Hifi amplifier biasing issues and output stage shorting? - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: October 12, 2020, 09:10:15 pm
content: Hi guys,Just wanted to clarify some doubts I had with biasing issues on SS amplifiers. I am really interested on working on amplifiers and anything related to audio. I have been reading on amplifiers.So here is the series of events on the project I am working on:Output transistor Q6050 was shorted and one half of the emitter resistor R6100 was open (left channel) and the B+ fuses were open.So I borrowed the two output transistors and the emitter resistor from a working channel which was not used. Replaced the fuses however I didn’t replace the driver transistors(I should have!).Powered on the unit and measured the biasing voltage at the test points and it was within specs given on the SM. However I should add that I didn’t monitor it long enough.Played some signal through it and it was working perfectly fine.So assembled the unit and returned it to the owner to be used until the replacement parts arrived.Checked it at his place, was working fine and after I got back he called me back saying that the amp had failed and that there was no output.I checked the amplifier again the same left channel had shorted. The two output transistors were shorted and the B+ voltage fuses were blown. The emitter resistor was intact this time though.As per what I have read online regarding what could blow the output stage (some reasons):The speaker being shorted - I checked the speakers and I didn’t measure a short across the terminals. However it had a crossover circuit and only what I could read was a capacitor charging and both speakers showed similar behaviour. The L and R speaker sounded same to me when it was working.Cold/broken solder joints - I have checked for these and didn’t come across any obvious points.Other components failed - I have checked almost all the components on the left channel and it measures ok.Biasing issues.I would like to clarify about the biasing circuit used on this amp. I have done some research on Vbe multiplier circuits but most of the circuits I came across the variable resistor is connected to the base of the Vbe multiplier. However in this unit the VR is connected to another transistor’s base Q6010. I am finding it a bit hard to grasp how this works.Could someone kindly explain.Q6010 is placed next to one of the drive transistor and it has got what looks like thermal compound applied to it.What is the reason for this?If it was a biasing issue and somehow it caused the output transistors to short out,note B-C-E were shorted- why didn’t it take out the drive transistors? The drive transistors still checks out ok.All of the components related to the biasing part of the channel checks out ok but hypothetically speaking what component failure for e.g.: the potentiometer’s contacts loosing connection, some kind of thermal effect messing up with the biasing circuitry’s transistors?.If its some kind of an issue which only causes the fault when power is applied what further troubleshooting should I be doing on this circuit? At the moment I am waiting for the replacement parts to arrive but meanwhile I would like to figure out what exactly caused the fault - could I power up the amp without the output transistors and see if the proper voltages are preset which is shown on the SM and lets say if the drive transistors are faulty, ‘leaky’ for instance would I be able to measure that?Based on what I have detailed above, what’s your opinion for the reason for the fault to occur?Thanks in advance for the help.P.S - I couldn’t find replacement for some of the transistors and resistor so here are the equivalents that I have ordered2SC1740S-S ——— 2SC22402SA1930 ——— A18370.22 Ohm 2W emitter resistors —— 0.22 Ohm 5WAlso note that there are some film capacitors which looks a bit black, I am not sure it was like it to start with. It measures ok especially when compared to a working circuit. Hope you can see it in the photo

author: magic
author_rank: Super Contributor
timestamp: October 12, 2020, 10:06:16 pm
content: If we replace Q6000, R6000 and R6010 with a short, the Q6010 circuit is a Vbe multiplier.The Q6000 circuit also is a Vbe multiplier and as such it behaves like a zener. It forces Q6010 to increase its collector by Q6000 circuit's "breakdown voltage".Total bias is the sum of two Vbe multipliers which operate at different temperatures. I suppose there is some chance that if the inner multiplier doesn't work correctly, you can compensate its error by readjusting the outer multiplier, but then things get bad when the amp warms up.Or it could be some other problem altogether causing it to blow up, dunno.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 13, 2020, 01:21:37 am
content: q6000 could be a constant current reulator too, you need to be stable in voltage and current for the biais adjustmentIs it a sony amplifier / tuner ? it's a direct coupled amplifier model, meaning if something goes wrong lots of parts may die.You have to remove all transistors, the biais one too, check them carefully on diode and ohms mode each pins and reverse polarity too, not in the circuit.Sometimes in the past the only solution i had was to replace all transistors ... check all resistors, and check for cold joints

author: SpecialK
author_rank: Regular Contributor
timestamp: October 13, 2020, 01:38:17 am
content: i believe it's an Onkyo HT-RC440 or similar model. Googled one of the transistors model number and it's silkscreen ID.

