Yamaha A-S500 Amplifier DC protection - Page 1
Forum section: electronics / repair

author: tiggerlator
author_rank: Contributor
timestamp: March 30, 2023, 08:16:45 am
content: HiI 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

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: March 30, 2023, 10:56:41 am
content: Without looking at the manual, did you check bias and DC offset adjustments?These would be the first things to check.Jeff

author: tiggerlator
author_rank: Contributor
timestamp: March 30, 2023, 02:58:05 pm
content: Sorry, i am no expert. Would these checks be in the service manual?thx

author: andy3055
author_rank: Super Contributor
timestamp: March 30, 2023, 05:01:22 pm
content: 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.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: March 31, 2023, 12:42:33 am
content: Yes the service manual should show how to CHECK these.If there is DC offset enough on any channel output it may notshow at the speaker terminals if it is in protect.Jeff

author: David_AVD
author_rank: Super Contributor
timestamp: March 31, 2023, 05:38:06 am
content: Exactly what was the diagnostic code /message?It could be an output DC fault, output over current fault or a power supply voltage fault.

author: tiggerlator
author_rank: Contributor
timestamp: March 31, 2023, 09:13:20 am
content: Thank you for your replies. I will try what has been suggested and report back.

author: tiggerlator
author_rank: Contributor
timestamp: March 31, 2023, 09:17:55 am
content: Quote from: David_AVD on March 31, 2023, 05:38:06 amThe fault code is 4 flashes on the power led reapting.

author: tiggerlator
author_rank: Contributor
timestamp: March 31, 2023, 09:25:12 am
content: Quote from: andy3055 on March 30, 2023, 05:01:22 pmTried 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 terminalsI am so sorry about the multi posts.

author: Martin72
author_rank: Super Contributor
timestamp: April 01, 2023, 10:49:48 pm
content: 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).

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 01, 2023, 11:08:53 pm
content: Quote from: tiggerlator on March 31, 2023, 09:25:12 amI 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 controlwith 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.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 12:13:45 am
content: Quote from: Martin72 on April 01, 2023, 10:49:48 pmYes, 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 terminalsuntil 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.

author: Martin72
author_rank: Super Contributor
timestamp: April 02, 2023, 12:23:10 am
content: 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.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 12:39:32 am
content: Quote from: Martin72 on April 02, 2023, 12:23:10 amThere 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 thischangingDC 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.

author: tiggerlator
author_rank: Contributor
timestamp: April 02, 2023, 07:39:00 am
content: Quote from: elecdonia on April 02, 2023, 12:39:32 amTurned 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 CB101Thanks for the help with this guys.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 03:44:48 pm
content: 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.

author: tiggerlator
author_rank: Contributor
timestamp: April 02, 2023, 03:51:29 pm
content: Quote from: elecdonia on April 02, 2023, 03:44:48 pmI will connect some stuff to it and try it now, gimme 10 mins. thxEdit.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.

author: Martin72
author_rank: Super Contributor
timestamp: April 02, 2023, 04:24:34 pm
content: Quote from: elecdonia on April 02, 2023, 03:44:48 pm+1, I hadn´t seen such fault in the last 25yrs of repairing.

author: tiggerlator
author_rank: Contributor
timestamp: April 02, 2023, 04:30:45 pm
content: See edits guys

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 02, 2023, 05:34:39 pm
content: 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.

author: tiggerlator
author_rank: Contributor
timestamp: April 02, 2023, 05:41:50 pm
content: Quote from: Audiorepair on April 02, 2023, 05:34:39 pmOn the input section boards?I guess the main board is all good?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 02, 2023, 05:50:32 pm
content: 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.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 06:24:47 pm
content: Your observations prove thatIC401is 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 boardand simply replace your faulty PC board with it. No soldering required.- This“Function(2)” PC board is the board with IC401on 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 embeddedinsideIC401.4) 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 itsinternal digital attenuatorsto 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 ofIC401(or replacement of“Function(2)” PC board) will fix it.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 06:39:20 pm
content: Quote from: tiggerlator on April 02, 2023, 05:41:50 pmAlthough those small 10uF electrolytic capacitors are known to fail, this isn’t what’s wrong with your amplifier.You have a faultyIC401. 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.QuoteYes, 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.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 07:03:24 pm
content: QuoteHere’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 youdo not touch the main volume control knob,If youdohear 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.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 02, 2023, 07:04:24 pm
content: Hmm,as I understand it, the OP has established the amp shuts down due to DC on the speaker output on one channel when moving the volume pot.He has observed this happening.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 circuit.But maybe you are correct.Personally, I would be looking at the caps as the culprit, this is cheap and easy to do and a very common problem in all sorts of units.And if that doesn't work, then the amp is toast anyway.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 07:15:37 pm
content: Quote from: Audiorepair on April 29, 1975, 11:30:26 pmIC401 operates on balanced +/-7V supply rails. But an internal fault might cause a full -7V to +7V step function at its analog output pin when the volume control gets rotated. This is large enough to be seen by the DC voltage detector at the power amplifier output, even after passing through several 10uF coupling capacitors.See my previous post for a description of the (really strange) way the volume control section works in this unit.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 02, 2023, 07:41:34 pm
content: Logic in fault finding often leads to trying to find the component that has failed.Real world experience fault finding teaches you that sometimes many, most or ALL of these 10uF capacitors may have failed.The cumulative result of all these failures being DC is able to get through to the Power Amp.

