There are many of these 10uF caps between IC401 and the power amp, including C103 and C104 on the power amp itself, so I don't get why you seem to think a faulty IC401 could possibly inject DC into the power amp input.
So I would carefully check that the +/- 7V rails are really on the chip itself before replacing the chip.
could i use it as a power amp, by using an input into CB101?
I will see if i can check the 10uf caps, and the voltages to ic104
This “signal” isn’t a steady unchanging DC offset. Rather, when the “volume” control is rotated, sharp pulses of several volts + or - pass through the cascade of 10uF capacitors and may still be large enough to produce pulses of perhaps + or - 30V at the speaker terminals.
Consider what the OP observed: “Rotating the volume control makes loud pops on the right channel and then triggers the protection mode.”
To simulate: Make this testing device: 1.5V battery, a series resistor between 1k and 10k ohms, and a momentary pushbutton switch. Connect it to an audio cable with a male RCA plug on the far end. Plug this cable into any line input of an integrated amplifier or receiver. Starting with amplifier’s volume control fully counterclockwise, gradually increase the volume while pushing and releasing the button. Slowly raise the volume. Try to avoid damaging your speakers or use a disposable junk speaker. How loud does it get? How high did you raise the volume before you decided it was unsafe to make it any louder?
To push things harder put a 100 ohm 5W resistor in series with the test speaker. This will act a an attenuator to prevent speaker damage. Then you can continue raising the volume until the pop (or bang) from pushing the button triggers the DC protection circuit in the amplifier.
Now, consider that the pulses generated by a malfunctioning digitally controlled attenuator inside the IC401 might produce a pulse (step function actually) as large as 1/2 of 7V. That’s 3.5V. Twice what the 1.5V test rig generates. How loud would that be? Would it trigger most DC protection circuits?
I will see if i can see any problems on any on the input PCB's, but there is a shed load of 10uf caps all over the input section, so "if" that is the fault, finding which one/s that are faulty will be a pita
I disconnected the R channel input from the CB101 connector and amp works just from left channel.
I disconnected the R channel input from the CB101 connector and amp works just from left channel.May I ask you to check some more things related to the left channel?
1) With R disconnected from CB101, then connect a line level audio signal to only the left channel RCA input jacks one at a time. Do not connect anything to right channel RCA jacks. Connect speaker to the left channel only. Nothing connected to R speaker outputs.
2) “Pure Direct” button must be released(off). Then switch on the power button.
3) Are you able to hear the audio source for each of the following inputs: CD, line 1, 2, 3, and Tuner? ( note that you must rotate the “source select” knob to match the input jack you are applying the audio signal to)
4) Does the large “Volume” knob work to control the left channel volume?
5) Does rotating the large Volume knob cause any loud pops from the left speaker?
6) Does the sound you hear from the left speaker seem “normal?” (Not distorted or noisy, the volume seems correct for the position of volume control knob, the source selector knob works properly for each line-level input)
7) With the large Volume knob at a middle setting, then rotate the “Loudness” knob counterclockwise. Is the “Loudness” knob able to control the volume? Does it make the volume go all the way to zero when fully counterclockwise? The goal of this test is to see if the “Loudness” control will work OK as a substitute for the large “Volume” control.
8 ) Did the “protection” activate at any time while doing these tests?
Please post the results of this test.
Could hear music from each line-level input on L channel
Large “Volume” knob works perfectly to control L channel
No pops, clicks or anything else, sound is crisp and clear
"Loudness knob" as volume control goes quiet but not to zero, i can still hear it but pretty quiet.
“Protection” did not activate at any time
QuoteCould hear music from each line-level input on L channel
Large “Volume” knob works perfectly to control L channel
No pops, clicks or anything else, sound is crisp and clear
"Loudness knob" as volume control goes quiet but not to zero, i can still hear it but pretty quiet.
“Protection” did not activate at any timeThanks! Excellent test results. These confirm the fault is only on the R channel and is caused by a fault inside IC401. No need to worry about any capacitors or other parts being the cause of the loud pops and protection mode.
I worked out a simple and reversible way to bypass IC401. It is done entirely on the function(2) PC board.
To gain access to the function(2) PC board I recommend removing the rear panel. Remove every screw from the rear panel. Note that some may be fine thread screws and others may be coarse thread. I make a diagram as I remove them to remind me where to put them back in during reassembly.
After removing the back panel it should be very easy to unplug the entire function(2) PC board and lift it out of the chassis.
With function(2) PC board on the workbench, locate jumper wires labeled J406 and J407. Clip each jumper wire in the middle and bend up the ends.