author: xavier60
author_rank: Super Contributor
timestamp: October 13, 2020, 12:52:37 pm
content: Two main possible reasons for that kind of failure. Even a brief open circuit in the idle current biasing circuit such as a fractured joint or bad contact of the trimpot's wiper will cause destructively high rail to rail current through the output transistors.There could be a problem at the owners place. Many people just don't realize the harm that short circuiting stray speaker lead strands can cause.When an output stage blow up occurs, because the VAS transistors are configured as current sourcing, damage rarely occurs to them, this also applies to earlier stage transistor.Transistors and other components in the bias circuit do get damaged.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 13, 2020, 08:30:24 pm
content: Thanks everyone for the input really do appreciate it.Quote from: coromonadalix on October 13, 2020, 01:21:37 amIts an Onkyo SR508, sorry I should've mentioned that in my first post. Yes I was also planning on taking the small transistors out of circuit and testing them. I have tested them in circuit and didn't come up with an obvious fault. I also measured them in ohms mode and compared the results with the working channel's transistors they seem to match upQuote from: xavier60 on October 13, 2020, 12:52:37 pmCan I ask please do correct me - for instance if the VR was open that should 'turn off' Q6010 right? Could that take out the Output transistors? If so why does it happen? I was under the impression that it'll just make the output sound distorted with cross over distortion.Sorry for the noob question - could you please elaborate on VAS stage of this amplifier, I know it stands for the voltage amplification stage.What is the purpose of Q5030 & Q5040?Why doesn't biasing issue take out the drive stage transistors like the output transistors? I haven't had much experience working on amps but from most of the stuff that I have read and seen the driver transistor seems to be intact even when the outputs short out, but everyone recommends to change them anyway.'IPRO' is current protection, shouldn't this protection be engaged when there is a bias issue? There are other two protection signals but I'm not sure what it stands for 'FLVPRO' and 'FLVOLH' the service manual is not really descriptive on this.Nice pointer about the strands causing shorts, I'll have a proper look at the speakers terminal at the owners place.Once again thanks so much for the help everyone, really do appreciate it.

author: xavier60
author_rank: Super Contributor
timestamp: October 13, 2020, 09:15:05 pm
content: Quote from: Yamin on October 13, 2020, 08:30:24 pmMost designs have one VBE multiplier so forget about Q6000 for now. The VBE multiplier's job is to clamp the voltage difference between the Bases of the driver transistors at something that will cause the correct idle current and to provide temperature compensation.The C-E clamping voltage becomes roughly the B-E turn on voltage, 0.6V , multiplied by the divider ratio. The 2mV/degree c drop in B-E turn on voltage also gets multiplied which conveniently provides temperature compensation for all of the series-ed B-E junctions in the output stage.The top VAS transistor is sourcing current. The bottom VAS transistor is sinking current. Without the voltage clamping action of the VBE multiplier, the Base of Q6030 will go to a high positive voltage and the Base of Q6040 will go to a high negative voltage. This is what will happen if the VR's wiper goes open.Because the VAS transistors are current source/sinking, they are inherently self protecting. You can't overload a current source by short circuiting it.I'm not to sure about the other stuff for now,

author: Yamin
author_rank: Frequent Contributor
timestamp: October 13, 2020, 09:34:59 pm
content: Thanks so much for the detailed explanation.

author: xavier60
author_rank: Super Contributor
timestamp: October 13, 2020, 09:41:08 pm
content: The current at witch a BJT fails is affected by temperature and C-E voltage. Look at the SOA curve, the maximum allow current drops steeply above a certain voltage.So it's not likely that an over-current protection circuit will always be effective.Most over-current protection circuits are voltage compensated, so that less current is needed to cause a trip as C-E voltage increases.This allows discrimination between high currents caused by normal speaker load and that caused by a short circuit on the output.The over-current protection in this amplifier does not appear to have voltage compensation.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 14, 2020, 08:11:46 pm
content: Quote from: xavier60 on October 13, 2020, 12:52:37 pmHi @xavier60, I was hoping to run something by you regarding the trimpot failure - this is quoted from Bob Cordell's book Designing Audio Power Amplifiers. This is under the Vbe circuit explanation -"When the Vbe multiplier is used as a bias spreader, R2 will be made adjustable with a trim pot. As R2 is made smaller the amount of bias voltage is increased. Notice that if for some reasonR2 fails open, the voltage across the Vbe multiplier falls to about one Vbe,failing in the safe direction."Would that also be applicable in this unit I am working on?

author: xavier60
author_rank: Super Contributor
timestamp: October 14, 2020, 08:32:18 pm
content: R6040 is configured as a potentiometer rather than a Variable Resistor. If the wiper loses contact with the resistive track, Q6010 will turn off.This problem mainly happens when a Trimpot is first moved after sitting in the same spot for all its life.As a precaution, I oil them and work them with the power off.The original position can often be felt because the moving contact usually makes a small dimple in track

author: Yamin
author_rank: Frequent Contributor
timestamp: October 14, 2020, 08:51:06 pm
content: Quote from: xavier60 on October 14, 2020, 08:32:18 pmThat's an interesting note, thanks for the tip. So would a momentary loss of contact cause the output transistor to short? I am sort of scared to move it now

author: xavier60
author_rank: Super Contributor
timestamp: October 14, 2020, 09:08:46 pm
content: Quote from: Yamin on October 14, 2020, 08:51:06 pmIt's possible but not likely since it has been worked now. If I suspect bias problems, I temporarily replace the Emitter resistors with something higher, 10 to 100 ohms.Edit: If you haven't moved it at all yet, then don't while powered up.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 14, 2020, 09:24:14 pm
content: Quote from: xavier60 on October 14, 2020, 09:08:46 pmNice tip about the emitter resistor, when you temporarily use a higher resistor and if the amp functions then you can be convinced that the issue is in the biasing right? Or is there any other thing I am missing? Could I know more about this procedure. Thanks so much

author: xavier60
author_rank: Super Contributor
timestamp: October 14, 2020, 09:39:15 pm
content: The temporary Emitter resistors allow amplifier function checks to be made more safely by limiting the fault current if something does goes wrong. It can be run into speakers only at very low level, it should sound normal if nothing is wrong.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 15, 2020, 12:31:48 am
content: Quote from: xavier60 on October 14, 2020, 09:39:15 pmThanks so much and thanks everyone really do appreciate it.I'd like to add one more point for completion sake, when I was first testing out the amplifier on my bench after installing the borrowed transistors and resistor from the unused channel I heard a relay click while I was playing music, nothing ill happened the amp was working fine. I turned off the amp and back on again was playing music and after couple of seconds I heard the relay click. I didn't figure out what it was. Its not the speaker relay which clicks when the amp is first turned on. What could this be?