author: Martin72
author_rank: Super Contributor
timestamp: April 02, 2023, 08:02:31 pm
content: I wish I had the amp here and could do some scope measurings.

author: David_AVD
author_rank: Super Contributor
timestamp: April 02, 2023, 08:47:38 pm
content: Another thought - I had a weird fault with a Denon amplifier a while back where one of the voltage rails to the selector / volume chip was missing and from memory it caused distortion.The actual +/- rails were ok from the regulators, but there was a corroded trace near the chip so it wasn't until I checked the voltages right at the chip that I found the issue.So I would carefully check that the +/- 7V rails are really on the chip itself before replacing the chip.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 08:58:32 pm
content: Quote from: David_AVD on April 02, 2023, 08:47:38 pmIt is a good idea to check the +/-7V power supply voltages on IC401. However, because one channel has no DC offset but the other channel does, this leads me to suspect a faultinsideIC401.I’m thinking that a power supply fault would cause both the left and right channels to have DC offset.Or, more correctly, a huge DC “step function” which occurs when the volume control is rotated.

author: tiggerlator
author_rank: Contributor
timestamp: April 02, 2023, 10:16:33 pm
content: Thanks for all the input guys.I repeat, there is no DC voltage on the L speaker output.If i cannot get the faulty IC repaired, 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

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 02, 2023, 10:52:06 pm
content: Actually I think it can be rewired to bypass the faulty IC401.One pairof line-level analog inputs could be re-routed to go through the "conventional" bank of 4 knobs in the middle of the panel: These controls are bass, treble, balance, and "loudness." None of this circuitry requires IC401.The loudness circuit can easily be revised to become a simple "volume" control circuit without the low-frequency boost at low volumesHowever the "Volume" control with the large knob and the selector switch with the large knob aren't usable without a functional IC401.

author: Martin72
author_rank: Super Contributor
timestamp: April 02, 2023, 11:00:12 pm
content: QuoteShould be no problem.QuoteDo this, but I´m afraid elecdonia are right.Between the volume out from the IC401 and the input of the power amp are three bypass caps(including the one in the power amp input), if I count it right.Chances are very low that all caps are defective (shorted) so dc components could disturb the power amp.Normally bypass caps are used to block dc components of the signal.For a very short time they let them through because of charging.It must be a kind of repitive signal which can pass the caps, this could be only coming from the IC401.Two "last chances":-Observe the solderings on the pcbs for faulty areas.-Check the transistor Q802 in the mute circuit.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 03, 2023, 04:12:41 am
content: 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?

author: tiggerlator
author_rank: Contributor
timestamp: April 03, 2023, 06:15:14 am
content: Quote from: elecdonia on April 03, 2023, 04:12:41 amIirc, the voltage on the right speaker was as high as -12v.Shame about the fault, i only paid £25 for it, assuming it would likeley jusy be a popped main tranny. As it is, the repair is probably beyond my skills, so i will either take it to a proper repair shop, or try and use it as a power amp only with a pre amp for input to it some how, as it is a 85wpc amp.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.Shame i do not live close to one of you pros.Thanks anyway, and i am kinda glad i gave you guys something to think about.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 03, 2023, 01:33:05 pm
content: QuoteWhile it’s true that these small electrolytic capacitors sometimes fail, I sincerely doubt they are an issue in this amplifier.A couple of posts back I suggested the input(2) PCB could be modified to provideoneset of line-level audio inputs along with the 4 controls in the middle of the front panel remaining functional (balance, bass, treble, loudness). This wouldn’t be a difficult job. It requires only a little bit of desoldering/soldering. The faulty IC401 can be left in place. IC401doesn’tneed to be removed in order to make this modification. If you are interested I will provide instructions for doing it.I searched ebay and saw listings for several A-S500 and similar models. Some were offered “for parts only” at reasonable prices ( < $50 USD ). My approach to repairing audio gear is to get at least two similar units. This makes “board swapping” possible.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 03, 2023, 02:36:48 pm
content: Quote from: tiggerlator on April 02, 2023, 03:51:29 pmMay I ask you to check some more things related to theleft channel?1) With Rdisconnectedfrom CB101, then connect a line level audio signal toonlythe 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.

