Hifi-amplifier powers on to white noise - Page 1
Forum section: electronics / beginners

author: sdancer
author_rank: Contributor
timestamp: October 08, 2023, 03:47:34 pm
content: Hi,I'm trying to troubleshoot an issue with an Hi-fi amplifier (Marantz SR3001 from 2006 or so). It will sometimes after being off for the night power on to white noise that's independent of the input after the slow start relay clicks. That noise persists for maybe 10-15 seconds, then it get audibly low-pass filtered, and eventually it will start operating normally with a loud "pop". During that noisy phase, there is little no no response to input. So far, it has never occurred during operation, only when powering up the amplifier.I do think that it is a power supply issue wheresomesupply is slow to get up to speed, "leaky capacitor" is way on top of the list. The block diagram with what I measured marked is attached. Luckily I found the entire service manual with schematics, and it's a very usual analogue construction with two-layer PCBs and a layout that makes sense.Since the noise comes up at proper and stable volume and the front panel works fine, I assume that the +/-48V on the power stage and at least the +5V supply/supplies are fine. Measuring a bunch of voltage rails all over the place didn't show anything out of the order and everything looks stable well before the soft start turns on the power stage, but since the problem is intermittent, I'm not sure if I was ever measuring into it. The most likely culprit in my opinion so far is a +/-8V supply derived from the generic +/-12V analogue supply that's used to power an NJW1157 (a nifty multi-channel volume control/audio signal switch). I guess if that was slow to start, it could feed the preamplifiers with adverse signals. Sadly there are no obviously popped caps because that would be too easy.I have now disconnected a video board that derives a +/-5.1V supply from the +/-8V through a resistive divider (because what the hell, variable linear regulators have been a thing for a while now), but apart from that I am out of ideas. So if anyone has good ideas where that behaviour would be likely to come from, I'm all ears.

author: David_AVD
author_rank: Super Contributor
timestamp: October 08, 2023, 08:52:24 pm
content: Try switching to the 7.1 inputs as soon as you hear the noise. This should bypass the DSP which is what could be making the noise.

author: sdancer
author_rank: Contributor
timestamp: October 08, 2023, 09:04:26 pm
content: I'll try that once I put the video PCB back in, because that has the remote control signal looped through it

author: sdancer
author_rank: Contributor
timestamp: October 10, 2023, 07:21:52 am
content: After disassembling and reassembling, it worked fine for a day, and today it came back up to noise. It took so long that eventually the receiver showed "PROTECT" on the display and only became operational after a power cycle. The noise it there both on the "direct" 7.1 input and a stereo input, but it sounds different (I think 7.1 had more high frequency content). Both signals go through the NJW1157.Since the unit recognised an issue, hooray, it's something monitored. The search goes on. I'll run it without most of the video and tuner parts I guess, because I find that high-power reverse zener supply on there scary, it runs off two big hot 2W resistors.

author: David_AVD
author_rank: Super Contributor
timestamp: October 10, 2023, 07:26:19 am
content: Hmmm... that seems to rule out the DSP which is a common cause of squeaks, pops, noise, etc in many receivers like yours.Something else to check for is any KIA branded TO-220 regulators (mainly the -15V ones, but can be others) as they go bad more often than they should.

author: magic
author_rank: Super Contributor
timestamp: October 10, 2023, 07:36:22 am
content: Protection?Can you check for output DC voltage from noisy channels when it happens?Can you turn it on on the bench and measure supply rails while it's noisy (but maybe before it shuts down, if it does)?editBTW, the "loud pop" sounds like something may be coming into operation after excessive delay. It probably could be seen with a scope so my first thought would be to find the source of it. In particular, is it generated by the power amp modules (I presume it happens on all channels, right?) which could point to perhaps some power amp supply rail coming up late, or is it present already in the input signal which places blame on the small signal circuitry upstream of the amplifiers.If there are volume controls, you can easily determine if the pop (or noise) is generated before or after the volume control.

author: sdancer
author_rank: Contributor
timestamp: October 10, 2023, 08:09:31 am
content: Quote from: David_AVD on October 10, 2023, 07:26:19 amDo they maybe have a failure mode where they need some time to warm up until they become operative? As I mentioned, the issue so far only occurs after the amp has been off for a few hours (usually over night). All rails I checked were bled within a few seconds, so a thermal issue seems likely here.Quote from: magic on October 10, 2023, 07:36:22 amYeah, the display just said "PROTECT", so something detected an issue and shut down at least the power section. Not that the maintenance- or user manual mention that indication anywhere. I'd love to measure things while the issue is present, and it should be possible (minus the video board because that's in the way), but when it happens I have maybe 20 seconds before it goes away.The issue happens across channels, so if it is the power stage, it would be something in the general supply of that. If I'm lucky, the whole thing boils down to the central +/-12V rails which are produced on a very convenient daughterboard.Maybe it's time to invest in some ice spray to cool things down? (Or just douse some regulators in alcohol...)