author: xavier60
author_rank: Super Contributor
timestamp: October 15, 2020, 12:56:22 am
content: Some amplifiers have some sort of switching tied in with the speaker A/B switch to change the rail voltage to the power amp stage.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 15, 2020, 10:19:02 pm
content: Quote from: xavier60 on October 15, 2020, 12:56:22 amI also have a suspicion that it was the 'voltage shifting' relays. RL6091 and RL6902. Whats the reason for this B+ voltage change? and when is it suppose to happen maybe at certain temp or volume? When I heard the relay click I wasn't changing the volume. The SM shows that the control signal for this is 'SEC1H' I can't figure what this is. Also one more important thing I just realized is that the two fuses which were blown were F6901D and F6902D those are on the low voltage path.

author: xavier60
author_rank: Super Contributor
timestamp: October 16, 2020, 08:21:16 am
content: Quote from: Yamin on October 15, 2020, 10:19:02 pmWhen going from 8 to a 4 ohm load, the output power will potentially almost double, putting too much stress on the amplifier as a whole.The idea is to reduce the rail voltages to what will cause the amplifier to develop approximately the same safe power level.'SEC1H' could relate to "Secondary", as in switching of the transformer's secondary windings.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 17, 2020, 05:08:12 pm
content: One more question:)I am going through Bob Cordell's book - Designing Audio Power Amplifiers. I am trying to figure out stages of the onkyo amplifier and trying to find similarities from the examples given in the book.(I haven't gotten far into the book yet)Please see the attach photo, the pink dotted line I have drawn isthe negative feedback path right?R5030,R5230 (why have two resistors in parallel would a lot of current be flowing in this path)&R5200forms the voltage divider for this path right?I read from somewhere that issues on this path could also take out the output transistors. I have tested the components on this path but would an issue on this path cause the output transistors to blow - or an issue this path would only introduce DC offset?Thanks for the help

author: Renate
author_rank: Super Contributor
timestamp: October 17, 2020, 11:40:41 pm
content: No, the gain is set by R5030, R5230 and R5040.R5030 || R 5230 = 60 kR5040 = 2.2 kGain = 60 k / 2.2 k + 1 = 28.2720 * log10(28.27) = 29 dB (just like the upper left hand corner of the schematic says)C5040 makes it that the DC gain is only 1 (we don't need or want DC gain)Have you tested this without speakers?If you want to run this without speakers you can replace the final output transistors (if they are pulled already or blown) with just a diode for the base-emitter part of the transistors.You can't drive a speaker like this, but it will work fine if you want to check voltages.Without the diodes the circuit won't balance.Have you checked the bias? "+BL" to "-BL" is critical.You know that there is a typo in the schematic? The collector of Q5040 ("-BL") should benegative1.1 V

author: Yamin
author_rank: Frequent Contributor
timestamp: October 18, 2020, 10:56:45 am
content: Quote from: Renate on October 17, 2020, 11:40:41 pmThanks so much, I am going to try that and thanks for pointing out the typo. I still haven't got the spares in yet. Last time I checked (the first time I 'repaired' and assembled the unit) I checked the +BL voltage and it measured around 54V. Same for the -BL.

author: Renate
author_rank: Super Contributor
timestamp: October 18, 2020, 12:32:30 pm
content: Well, you can't just throw more transistors at it, especially since I believe that you are waiting for them.As I said, replace the output transistors with two diodes, one in each position.Just tack solder them from base to emitter of where the output transistors aren't.Put them so that the diode symbol matches the little arrow on the transistor symbol,Q6050: base = anode, emitter = cathodeQ6040: base = cathode, emitter = anodeThis is like replacing the output transistors with transistors with a Beta of 1.It makes the drivers do all the work.Which is why you should not connect a speaker directly.But it will close the loop and the voltages should be sane.Because there will be less drop over the output emitter resistors the idle current through the drivers will be a bit higher, but not bad.You should also be able to see that the bias adjustment works by looking at the voltage across R6080 & R6090 when adjusting the bias.(Does the manual tell you how to adjust the bias? Probably monitoring P6080? Donotfollow their procedure without the real output transistors!)Note: Many measurements need precision and measuring to ground and subtracting is not accurate. You have to measure across components.If the output voltage balances to zero now you could hook the output to a 500 ohm resistor and connect that to a speaker.You should be able to play music softly using just the drivers.There's no chance that big things will burn because the output transistors are gone.In worst case you will burn up R6080 & R6090If everything appears (and sounds) normal, put your voltmeter between +BL and -BL and put on a long playing record.It could be that your problem is thermal though and it may not show unless the heatsink is warmed up, (maybe gently with a hot air gun?)