author: tiggerlator
author_rank: Contributor
timestamp: April 03, 2023, 04:09:41 pm
content: Quote from: elecdonia on April 03, 2023, 02:36:48 pm3- 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) YES. Could hear music on all of these inputs L channel4) Does the large “Volume” knob work to control the left channel volume? Yes, perfectly.5) Does rotating the large Volume knob cause any loud pops from the left speaker? No, no pops, clicks or anything else, sound is crisp and clear, see 67) 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. It goes quiet but not to zero, i can still hear it but pretty quiet.8 ) Did the “protection” activate at any time while doing these tests? No it did notThe right channel of the power amp section is working fine on both channels. the amp only goes into protection if the R channel input is connected to the power amp input. I connected the L channel to the R channel power amp input and did the same tests, and again was all fine.If you need me to try anything else let me know.Thanks

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 03, 2023, 06:46:07 pm
content: QuoteThanks! Excellent test results. These confirm the fault isonlyon the R channel and is caused by a fault insideIC401. 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 bypassIC401. It is doneentirelyon the function(2) PC board.To gain access to the function(2) PC board I recommendremovingthe rear panel. Removeeveryscrew 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 labeledJ406andJ407. Clip each jumper wire in the middle and bend up the ends.Next locate 10uF electrolytic capacitorsC461andC460. They are near the diagonal connectorW401. Unsolder the+ wireof each capacitor from the PC board and lift these wires up out of the PC board. Note that the wires to unsolder are the wiresfarthest awayfrom connector W401. Note that the- wirefrom each capacitorremainsconnected as it was to begin with. You willonlybe unsoldering the+ wireof 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 brushingrosin fluxonto the PC board pads you will be desolderingbefore you apply heat.There's plenty of instructions fordesolderinghere 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 fromupper end of J406 to + end of 10uF capacitor C461Other new jumper goes fromupper 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 workingFunction(2) PC boardthis will fix the original problem and restorecompletefunctionality to the amplifier.

author: tiggerlator
author_rank: Contributor
timestamp: April 03, 2023, 08:54:04 pm
content: Quote from: elecdonia on April 03, 2023, 06:46:07 pmThanks 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?

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 03, 2023, 09:29:31 pm
content: Quote from: tiggerlator on April 03, 2023, 08:54:04 pmYes, 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.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 03, 2023, 10:19:05 pm
content: QuoteWhat probably destroyed IC401 in this amplifier was from the user disconnecting or reconnecting line-level cableswhilethe AC mains was connected.Today nearly all integrated amplifiers and receivers contain line-level input switching and control chips similar to IC401.A major problem with external cables connected directly to low-voltage line-level audio switching chips such as IC401 is thatESD (static electricity)andexcessive input signalscan easily damage the IC chip.My advice to owners of modern solid-state audio gear is "always switch everything off" anddisconnectall AC mains supply cords prior to connecting or disconnecting line-level audio cables.But of course we sometimes forget to disconnect the mains cables... Therefore, experienced circuit designers place ESD protection circuits between external cables and sensitive IC chips. Unfortunately the ESD protection for IC401 audio inputs consistsonlyof 470 ohm series resistors (R401, R408). The lines on the attached schematic labeledL1ILandL1IRconnect directly to pins on IC401. 470 ohms isn't anywhere near a large enough series resistance to provide useful protection. I would change these series resistors to 4.7k ohms. This won't cause a noticeable reduction in gain or loudness. I would also consider adding a pair of diodes or zeners connected so that the input voltage cannot go above +7V or below -7V. A diode protection circuit contains 2 diodes which clamp maximum input voltage to the supply rails. A zener protection circuit has a pair of back-to-back 5.6V zeners in series between the input and chassis ground. Replacing a diode is a lot easier than changing out a 100-pin IC chip which might not even be available as a replacement part.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 03, 2023, 10:36:21 pm
content: This is a block diagram ofIC401.It looks...complicated11 stereo line level input channels8 output channels.176 analog switches inside IC401 are required just to handle the input selection.Additionally there is a whole bunch of programmable routing for front/rear left/right, center channel, subwoofer output, etc.Finally each of the 8 output channels has a digitally controlled attenuator having a 130dB range in 0.5dB steps.The strangest thing I find about this A-S500 is thatonly2 channels of IC401 are used. Yamaha could have selected a much smaller, less expensive, and more standardized analog input switching chip. As for the digital attenuator I would think plenty of 2-channel devices are available to choose from.