author: magic
author_rank: Super Contributor
timestamp: October 10, 2023, 08:43:19 am
content: Well, you could permanently connect a DMM to one of the output channels (or some supply rail or whatever) and try to turn on the amp every few hours. After a few days you may have a rough idea what's going on in there...BTW, do you have any schematic of the power amp analog stuff? A complete list of everything shared between channels would be nice to have.Another troubleshooting angle is to temporarily give up one channel and wire its input to ground. Wait until the problem happens again and see if there is noise in the disabled channel. This will tell with certainty whether the culprit is the power amps or the input circuitry.editI see that subwoofer output is line level, there is no power amp for that inside this box. Obvious question: is it noisy too, or not?

author: Zenith
author_rank: Frequent Contributor
timestamp: October 10, 2023, 08:44:18 am
content: I've seen this sort of intermittent start up problem caused by soldered joints which have gone bad. They don't conduct properly when they are cold but do when they warm up. This can also be caused by broken PCB tracks.Look over the board with a magnifying glass and remake any joints that look suspicious. Sometimes you can see that a joint has cracks in.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: October 10, 2023, 08:47:24 am
content: Educated suspicion, could the protect message be generated by the state of the P_LINE_FAIL or P_AMP_FAIL lines?Does the noise burst/pop occur on all of the speaker outputs, including the surround jacks?@zenith Agreed. Maybe reflow some of the 'hot part' terminations? Transformers, power rectifiers, etc

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2023, 09:25:37 am
content: Quote from: Zenith on October 10, 2023, 08:44:18 amThe problem sounds like a "high open", which is a connection which is open until sufficient current passes through it, and can often be a cracked solder joint.Pushing on the board in various spots may reveal the approximate location of the high open.

author: David_AVD
author_rank: Super Contributor
timestamp: October 10, 2023, 10:31:29 am
content: Quote from: sdancer on October 10, 2023, 08:09:31 amQuite possibly, but mostly they fail after warm up. DSP faults can go either way. ie. Start fine then fail once they get warm, or start bad and come good after they're warm.

author: sdancer
author_rank: Contributor
timestamp: October 10, 2023, 10:44:10 am
content: The service manual with full circuit diagrams and parts listsfloats around the web, that helps a lot.I'll have a go at the subwoofer output and see if I can hook up some diagnostic leads in other strategic places. The main regulator board would be a good candidate for cracked joints seeing how it is mounted, so that's getting quality lead solder the next time I'm in there :-)

author: sdancer
author_rank: Contributor
timestamp: October 10, 2023, 11:06:13 am
content: Quote from: AndyBeez on October 10, 2023, 08:47:24 amI suppose so.From the looks of it, the P_AMP_FAIL signal is a mixture of DC detection on the outputs and a thermal sensor on the main heatsink, which is less suspicious in my opinion (continued operation is fine).P_LINE_FAIL however is an imbalance detection of the +/-12V low-power "line level" supply and the +/-48V power amp supply.I'll definitely track those down and see if I can get a scope attached to P_LINE_FAIL and the associated voltages.Circuitry looks like this, just in case I'm way off:QuoteI haven't tested them all, but it's present on front L/R and surround back L so far.

author: Zenith
author_rank: Frequent Contributor
timestamp: October 10, 2023, 11:14:10 am
content: Don't forget to look for broken PCB tracks and dodgy connectors as well.These sorts of faults can have you tearing your hair out. They may not follow a clear pattern. Bad soldered joints can look perfectly good from the solder side and you won't see cracks, but they are not, or it's a plated hole and the solder hasn't flowed through properly to the other side. Cracked circuit tracks can be hard to spot.It may not be that, it may be something such as a resistor which misbehaves when cold, or the DSP as mentioned previously. Making sure the voltage supply lines are as they should be, is always a good place to start, because if they are not, investigating the rest of the system is an exercise in frustration.

author: sdancer
author_rank: Contributor
timestamp: October 10, 2023, 11:30:25 am
content: Quote from: Zenith on October 10, 2023, 11:14:10 amOh don't worry, I'm already mostly done with my hair. I was hoping for some easily recognisable cooked capacitor, and now I'm still fairly certain that it's a "simple" power supply issue; the digital parts are working too well for anything around the DSP or CPU as far as the front panel goes, and the noise and "pop" are also "too analogue" somehow?Anyway, a bigthank youto everyone so far

author: magic
author_rank: Super Contributor
timestamp: October 10, 2023, 01:03:17 pm
content: I suspect it's not overheating, but to be sure you could check heatsink temperature when/if it enters protection again.Power amps have AC coupled inputs, so if you find DC on the outputs you can be sure it's their fault, somehow.But the only thing they seem to share is power rails.Rail voltage may not necessarily be the same at the P_LINE_FAIL circuit and at the power amps+12V has to be at least somewhat functional for the protection circuits to trip.There is also positive rail overcurrent monitor, right above the P_LINE_FAIL detector on your image.