author: Yamin
author_rank: Frequent Contributor
timestamp: October 18, 2020, 10:37:43 pm
content: Quote from: Renate on October 18, 2020, 12:32:30 pmYep I am going to try the procedure while I am waiting for the parts to arrive. Have to figure out a way to connect everything up and orient the main board in a way so that I would be able to take the voltage measurements. Such a nightmare.Thanks

author: Renate
author_rank: Super Contributor
timestamp: October 18, 2020, 10:50:59 pm
content: Quote from: Renate on October 18, 2020, 12:32:30 pmMmm, actually it means a Beta of zero.Beta = Ic / IbIn this case Ic = 0, therefore Beta = 0I was thinking Ie / Ib which is not used for anything.

author: xavier60
author_rank: Super Contributor
timestamp: October 19, 2020, 12:20:55 am
content: A problem with the diode idea is that the drivers will still be damaged if there is a bias problem. 100 ohm resistors would be safer.I usually just put a jumper across the B-E pads of the NPN output BJT. Both output BJTs removed.

author: Renate
author_rank: Super Contributor
timestamp: October 19, 2020, 12:12:45 pm
content: Yeah, the diode idea is riskier, but I still think that it's a better approximation to how the circuit is supposed to work.I'd rather throw a 0.5 A amp fuse inline with one (or both) diodes.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 19, 2020, 07:48:10 pm
content: Some further developments/discoveries, after some more digging around. I have soldered the diodes as suggest (1N4148)I noticed that C6040 and C5050 (caps across the base’s of OT & drivers) reads slightly higher ESR value compared to a good channel. It was around 1.9Ohms. My meter indicates that cap is good <47 uF cap. Its sort of in the good zone so to speak. It says ‘audio’ on the capacitor’s label - I am pretty sure that I would not be able to source one of those from the local shop. First of all could this be an issue enough to blow the OT if so could I replace it with a non-‘audio’ capacitor?I took out the driver transistors out of circuit and measured the resistances and compared it to new transistors and they were identical. So I am planning to keep the old one, mainly because I am not sure whether the new transistors I got from the local shop is a genuine part. What are your thoughts on keeping the old drive transistors as they are measuring ok.I took out Q6010 out of circuit to measure it - there was thermal compound on it but it had more of a silicone like consistency. The compound had dripped onto the leads of the transistor. There was no dripping on to the leads of the same transistors on other channels. The compound when measured was not conductive like how some glues becomes conductive after time. Since 2SC1740S are obsolete the replacement suggest by people were 2SC2240. I measured resistance measurement with the one I took out and compared it to a new 2SC2240 and they were identical as well except for one measurement. Measured across the C and E using an analog meter at x10k (the highest my meter goes to) red probe on C and black probe on E - the old one appears to be more ‘conductive’ than the new one. When the probes are reversed there is no ‘conduction’ on either one. I did clean the compound off the leads before taking the readings.I haven’t powered the amp yet to measure the voltages as I have to partly assemble the unit to do so. Will continue tomorrow.Thanks again for the time.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 22, 2020, 10:24:00 am
content: Right so I powered up with the amp, with the didoes in place. Left it for maybe half an hour so nothing blewSo I replaced them with transistors borrowed from yet another unused channel and I wanted to monitor anything happening on the bias. So I connected some jumper wires from the test point and had series 100 ohm resistors on each wire. Powered up the amp - and it seems to follow what the SM described. However at one point my heart sank when I saw 254mv! I quickly turned off the amp. I touched the 'test' resistors and they seem to be slightly hot as well :S. But after thinking about it for a while and checking whether anything had shorted I realised that it was due to the fact that my test probes weren't making proper contact with the test point. PHEW!So I powered back the amp again and monitored the biasing and it followed almost exactly what the SM describes. 2.5mV on startup and after couple of minutes goes to around 9 to 11mV and stabilizes. So is it safe to say that the biasing is ok?Another thing I want to remark is I measured something around 55mV of DC offset on the right channel and something around 30mV on the left channel. There are no adjust points for the DC offset. My questions are:1.Does DC offset got anything to do with the biasing or biasing circuitry?2.Are the above mentioned values acceptable for the DC offset?

author: xavier60
author_rank: Super Contributor
timestamp: October 22, 2020, 10:33:46 am
content: I would call 50mV borderline acceptable.The input pair transistors should be passing about the same current to minimise offset.Any residual offset should only be due to transistor mismatches.Edit: DC offset and idle current biasing usually don't interact.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 22, 2020, 10:40:15 am
content: I wouldn't be happy with 50mV DC offset, I would consider that to be an unsolved problem.Well, in light of the fact that the amp has blown up twice, that is.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 22, 2020, 10:44:39 am
content: Quote from: xavier60 on October 19, 2020, 12:20:55 amI also wanted to ask - you recommended using 100 ohm resistor instead of of the didoes (in the same place)?Could you please explain about the jumper across on Base to emitter scenario as well, its on both the output transistors right? In that case I assume there is not 100 ohm resistor right?Follow up on my last post - could the DC offset blow up the output transistors, maybe something in the crossover circuit of the speakers shunting to ground and putting a short across the terminals?Thanks

author: Yamin
author_rank: Frequent Contributor
timestamp: October 22, 2020, 10:47:33 am
content: Quote from: Audiorepair on October 22, 2020, 10:40:15 amCould that be taking out my output transistors?I measured the 50mV on the right channel which wasn't shorting out initially.What would be an acceptable range? I know ideally it should be 0mV.Thanks

author: xavier60
author_rank: Super Contributor
timestamp: October 22, 2020, 10:51:39 am
content: Quote from: Yamin on October 22, 2020, 10:44:39 amYes, the 100 ohm resistors would go in the same place as the diodes.It doesn't matter witch B-E pads are jumpered but the NPN like I said is best for the readings making more sense.Using voltage measurements, calculate the current through Q5000 and Q5010.