author: Martin72
author_rank: Super Contributor
timestamp: April 03, 2023, 11:39:11 pm
content: Burr Brown PGA2310 or wolfson WM8816, but if you already have a chip in stock(for mutlichannel amps) produced by your own....

author: tiggerlator
author_rank: Contributor
timestamp: April 04, 2023, 07:03:06 am
content: Quote from: elecdonia on April 03, 2023, 09:29:31 pmRight, connected back up with both L/R channels of CB101 connected to the power maain 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.Simon

author: tiggerlator
author_rank: Contributor
timestamp: April 04, 2023, 12:57:16 pm
content: 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?thanks

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 04, 2023, 10:10:29 pm
content: QuoteHow 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 onlyoneline-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” knobno longer operatebecause 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 performedinsideIC401.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 donothave a volume control. Rather, their gain is fixed, often at 29dB.Integrated amplifier:The “preamplifier” and “power amplifier” are combined in one physical unit.

author: tiggerlator
author_rank: Contributor
timestamp: April 05, 2023, 06:50:11 am
content: Quote from: elecdonia on April 04, 2023, 10:10:29 pmproblem 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.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 05, 2023, 02:57:00 pm
content: If connecting R signal to either L or R input of power amplifier at CB101 causes "protection" then we need to check a couple of items on the other end of the cable that connects to CB101.Also it isveryimportant to make sure the "Pure Direct" button is released.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 05, 2023, 08:15:17 pm
content: QuoteThere 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 shouldnothave 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 lookexactlylike 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 namedOPERATION(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 ?

author: tiggerlator
author_rank: Contributor
timestamp: April 06, 2023, 05:44:01 pm
content: Quote from: elecdonia on April 05, 2023, 08:15:17 pmThe 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 on zero still produces sound when turning the loudness control up. Both channels of the power section work, the problem is the R output from the pre section.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 06, 2023, 09:42:07 pm
content: Quote from: tiggerlator on April 06, 2023, 05:44:01 pmOK let's try this:1) Set main "VOLUME" control (large knob) to minimum (set to "0," which is fully counterclockwise)2) Then rotate "LOUDNESS" control over its full range. Sound level should get louder as you rotate "LOUDNESS" control clockwise. In fact the normal setting for "LOUDNESS" isfullyclockwise. This is where the "LOUDNESS" bar knob is straight up and down. Rotating "LOUDNESS" knob counterclockwiseshouldreduce the volume, but I don't expect it to reduce it all the way down to nothing.3) It is possible that you won't hear any sound at all with main "VOLUME" (large knob) fully counterclockwise, regardless of what position "LOUDNESS" knob is at.Please tell me whether this is case (or not)?4) If there is no sound with main "VOLUME" fully counterclockwise, then turn it up by 1/4 turn clockwise. Does this get the sound to start coming out?Also try setting main "VOLUME" control to 1/2 volume, with pointer on large "VOLUME" knob straight up. What does this do?Does the sound level change any at all when you rotate the large main "VOLUME" knob back and forth between 1/4 and 1/2?One possibility is that you won't hear anything at all until main "VOLUME" is above 0. Perhaps main "VOLUME" totally shuts off the sound when at 0, but at all settings above 0 the volume doesn't really change. In other words, no difference between 1/4 and 1/2 setting of main "VOLUME" control knob.5) Repeat steps 1-2-3-4 with "PURE DIRECT" button bothpressedandreleased. I don't know which position (buttoninor buttonout) corresponds to the mode where the BASS TREBLE BALANCE LOUDNESS controls are active and functional.Details:- The "PURE DIRECT" switch is supposed tototally bypassthese four controls (BASS TREBLE BALANCE LOUDNESS) inoneof its two positions. In this modenoneof these four controls should do anything at all- In other position of "PURE DIRECT" button then the BASS TREBLE BALANCE LOUDNESS controls should all "do something" when rotated over their full ranges.We want to set "PURE DIRECT" button so that BASS TREBLE BALANCE LOUDNESS are allworkingQuoteCan you explain"what changes about the sound"at the time when the main "VOLUME control knob is is moved just above or below position "1" ?Is it like this?When you rotate large main "VOLUME" knob to "above 1" then the sound abruptly cuts out?Does this "1" position correspond to the pointer on large main "VOLUME" knob being near the letter "E" on the word "VOLUME" ?QuoteUnderstood. For now let's do all testing with the R wire disconnected from CB101. Goal is to get the L channel to be as functional as possible. After that then we will sort out the problem which R channel preamp section still has.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 07, 2023, 06:05:43 pm
content: Have you actually checked any of these 10uF caps to at least rule them out as the suspect?

