialized “audio muting” transistor. Original part number is 2SD2704. I’ll check and post a list of part numbers which will work to replace it in case it is difficult to find an exact replacement.
And if you haven’t already done so, you should reverse the modification of FUNCTION(2) PC board, restoring it back to its original wiring:
Unsolder and remove the twisted pair of jumper wires that were added
Reconnect the free ends of C460 and C461 back to the PC board as they were to begin with
Use short pieces of wire to reconnect J406 and J407 as they were originally