author: xavier60
author_rank: Super Contributor
timestamp: October 22, 2020, 11:17:41 am
content: The input differential pair,Q5000 and Q5010, current should be close to 1.5mA each.8mA each for the VA transistors.

author: Renate
author_rank: Super Contributor
timestamp: October 22, 2020, 11:49:15 am
content: We've kind of been going on the presumption that it's a bias failure.Since only the top output transistor blew (is blowing), might it be a DC balance failure going positive and dumping current into the speaker?When it blew did it just go silent or was there a "whump" or bad hum for a second?How about checking leakage in C5040?When you remove it, you can check your output DC offset without it.(With that cap removed you have reduced your amplifier for both DC and audio to a gain of 1.)Still, even if C5040 was shorted it would only amplify the DC offset of the input pair by 28 times.Maybe Q5040 current sink intermittent failing would nail this thing positive and kill Q6050 (top output).Check across D5000 for 5.6 VSomething shorting there would make this to go all positive.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 22, 2020, 08:32:20 pm
content: Quote from: xavier60 on October 22, 2020, 10:51:39 amSorry @xavier60 I am a little bit confused. It was suggested to use two diodes for the two transistor’s B-E (Q6050 & Q6060). So I should use two 100 ohms instead as you suggest right?With the B-E jumper option- you suggest connecting only one transistor’s B-E via jumper right (the NPN)? So the PNP transistor’s B-E pad should be left open or should I connect a 100 ohm resistor or diode there?Would I be able to measure across the test points to see whether the bias is holding up with the above mentioned procedure?Sorry again for the noob question, really do appreciate the time.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 22, 2020, 08:33:17 pm
content: Quote from: Renate on October 22, 2020, 11:49:15 amYes I’ve been trying to cross-out the possible reasons for this failure. I had a feeling that it might be something to do with the biasing as I have checked all the other factors to my best knowledge initially.The first time the NPN was shorted, but the second time the two transistors had shorted out.The borrowed output transistors are holding up so far (like last time). This time I want to run it on my bench long enough till I am confident that there are no issues with the amp. I will obviously return the amp with the new components soldered in and after running it again.I wasn’t there when the amp failed but the owner did mention that there was a ‘pop’ sound when it went out.I have checked C5040 with an ESR meter and it seems to be in order. I could remove it to make sure. If there was an issue with the capacitor being leaky what should I expect to see when I power up without it like you suggested?I did check across D5000 and it seems to show proper forward voltage drop. I’ll check it after powering up for the 5.6V drop across it.Any suggestion on how to suss out the possible intermittent failing Q5040?Also please note that I measured the 50mV at the right channel output the left channel output measured around 30mV - the left channel was what had failed before. What would be the acceptable DC on the outputs?Thanks for the help.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 22, 2020, 09:05:20 pm
content: At this point I would be thinking this:It's possible one transistor fails over time.There are 9 transistors in the pre-amp stage before the main power transistors.Replace all 9, probably half an hours work and a couple of dollars in parts.Way, way less than you have spent on this already.If, for some reason, this doesn't fix the problem, you will KNOW that it is not due to any of the transistors, and you can use this fact to test the rest of the circuitry that might be bad.Currently you know nothing other than it is not the output transistors, you need to establish some facts.

author: xavier60
author_rank: Super Contributor
timestamp: October 22, 2020, 09:44:49 pm
content: Quote from: Yamin on October 22, 2020, 08:32:20 pmUsing two 100 ohms instead of diodes will give better protection to the driver transistors if the bias circuit develops a fault while testing.Don't bother with the B-E jumper idea, it makes things too confusing.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 23, 2020, 06:07:45 pm
content: Quote from: Audiorepair on October 22, 2020, 09:05:20 pmThanks @Audiorepair this was my plan at one point too, I tried sourcing the parts but it turns out the exact parts are now obsolete and the only ones available are from not so reputable shops so I was afraid of putting in counterfeits. I tried looking for equivalent parts but it always turns out it's not an exact match if one parameter is superior or matched the other parameter would be inferior to the original. But that's an excellent suggestion

author: Yamin
author_rank: Frequent Contributor
timestamp: October 23, 2020, 06:08:24 pm
content: Quote from: xavier60 on October 22, 2020, 09:44:49 pmAh thanks so much for clearing it up

author: Yamin
author_rank: Frequent Contributor
timestamp: October 23, 2020, 06:11:57 pm
content: Guys so the mystery of this amplifier's plots thickens! just realised something very interesting -When I checked the amplifier last time I used all the analog inputs to input my signal and even this time I have been using the analog inputs to test the amp.The owner however uses the HDMI ports. He has got his DVD player, TV and some other devices connected to the HDMI ports and the amp has got some kind of automatic function of detecting which device is currently being used - so I guess its switching from one to another. The owner asked me even at that time could that have caused the amp to blow up and I thought that its highly unlikely, since the fault is on the output side of things.Reading online about Onkyo amps many have had issues with the HDMI board, but none have reported of the outputs blowing. Do you guys think that its a possibility? the input is saturating the outputs during switching? I was under the impression that these amps would be designed in such a way that there would be some sort of 'limiting' if the input is driven hard.Thanks so much for the help guys.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 23, 2020, 07:12:11 pm
content: It looks like there are 3 protection lines on the speaker output.IPRO, providing protection against excessive output current.FLVPRO providing protection against escessive output voltage, I suspect.FLVOLH providing some kind of I don't know what signal.(looks like high DC, but why no low DC?)The amp is well protected.Replace the transistors. One of them probably fails randomly.And if in doing so your DC offset falls considerably below 50mV, then that might be a clue that one of them was actually duff.You could have done that instead of thinking about and composing this post.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 23, 2020, 08:36:25 pm
content: One other thing,the fact that the working side also has a high DC offset, may not actually be an indicator of normality, but might just mean this channel is also on the verge of self destruct, due to the same component failure.