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 07, 2023, 07:46:35 pm
content: Quote from: Audiorepair on April 07, 2023, 06:05:43 pmThis 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 theextreme complexity of the circuit design employed in this integrated amplifier.I'm beginning to suspect my previous 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 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 donotpass 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.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 07, 2023, 07:56:05 pm
content: Quote from: elecdonia on April 07, 2023, 07:46:35 pmOK, 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.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 07, 2023, 09:48:47 pm
content: QuoteAs I mentioned earlier, let's keep the R channel wiredisconnectedfrom 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 shouldneverhave 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 voltmeterwhilerotating the large volume knob should be informative.Do you have any access to an oscilloscope?

author: Martin72
author_rank: Super Contributor
timestamp: April 07, 2023, 10:08:23 pm
content: Quote from: elecdonia on April 07, 2023, 07:46:35 pmTherefore I´ve wrote :Quotehttps://www.eevblog.com/forum/repair/yamaha-a-s500-amplifier-dc-protection/msg4792805/#msg4792805But no feedback.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 07, 2023, 10:34:05 pm
content: Quote from: Audiorepair on April 07, 2023, 07:56:05 pmI admire your persistence in suggesting these 10uF capacitors areimportant. Here's what I just thought of:TheDC voltageateachend of each 10uF capacitor needs to be measured.I'm not thinkingonlyof faulty 10uF capacitors, but rather I'm thinking these 10uF capacitors are theperfect locationto probe for unwanted DC voltages. They are "through-hole" parts which are easy to locate on the PC boards and it will be easy touch their connecting wires with the probe of a DC voltmeter.I fully agree that small electrolytic capacitors are "prime suspects" for many faults. But I also have a strong suspicion that "something else" is wrong with this Yamaha amplifier.I've looked at the schematic for this amplifier over and over: All of its audio circuits operate on balanced +/- power supply rails, either +/-15V for the opamps or +/-7V for IC401. If anything is wrong with these power supplies it will affect both channels equally. However the L channel always works perfectly yet the R channel always generates this horrific loud pop whenever the volume control rotates. Therefore I don't suspect a power supply fault.The entire audio signal path itself should be at 0V DCdue to the use of these equal +/- supplies. So while reading your posts about the 10uF capacitors the thought popped into my head that"both ends of each 10uF capacitor should be at 0V DC."When a non-zero DC voltage is located this willpinpointa bad opamp or muting transistor. Or perhaps IC401 is actually at fault?So I think the OP will need to test both the + and - wire of each 10uF capacitor for DC voltage. There should be 0V on both ends of all of them.Unfortunately some of these 10uF capacitors are hidden away on the OPERATION(2) PC board which is just behind the front panel. It might be difficult to probe the capacitors on the OPERATION(2) PC board, but thisiswhat needs to be done next.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 07, 2023, 10:44:15 pm
content: Quote from: Martin72 on April 07, 2023, 10:08:23 pmBINGO!I don't know how I missed you mentioning Q802 previously. I discovered it myself while going over the schematic. These muting transistors are precisely at the other end of the cable which feeds audio into the power amp board. Of course they are tiny SMT devices tucked away on a PC board OPERATON(2) which isn't exactly easy to reach. But they are certainly in a place where a failed transistor could put major DC transients into the power amp input. It is also possible that the MCU sends brief muting pulses to Q801/Q802 whenever the main volume control is rotated or when the selector switch moves.Logic-level control signal PREMT which drives Q801/Q802 is large enough to trigger protection if it gets onto the POR line. That will never happen if Q802 is good.

author: Martin72
author_rank: Super Contributor
timestamp: April 07, 2023, 11:44:21 pm
content: I looked over again...Hm-hm...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...

author: mbz
author_rank: Contributor
timestamp: April 08, 2023, 12:26:23 am
content: 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