author: sdancer
author_rank: Contributor
timestamp: October 10, 2023, 01:57:54 pm
content: Quote from: magic on October 10, 2023, 01:03:17 pmThat appears to be an output overcurrent protection, the "OVER" network is connected to monitor circuits on each speaker output that can pull it low momentarily during a negative swing in an overcurrent situation. I think. Ultimately that would also end up pulling the P_AMP_FAIL line low. For now I'll focus on things that are unlikely to burn down the house

author: sdancer
author_rank: Contributor
timestamp: October 11, 2023, 07:15:44 am
content: Good news, everyone!The noise is also present on the SUB channel straight from the preamplifier board; additionally, while the volume control doesn't have an effect on the noise itself, there is an audible popping noise with each volume change (it's a discrete control), and reducing it to -∞ kills it; so it happens before the preamp mute circuit. Since the volume control and mute signals are generated by the CPU and work, I can focus on the more easily accessible parts 🥳

author: magic
author_rank: Super Contributor
timestamp: October 11, 2023, 07:38:51 am
content: That's progressAlthough, I think power amp issue would be easier to fix because it would have to be either missing positive rail or missing negative rail

author: sdancer
author_rank: Contributor
timestamp: October 14, 2023, 01:58:32 pm
content: I've had some fun.I'm pretty sure now that the reverse-biased zener supply has gone fishy. It turns on to a distinct whiff of burning electronics, the board has visible discolouration around the resistors, and I could power up the amp with the circuit detached (worked fine), then turned it off, plugged that board back in, and it turned back on to noise. It's dropping the 8V supplies over a pair of 2W resistors that get hot to the touch.I'm really not sure why they didn't put in a pair of 5V regulators; it can't be that sensitive to those 100mV, and considering Marantz isn't a budget brand and the rest of the unit seems pretty decent, it's just a weird decision to do it that way.

author: jmelson
author_rank: Super Contributor
timestamp: October 14, 2023, 05:49:28 pm
content: Two Zeners in PARALLEL? Crazy! All the power is being dissipated by the one with the lowest Zener voltage, unless they are somehow matched at the factory. And, they probably won't STAY matched. I'd replace the Zeners and the electrolytic caps in that circuit. Assuming one Zener is taking all the current, that one is dissipating probably close to 2 W, depending on the other load. Perhaps replacing this circuit with a regulator chip would be a good idea.Jon

author: sdancer
author_rank: Contributor
timestamp: October 14, 2023, 08:13:35 pm
content: Quote from: jmelson on October 14, 2023, 05:49:28 pmThing is, that supply only handles the video switching, and I don't use that. In fact, the whole receiver only takes one stereo input from a computer these days. So for now I just taped the ribbon cable to the board and left it INOP, if that "solves" the problem I'm happy even if I'd hoped for some repair jobIf it doesn't solve the problem, then it's something else triggering the short-circuit protection on the -8V preamp supply, and that's something I can probably manage unless it's one of the SMD opamps, and even then I can remove a channel or six. At least I saw a stable but slightly noisy -12V supply, and a wildly oscillating -8V regulator output which looked like a short-circuit protection flapping violently.But making up a supply for a part I never use is money and energy I'd rather spend on one of my other dream projects, like completely redoing an old but working all-in-one with a record player on top that sounded like a thunderstorm when I tried turning it onI'll keep you all excessively updated on how things develop. And again, thanks for all your support

author: magic
author_rank: Super Contributor
timestamp: October 14, 2023, 08:31:34 pm
content: I suspect it would either be a trivial fix with 7805, or it wouldn't, if the reason they didn't use 7805 was because the regulator must dissipate too much power. I guess resistors are cheaper than heatsinks.Parallel zeners could share current if they have positive thermal coefficient of breakdown voltage. Unfortunately, 5.1~5.6V is where TC is near zero so it's a crapshoot. At a higher voltage perhaps it would work.

author: sdancer
author_rank: Contributor
timestamp: October 14, 2023, 09:08:31 pm
content: Quote from: magic on October 14, 2023, 08:31:34 pmThe whole thing is driven from +/-8V TO220FP regulators sitting sandwiched between two boards and cooled by the cheapest stamped aluminium heatsinks I could find at a parts supplier. Since that hasn't burnt down for 17 years, I doubt heatsinking the 5V rail on a board that's right below the perforated case would have exceeded a heat budget. Most likely it was just cost-cutting.

author: Kim Christensen
author_rank: Super Contributor
timestamp: October 14, 2023, 10:05:17 pm
content: I have a Denon 5.1 amp that did the same thing:White noise upon power on, especially in cooler temperatures, and if it got cold enough in the basement, the protect circuit would engage.On mine it wasn't the DSP. It was a transistor in the AF power amp that was failing (losing it's gain/beta) and the channel would make this noise which would fade away as it warmed up. But if it didn't heat up quick enough, the output would have a DC offset for too long and the protect circuit would kick in before the speakers were engaged.I've fixed this amp 3 times as transistor after transistor would go in different channels.