author: dietert1
author_rank: Super Contributor
timestamp: October 23, 2020, 09:30:41 pm
content: Have seen quite some amplifiers and when i look at the schematic, i am wondering how this amplifier is meant to be RF stable. This must be a very old design and it appears strange. Amplifiers can be killed by RF oscillation and dynamic instability will cause horrible sound quality.There should be a 1 nF cap over R5120. There should be a miller cap on Q5030 (maybe 100 pF+3K3) and there should be a capacitive voltage divider in the feedback path (e.g. 2.7 nF parallel to R5040 and 100 pF parallel to R5030). C5080 can go. Next thing to check is the presence of a snubber parallel to the output. It may appear outside of the schematic shown. Many amplifiers also include an inductor in the output path.Recommended reading:apexanalog.com/resources/appnotes/an19u.pdfRegards, Dieter

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 23, 2020, 10:09:05 pm
content: What about the grounding?The schematic as posted shows the power amp ground and the pre-amp ground separate and dissapearing stage left, and no speaker ground indicated.If there is a problem here where they don't all three connect properly, then, er, er, I dunno what might happen to the power amp.Maybe DC offset or something.Its ground might be floating.

author: Renate
author_rank: Super Contributor
timestamp: October 24, 2020, 03:17:17 pm
content: Quote from: dietert1 on October 23, 2020, 09:30:41 pmYeah, that would be more typical.Still, there is C5080 to cut the high frequency.

author: dietert1
author_rank: Super Contributor
timestamp: October 24, 2020, 08:23:22 pm
content: Yes, that capacitor may have the intention to limit high frequency gain, but in my understanding that "solution" is invalid. It's exactly what you can't do: Feedback over the full loop gain without bandwidth limit. A perfect way to make it oscillate. I mean there is always a frequency where negative feedback becomes positive feedback due to delays.Just imagine some clever ONKYO engineer determine a capacitance value experimentally such that the input capacitance of Q5010 makes it a capacitive divider and it remains roughly stable, but that is a bad approach. Though it appeared in many amplifiers, typically with a 10 pf to 20 pF capacitor.Regards, Dieter

author: Yamin
author_rank: Frequent Contributor
timestamp: October 24, 2020, 08:50:48 pm
content: Quote from: dietert1 on October 23, 2020, 09:30:41 pmThanks so much @dietert1, very informative.There is a snubber like circuitry on the outputs.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 24, 2020, 08:54:25 pm
content: Quote from: Audiorepair on October 23, 2020, 10:09:05 pmThanks @Audiorepair - I was hoping that I could run something by you. When checking for DC offset should I have connected a dummy load or not. Some people are against it and some people are for it. In my case when I took the measurement I didn't have a load connected.Is it wise to connect a resistive dummy load or should I have connected a speaker.Thanks

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 24, 2020, 09:11:07 pm
content: If you connect a dummy load and still have DC offset, then you still have a problem, IMHO.Snubbers, capacitors, RF, are not part of this equation.Have you checked that Speaker negative, input signal ground, Power Amp ground and almost certainly chassis, are all connected together by zero ohms?

author: dietert1
author_rank: Super Contributor
timestamp: October 24, 2020, 09:28:17 pm
content: Output offset may result from RF oscillation, too.The schematic shown of the output terminal board has something very unusual once more: 10 nF caps across the output terminals. The usual snubber has a 100nF capacitor in series with a low ohms resistor, like 5 or 10 Ohms. There is a special name for that when used with amplifiers, just don't remember.Also i would check whether the amplifier has the inductors or the jumpers, the schematic shows both options.Regards, Dieter

author: Yamin
author_rank: Frequent Contributor
timestamp: October 24, 2020, 09:57:00 pm
content: Right guys, so I may have discovered what potentially has caused the amp to fail -I had the amp turned on was monitoring the bias test points, the voltage measured was exactly what the service manual states 2.5mV at start-up and goes to around 9mV. After about 5 minutes in I heard the relay click I have mentioned about this in a previous post. The moment the relay clicked the voltage at the bias test point varied a bit (it didn’t go over 9mV) and it started going down! It went down till 2.5mV and remained there.I have now confirmed which relay it is too! Its the relay which switches the B+ voltage. Here is what I measured across the bridge rectifier for the B+ voltage -when the amp is turned on the relay clicks and it measures around 110V after about 5 minutes the relay clicks and the measurement now is 54V. I turned off the amp and immediately turned it back on again. The same thing happens it starts with 110V and exactly around 5 minutes relay clicks and the voltage is 54V. The relays are controlled by a signal from the controller ’SECH1’ not really sure in what case this is suppose to turn on and off.There are people who have reported about hearing random relay clicks from Onkyo receivers in AV forums. No one has identified the issue but someone has suggested that messing around with the settings (software) - specifically changing the impedance to 4 ohms gets rid of the clicking even though the speaker that he was using was stated at 8ohms.Could I be on the right track do you think that this might have caused the amp to fail at the owners place? He has got Left and Right tower speakers as well as the centre and the rear speakers too. While I have only one 8ohm speaker connected on my bench which I’m using to test the amp.Thanks for the help.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 24, 2020, 10:03:10 pm
content: Quote from: dietert1 on October 24, 2020, 09:28:17 pmIt has got the two. I'll try to take a picture tomorrow