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 08, 2023, 03:28:29 am
content: Quote from: Martin72 on April 07, 2023, 11:44:21 pmI suspect the MCU sends a brief "mute" command whenever the main volume control or the input selector switch are rotated. By "brief" I mean <20msec. Still, 20msec is a long enough pulse to make a big loud pop in the audio signal if Q802 C (collector) is open circuit. The B-E junction will act like a forward-biased diode to couple the logic level pulse onto the audio line. Note that on the schematic Q802 is drawn as an NPN transistor with C grounded and E connected to the audio line. Also the schematic shows both E and C are 0V when B is +0.7V. The +0.7V on B implies the "mute" state is active.These "muting" transistors are kind of strange. Their emitter and collector are interchangeable. Some even have a spec named "reverse hfe" which implies that when the B-C junction is forward biased then the reverse-biased B-E junction will conduct. Again, this Yamaha has Q802 C (collector) grounded with E (emitter) is connected to the L (or R) audio line. This seems strange, but it's plausible if the B and C terminals on this particular transistor areinterchangeable.I don't recollect needing to replace many of these muting transistors. It would be fascinating if the root cause turns out to be something like poor soldering on the emitter of Q802 or a broken foil on the PC board. Earlier I was all worried that IC401 was faulty (switching and attenuator IC) . IC401 is an insanely complex chip and the line input RCA jacks go straight into it through 470 ohm series resistors which don't provide much ESD protection.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 08, 2023, 03:38:02 am
content: Quote from: mbz on April 08, 2023, 12:26:23 ami wonder how the B or E voltage on Q802 could get negative?The schematic shows +0.7V on Q802 B. I expect this to be the case when the amplifier is in the "muted" state.Schematic also shows both E and C should be 0V DC.One (very odd) thing about this muting circuit is that C is grounded, with E connected to the audio signal line that gets muted. Evidently these muting transistors are designed so that E and C pins are interchangeable.As a quick test to confirm this Q802 fault I suggest the OP should remove Q802 from the OPERATION(2) PC board and then test the amplifier again.There won't be any muting on the R channel with Q802 removed, but there also won't be any DC getting into the R channel audio.

author: mbz
author_rank: Contributor
timestamp: April 08, 2023, 04:34:29 am
content: yep, remove Q802 would be a good test

author: tiggerlator
author_rank: Contributor
timestamp: April 08, 2023, 05:59:00 am
content: Thanks guys.I will read through these replies and test what you have suggested and report back today ok.Thanks a lot for your timeSimon

author: tiggerlator
author_rank: Contributor
timestamp: April 08, 2023, 06:12:28 am
content: Quote from: elecdonia on April 07, 2023, 09:48:47 pmThere is -2.87v DC on the R wire of CB101 with the volume turned up. the voltage is constant, amp did not go protect as i had the wire disconnected from the CB101 connector before the power amp. There is 0v at the L signal wire.

author: tiggerlator
author_rank: Contributor
timestamp: April 08, 2023, 06:16:03 am
content: Quote from: Martin72 on April 07, 2023, 10:08:23 pmSo sorry i missed that.

author: tiggerlator
author_rank: Contributor
timestamp: April 08, 2023, 06:43:49 am
content: 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 does it need that Q802? Can i use it without it?If so i can probably find one somewhere.Can't thank you guys enough.Only paid £25 for this amp, and it is seemingly a very good one. I have tested and there is seemingly no problem without Q802, pure direct works too. .

author: mbz
author_rank: Contributor
timestamp: April 08, 2023, 08:24:04 am
content: 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.

author: tiggerlator
author_rank: Contributor
timestamp: April 08, 2023, 08:52:54 am
content: Quote from: mbz on April 08, 2023, 08:24:04 amJust checked the speaker outs, there is -11mv on R and nothing on L. Turning the volume up does not increase it. The amp works seemingly fine, no pops on power on or off, sounds really nice, the best amp i have now by miles.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 08, 2023, 09:12:13 am
content: Quote from: tiggerlator on April 08, 2023, 06:43:49 amThis 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 is2SD2704. 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 addedReconnect the free ends of C460 and C461 back to the PC board as they were to begin withUse short pieces of wire to reconnect J406 and J407 as they were originally

author: tiggerlator
author_rank: Contributor
timestamp: April 08, 2023, 09:17:27 am
content: Quote from: elecdonia on April 08, 2023, 09:12:13 amI 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?Thanks

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 08, 2023, 09:34:31 am
content: Quote from: tiggerlator on April 08, 2023, 09:17:27 amYes, go ahead and use it.