Next locate 10uF electrolytic capacitors C461 and C460. They are near the diagonal connector W401. Unsolder the + wire of each capacitor from the PC board and lift these wires up out of the PC board. Note that the wires to unsolder are the wires farthest away from connector W401. Note that the - wire from each capacitor remains connected as it was to begin with. You will only be unsoldering the + wire of each capacitor.
I use "solder wick" to remove as much solder as possible before attempting to pull the wire out of the hole in the PC board. I also recommend brushing rosin flux onto the PC board pads you will be desoldering before you apply heat. There's plenty of instructions for desoldering here on eevblog if you haven't done much of this yet. The goal is to unsolder these capacitor wires without damaging the foil pads on the PC board.
Final step is to solder on two new jumper wires. You may use thin insulated wires of the type often used for breadboard circuit construction.
One new jumper wire goes from upper end of J406 to + end of 10uF capacitor C461
Other new jumper goes from upper end of J407 to + end of 10uF capacitor C460.
That's it.
Now you will be able to use the "Line 1" inputs (the function selector switch no longer does anything).
The "Pure Direct" switch must stay in "off" mode.
The "Loudness" control is now the volume control.
Note: There is another small (reversible) mod which lets the "Loudness" control reduce volume all the way to zero. This mod requires soldering a couple of jumper wires onto the PC board which holds the "Loudness" control. But this mod is optional if you can tolerate being unable to reduce the volume to zero.
Ultimately, if you manage to find a working Function(2) PC board this will fix the original problem and restore complete functionality to the amplifier.
Thanks for this. I have done the mod, and will reinstall it into the amp, and let you know how it goes.
I am guessing i can reconnect CB10 with both L and R connected?
The person i got it off said it was working and his wife unplugged it, now it has this fault.
Thanks for this. I have done the mod, and will reinstall it into the amp, and let you know how it goes.
I am guessing i can reconnect CB10 with both L and R connected?Yes, go ahead and reassemble CB10 so that both channels are connected to the power amplifier stages.
Your rewiring on the Function(2) PC board looks perfect to me. I would have done it the same way. Should there ever be an opportunity to obtain a replacement IC401 chip it will be easy to restore the original configuration.
I do still think the best possible way to handle the IC401 issue is to keep searching for a "parts" amplifier or for somebody selling a Function(2) PC board. Like you said in an earlier post, the most common failures of gear like this Yamaha A-S500 are likely to be mains transformers with open internal thermal cutouts or toasted main PC boards due to failure of one or more output transistors. In either case the Function(2) PC board would likely be undamaged.
I noticed the power section is on a seperate board. Would it be possible to use it just as a power amp with a dual phono input from a pre amp/signal? How would i do this? or would the protection system make this not possible?
QuoteI noticed the power section is on a seperate board. Would it be possible to use it just as a power amp with a dual phono input from a pre amp/signal? How would i do this? or would the protection system make this not possible?How to use a separate preamp/control unit with your modified Yamaha A-S500:
Connect the “Line Outputs” from the preamp/control unit to “Line1” of the Yamaha A-S500. Keep the bass, treble, and balance controls centered on the A-S500. Set the A-S500 “Loudness” control at full clockwise or reduce it by 1/4 turn if the system plays louder than you want it to. Note that you will use the selector, volume control, etc. on the preamp/control unit. The A-S500 will act only as the “power amplifier” with this setup. You will control everything with the preamp/control unit.
The mod you just installed into the A-S500 also gives it limited “integrated amplifier” functionality for using it all by itself, but with only one line-level input from the pair of RCA jacks labeled “Line 1.” The 4 controls in the middle of the A-S500 front panel (Bass, Treble, Balance, Loudness) are also still fully functional.
However, the A-S500 “Input selector” control and the large “Volume” knob no longer operate because they require IC401.
The Yamaha A-S500 contains a “phono stage” but since the function selector switch is inoperative without IC401 it would require some additional rewiring to make use of the A-S500 phono stage. Note that all of the source input switching and selection is performed inside IC401.
Glossary:
Preamplifier (also known as “Control Amplifier): Provides source selection switching (CD, Tuner, Tape, Aux, Phono) volume control, and optionally bass/treble controls and balance control. Many preamplifiers include a “phono stage.”
Phono stage: A specialized low-noise high-gain equalized preamplifier stage which is used only with a turntable. This stage takes the 1-5mV low-level signal from a phono cartridge and raises it to “line-level.” Phono stages have a fixed gain. They don’t have their own volume control. They simply convert the millivolt level signal from a turntable into a standard “line-level” signal.
Power Amplifier: Takes “line-level” input signal and raises it to a much more powerful signal to drive a loudspeaker. Most power amplifiers do not have a volume control. Rather, their gain is fixed, often at 29dB.
Integrated amplifier: The “preamplifier” and “power amplifier” are combined in one physical unit.