author: xavier60
author_rank: Super Contributor
timestamp: October 24, 2020, 11:13:00 pm
content: I'd say that the control system automatically switchs to low impedance(low voltage) after sensing low output demand over the 5 minutes.This causes the output stages to become more power efficient and lower operating temperature.Any idea what the owner's listening habits are? Loud?And his speaker impedance?

author: Yamin
author_rank: Frequent Contributor
timestamp: October 24, 2020, 11:40:06 pm
content: Quote from: xavier60 on October 24, 2020, 11:13:00 pmI'll have to check his setup's specs. He listens at a moderate level normally. One thing I would remark is that the volume needs to be turned around 50 to get any usable level. I know its an arbitrary number but seems the lower part of the volume range is rather a waste. He also sets his volume around 53.Couple of questions arises now as to how the amp is detecting lower power draw? Maybe an issue on the detection circuit causing this problem, but I can't imagine how switching to low voltage could blow up the output transistors.Both of the times when the output blew the fuses marked on the 'low voltage' was blown - and here's another link that I just thought about. When I was testing the amp I heard the relay clicking on and off and it had no detrimental effect at my place. When I took the amp to the owner's place I didn't run the amp for 5 mins or enough for the relay to click. Its only after when I got back, he would have used it for more than 5 mins. Getting very interesting.But still can't think why switching to low voltage could take out the transistorThanks so much

author: xavier60
author_rank: Super Contributor
timestamp: October 24, 2020, 11:53:58 pm
content: The switching to low voltage is very likely unrelated to the failures.Output level would be sensed via the FLVOLH line or something.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 25, 2020, 12:12:36 am
content: Quote from: xavier60 on October 24, 2020, 11:53:58 pmThanks @xavier60 I would imagine so too, but during the relay click the voltage at the bias test point does a funny 'wobble' and then goes down. Would under biasing cause a high current draw enough to take out the OT and the fuse?

author: xavier60
author_rank: Super Contributor
timestamp: October 25, 2020, 12:22:29 am
content: Quote from: Yamin on October 25, 2020, 12:12:36 amI can't fully explain that except that it would have something to do with temperature being sensed in 2 places, changing at different rates as the rail voltage changes.The only things I would suspect are the Pot which has been already covered, and solder joints. I would identify all joints in the bias circuit that are capable of causing loss of control, examine them, then re-solder using a safe flux.

author: Yamin
author_rank: Frequent Contributor
timestamp: November 05, 2020, 08:06:25 pm
content: Finally I have figured out the why the relay randomly clicks - I was wrong, my speaker actually is rated at 4 ohms. When I changed the settings for 4 ohms from the amp the relay issues was solved. When 4 ohms is selected the the B+ voltage remains at the lower voltage setting that is 52V across the rectifier.The owner uses a KEF Q900 the impedance rating is stated as 8 ohms and the amp was set at the proper setting for that, but I do have a feeling the relay would have clicked at his place too because the fuses which were blown was on the low voltage line.I was able to get my hands on a scope and I measured the outputs of the DAC since the problem seemed to affect the left channel only and the HDMI inputs were used when the amp failed. I was hoping that there was an issue on the LPF and some high frequency content was going into the amplifier section. But this was not the case a clean signal was seen on the outputs of the DAC-damn! Can't think of what else to look for :S

author: Audiorepair
author_rank: Frequent Contributor
timestamp: November 05, 2020, 09:51:08 pm
content: Quote from: Audiorepair on October 24, 2020, 09:11:07 pm

author: Yamin
author_rank: Frequent Contributor
timestamp: November 06, 2020, 08:49:21 pm
content: Quote from: Audiorepair on November 05, 2020, 09:51:08 pmYep they all seem to be connected. However I have got the back panel removed at the moment some chassis connection are not made. I'll assemble the unit and re confirm.The amp threw another curve ball today. Discovered this charring near to the +/- 15V regulators. The +/-7V for the R2A15218FP IC (used for switching between sources) is also derived from here. Couple of capacitors around that area had gone out of value and reading high ESR value. This was hidden under all the boards had to disassemble everything to discover this. Going to try and replace them all before assembling.

author: TurboTom
author_rank: Super Contributor
timestamp: November 07, 2020, 11:28:40 am
content: These are two zener regulators and the corresponding ballast resistors. They may get (very...) warm. If the voltage is on spec, the charring should be tolerable. Actually, you don't need that high temperature to cause the observed kind of discoloration over a few years' time. One thing that would worry me much more are the electrolytics in close proximity of the "hot" resistors. This kind of arrangement can almost be considered to be "planned obsolescense" since the caps will dry out over a few years and become high-ESR. This may or may not cause additional trouble, depending on their usage. I'ld probably simply go and replace them all, they are small enough to be no-brainers price-wise. Preferably, you should use 105°C rated types as replacements. Just noticed -- in the second photo, you already removed some. Right so!Did you check the mains voltage present at you friend's home (also for surges)? Often, this consumer electronics isn't designed for a high margin of mains overvoltage. This may also contribute to too much dissipation in these shunt regulators and possibly contribute to the failure of the output stage. The reported 110V total supply voltage to the power amps appears a little high as well, though only marginally.