author: tiggerlator
author_rank: Contributor
timestamp: April 08, 2023, 11:01:44 am
content: Quote from: elecdonia on April 08, 2023, 09:12:13 amThanks again.I will replace that Q802 as soon as i find a replacement.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 08, 2023, 02:30:02 pm
content: Info and spec sheets for2SD2704 (K)audio muting transistor:https://www.rohm.com/products/transistors/bipolar-transistors/standard-bipolar-transistors/2sd2704k-product#productDetailThe data sheet confirms the 2SD2704 issymmetrical: E (emitter) and C (collector) areinterchangeablewhen used in a shunt muting circuit. Note that B (base) drive signal is always positive with respect to either E or C.After this lengthy Yamaha A-S500 discussion I am ordering a strip of these2SD2704transistors. I expect this part will work well in all similar line-level audio muting circuits. I already have other “muting” transistors on hand but they are all TO-92 “through-hole” types. However, these days one must also keep on-hand a good selection of commonly used small-signal transistors in surface-mount SMT packages. The 2SD2704 package is SOT-346, also known as SC-59.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 08, 2023, 02:53:41 pm
content: Quote from: tiggerlator on April 08, 2023, 09:17:27 amThis is a fully acceptable DC offset voltage.This small DC voltage which appears at the speaker terminals of an amplifier or receiver during “no signal” conditions is technically known as the “offset voltage.”The standard spec for DC offset voltage is“less than +/- 50mV.”In practice I have found amplifiers with DC offset < +/- 100mV to be borderline acceptable.Anythinglargerthan +/- 100mV indicates a fault within the power amplifier circuitry which needs correction.“DC protection” circuits typically switch off the speaker relay (or shut down the amplifier power supply) when the DC voltage at the speaker terminals exceeds +/- 500mv ( this is +/- 0.5V).Failure of one or audio output stage power transistors often results in the entire main power supply voltage appearing at the output. This could be anywhere from +/- 20V up to 80V DC (or more) depending on the size of the amplifier. In this case the speaker relay will not close and amplifiers/receivers with VFD displays may show a “protection” warning message.Some amplifiers have internal “offset voltage” trimmer resistors located on the power amplifier driver PC boards. These permit nulling the DC offset voltage to 0V. However, if the offset trimmer resistor must be rotated all the way to one end or the other, then this indicates more diagnosis and testing is required.

author: tiggerlator
author_rank: Contributor
timestamp: April 08, 2023, 09:43:16 pm
content: Quote from: elecdonia on April 08, 2023, 02:53:41 pmFunny thing is, i ordered a pair of matched output transistors just in case one channel had gone, so got a spare set in case one channel goes.I will have a look for a 2SD2704 after the bank holiday.

author: Martin72
author_rank: Super Contributor
timestamp: April 08, 2023, 11:02:05 pm
content: Quote from: elecdonia on April 08, 2023, 03:28:29 amTherefore I´ve wrote once, it must be a kind of repetive signal to go through all the bypass caps...But hey, we´ve found the failure without having the amp here on our desk..It´s the hardest kind of analyzis IMHO.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 08, 2023, 11:38:32 pm
content: Quote from: Martin72 on April 08, 2023, 11:02:05 pmIn the future I will pay more attention to muting transistors locatedmid-wayalong the audio path.I think what caught me off-guard with this Yamaha A-S500 is that most integrated amplifiersdon'thave any such muting transistors. They rely on the speaker relay for muting (andonlyon the speaker relay for muting).Line-level muting circuits similar to Q801/Q802 are often found in preamplifiers, cassette decks, AM-FM tuners, and occasionally CD players. If the device has line-level outputs then there will be a muting circuit prior to the output jacks (if it is a well-designed item).

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 08, 2023, 11:48:24 pm
content: Quote from: tiggerlator on April 08, 2023, 09:43:16 pmI'll take a closer look at the amplifier schematic and at the data sheets for those output transistors. This integrated amplifier looks like a "well above average" item in terms of its features, design, and general appearance. I understand why you are pleased to get it into working condition.Speaking of output transistors, the PC board layout shown in the A-S500 service manual indicates this amplifier can have twice as many output transistors: A pair of NPN and a pair of PNP per channel. That would be a grand total of 8 output transistors rather than the existing 4 output transistors. These locations on the PC board and heatsink are unused (aka "unpopulated" ) in the A-S500. Adding additional output transistors in parallel not only increases the reliability but also reduces distortion and non-linearity when driving difficult loads. Perhaps Yamaha uses the same PC board in a more costly model with higher output wattage?