author: dietert1
author_rank: Super Contributor
timestamp: November 07, 2020, 03:32:21 pm
content: It's a common design problem, selecting resistors with a nominal max power of say 2 W. Running at 1 W continuous the resistor body will reach several hundred °C. The resistor does not unsolder itself only because its wires are made of steel with low thermal conductivity. But you never know..One nice mod i have done often is using a chassis mounted resistor as a replacement that dumps the heat into a large piece of metal.Another common problem in many so-called high end amplifiers is the lack of low voltage transformer secondaries. The typical construction derived the +/- 15 V for the preamplifier from the +/- 60 or +/- 70 V supplies of the power amplifier, which caused those heat related problems, even when running the amplifier at low volume. A shame, since this was about saving some bucks during production of equipment to besold for thousands.Regards, Dieter

author: Yamin
author_rank: Frequent Contributor
timestamp: December 13, 2020, 06:29:54 pm
content: Quote from: dietert1 on November 07, 2020, 03:32:21 pmThat sounds like a nice Mod, would be great to see a photo if you can share. Is these resistors used as a fusable resistor?Thanks so much everyone for the help. Learned alot! Just an update - I changed the caps which have gone out of value. The random relay clicking has gone. Come to think of it the random relay click could have been an issue of not installing the back cover as it serves as a ground connection. I have been using the amp and so far so good

author: dietert1
author_rank: Super Contributor
timestamp: December 13, 2020, 07:05:06 pm
content: Those chassis-mountable resistors are aluminum housed and named RH010(ohms-value), cost about US$ 3 or 4 each. One example:https://de.rs-online.com/web/p/widerstande-durchsteckmontage/8459655/?sra=pmpnThe 10 W version is pretty small and it is easy to find a good mounting position, maybe on the main heatsink of the amp.Regards, Dieter

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 13, 2020, 08:13:35 pm
content: Quote from: dietert1 on December 13, 2020, 07:05:06 pmI've always had a bit of an issue with this type of resistor.They claim 10 Watts, but then if you look at the datasheet you will find this is only if they are atually chassis mounted.In free air, they are actually rated 6 Watts at 25 C, and should they get hot, only 4.8 Watts at 70 C.I've always thought claiming them a 10 Watt resistor is a little misleading at best.https://docs.rs-online.com/b9cf/0900766b81410739.pdf

author: dietert1
author_rank: Super Contributor
timestamp: December 13, 2020, 08:25:50 pm
content: What? He was asking for the chassis-mountable resistors i proposed. Those resistors are not meant to be used in free air.Free air resistors are made from completely different materials, like ceramics, cement and the like that withstand 800 °C. But nobody wants such temperatures inside an amplifier, possibly close to electrolytic capacitors and semiconductors.Regards, DieterPS: They are wire wound with welded copper terminals and low TC, typical 5 ppm/K, sometimes with a flat min/max at temperatures of 20 to 40 °C (selected ones). Also interesting for precision applications where you want to keep a resistor at a well defined temperature.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 13, 2020, 08:44:25 pm
content: Calm down, Dieter, I wasn't attacking you.I was pointing out that on first appearance these appear to be 10 Watt resistors.One might think one could buy one, stick it in free air, and it will withstand 10 Watts.It won't. That is my point.

author: Yamin
author_rank: Frequent Contributor
timestamp: December 13, 2020, 08:50:22 pm
content: Quote from: dietert1 on December 13, 2020, 07:05:06 pmThanks @Dieter for the link and info, so in the circuit what purpose does the resistors serve? They are not suppose to act like fuses right?

author: dietert1
author_rank: Super Contributor
timestamp: December 13, 2020, 09:05:38 pm
content: On your image Char1.jpg we can see that the circuit board gets very hot and is near black around the four resistors R4175.. R4177... Those are resistors that burn power when deriving the OpAmp supplies (+/-15 V) from the main power rails (probably around +/- 50 V or even higher). If you replace those resistors by chassis-mountable resistors, you take the heat away without creating high temperature spots. Nearby semiconductors and electrolytic capacitors will work better/last longer.The bad design shown in your photo is present in many so called high-end amplifiers. The best way to avoid it is a mains transformer with lower voltage secondary windings (helpers). Chassis-mountable resistors are a way to fix the problem later on.Regards, Dieter

author: Renate
author_rank: Super Contributor
timestamp: December 14, 2020, 01:12:55 pm
content: Quote from: dietert1 on November 07, 2020, 03:32:21 pmYeah, the low voltage sections often seem an oversight.OTOH, preamp sections take less current these days with low power op amps.There is one silver lining to using high V for low V.It gives you more time to see that the voltage is collapsing when the power cord is pulled and mute before the op amps start making weird noises.

author: dietert1
author_rank: Super Contributor
timestamp: December 14, 2020, 03:55:52 pm
content: Yes, a lot of last century equipment did not include power status monitoring. They used timers and the like, so decent behaviour on power on or power off or brown out is not guaranteed.Regards, Dieter