author: Martin72
author_rank: Super Contributor
timestamp: April 09, 2023, 12:09:52 am
content: Hi,The circuits in the yamaha are a kind of uncommon in several ways, no doubt - A few yrs ago, I´ve examined the standby circuit of the amp:http://www.hifi-forum.de/index.php?action=browseT&forum_id=220&thread=7827&postID=1#1(Sorry, only in german)QuotePlus two more, near the µP (Pre-mute and SWT)

author: mbz
author_rank: Contributor
timestamp: April 09, 2023, 12:17:17 am
content: The A-S700 appears to use the same board with paired NPN/PNP outputs. Be a simple upgrade assuming same heat sink but think I would be enjoying the amp and count my blessings that it cost little money, it's working normal and sounds good...

author: tiggerlator
author_rank: Contributor
timestamp: April 09, 2023, 06:40:50 am
content: Quote from: elecdonia on April 08, 2023, 11:48:24 pmSo it would be possible to add in the other 4? I wonder if they use that same output section for higher cost amps? there are some higher cost A-xxxx series ampsIt is a very nice sounding amp, and gets reviewed pretty well.Quote from: mbz on April 09, 2023, 12:17:17 amYes i will do that. After looking it would be tricky as there are lots of support components for the other main output pairs missing on this board.And it is a fine amp for £25

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 10, 2023, 09:09:18 pm
content: Quote from: Martin72 on April 09, 2023, 12:09:52 amI was able to read the article (Google did a great job with translating Deutsch -> English).It certainly is an unusual circuit design for a standby power supply. I own a couple of Yamaha home-theatre receivers. Now I will take a close look at their standby power supplies. Although my units have been free of power supply issues I am now very curious about this circuit design.I did need to replace the TI DSP chips in a couple of these Yamaha home-theatre receivers. There was a period where TI had troubles with their DSP chip wafer fab processes. This caused their DSP chips to failintermittentlyafter ~4 years. Several major manufacturers used TI DSP chips which developed this fault. Onkyo offered free repair including for "out-of-warranty" units. Onkyo repaired my son's receiver free of charge. Onkyo even supplied a pre-paid shipping box. They fixed our unit by installing a brand-new DSP/HDMI board in it.I don't know whether Yamaha offered any such extended free service? I was able to buy several Yamaha home-theatre receivers for < $50 USD each due to them having the "intermittent DSP audio fault." Improved replacement DSP chips were available from a 3d party for $15 each.

author: Martin72
author_rank: Super Contributor
timestamp: April 10, 2023, 10:14:44 pm
content: QuoteWon´t surprise me if you will find something similar, my former soundbar from yamaha (YSP..Don´t know it anymore) had this circuit too.

author: elecdonia
author_rank: Frequent Contributor
timestamp: April 11, 2023, 01:29:47 pm
content: Quote from: Martin72 on April 10, 2023, 10:14:44 pmI believe the Yamaha A-S500 service manual includes schematics and part numbers for both 120V and 240V mains. It is likely the low-voltage portion of the driver circuit for the backup power supply is similar for both 120V and 240V mains. The difference will be the part number for the small step-down transformer.Here in the USA nearly all home audio products are fitted with power supplies which accept 120Vonly. Are most audio items sold in Europe fitted for 240V only?

author: Martin72
author_rank: Super Contributor
timestamp: April 11, 2023, 09:54:38 pm
content: I think it will be more like that today.In the 90s I had my "high time" what repair of hi-fi equipment concerns(today I pick from time to time some up, when it´s interesting to me, pm the user in a forum, offer to repair it without additional costs except shipping/spares).I had done this as a hobby and as a schoolboy / student on the side, for my friends.And at that time, most of the equipment for sale here had an input voltage selector switch on the back.Later this became rarer and rarer, other transformers were explicitly installed instead of universal ones.In most European countries 230Vac is the standard, in England it's 240Vac, as far as I know.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: April 11, 2023, 10:19:04 pm
content: UK mains voltage is specified as 230v, +10%/-6%.It was changed from the (previously marked on equipment) 240v in 2009, to ensure that all equipment meant for Europe and the UK was compatible in both places.So basically manufacturers have to make sure their equipment works within these limits.I believe it is the same in Australia.

author: tiggerlator
author_rank: Contributor
timestamp: April 12, 2023, 08:47:43 am
content: I am glad i gave you guys something interesting to think about anyway.

author: Ajuztam
author_rank: Newbie
timestamp: December 14, 2023, 12:36:22 pm
content: Hi. Sorry for digging out an old post. I have the very same amp that my friend gave me and when I power it on it flashes constantly so according to manual it's APD (automatic power down). When I enter Protection Cancel Mode if flashes Line2 and powed led flashes 2 times pause 2 times and so on. Any advice on where to start a repair. Thanks