Subwoofer amp: Want to repair or override "lazy" standby/power sensor - Page 1
Forum section: electronics / beginners

author: 2222
author_rank: Contributor
timestamp: September 08, 2023, 08:46:57 pm
content: First time poster here.This is for a AR model S10Ho powered subwoofer. Its power switch is off/auto (standby). In auto mode it is getting lazy at sensing input. It appears to power up in auto mode and stay in standby mode but I have to trick it (by sending an overly loud signal to its input) to wake up its amplified output to the speaker. Otherwise it stays on standby mode, even with the gain knob turned all the way up. In between songs or loud passages it often reverts back to standby mode and needs to be "shocked" back to "on" mode again.It is connected via RCA cable from the receiver Sub Out jack to the AR subwoofer RCA Sub Mono In jack. Trying different cables does not affect the situation. Trying a different receiver, no different.Is there a way to jumper the autosense section so that on is full on? I have the power cord plugged into switched outlet on the back of the stereo receiver anyways so it would be switched off when not in use.There are two circuit boards, one I'm guessing is the amp section, the other is the control section?My soldering skills are sufficient for soldering jumper wires, replacing discreet components (caps, resistors, diodes, relays).Including some rudimentary photos, I can take more/better as needed.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2023, 09:38:04 pm
content: Do you normally run it with the gain all the way up as pictured? When it comes out of standby, is there a relay that clicks?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 08, 2023, 10:50:49 pm
content: Make sure you don't have the crossover in your amp set to too low of a frequency. Try a subwoofer specific RCA cable. I've had this issue before and I eventually found a non-sub cable that would trigger the woofer, but I recommend a real sub cable.Also, I would not! power the subwoofer with a power jack from the back of your amp. When the woofer hits, it will be drawing power away from your amp's actual amplifier.And make sure everything is very clean.

author: bdunham7
author_rank: Super Contributor
timestamp: September 08, 2023, 11:12:09 pm
content: Quote from: JustMeHere on September 08, 2023, 10:50:49 pmI try not to question actual experience too much, but what possible reason could there be for an audio cable terminated in RCA to 'not work'? I suppose you could check it with an ohmmeter to make sure it has low resistance and no leakage, but I've never heard of a 'subwoofer specific cable' outside of the lunatic fringe that is high-end audio.As far as power, this sub isn't very powerful so an amp or two at the mains plug is all you'll see. That shouldn't drop the mains voltage at the amplifier very much. That could be tested as well.I'm going to suggest that the OP figure out a way to totally disable the standby feature and leave everything else the way it is.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 08, 2023, 11:39:41 pm
content: Trace the wires from the "Red-Standby/Green-On" LED to the circuit board. That'll be the area where the audio sense / timeout circuitry is located. If you can figure out what switches the LED you'll also figure out what's switching the amp between standby and On.

author: 2222
author_rank: Contributor
timestamp: September 08, 2023, 11:43:25 pm
content: Quote from: bdunham7 on September 08, 2023, 09:38:04 pmI usually run it with the gain at the 1 - 2 o'clock position. Turning it all the way up sometimes helps to kick it on, then I can turn it back down.Have not noticed a relay click but will investigate further.Quote from: JustMeHere on September 08, 2023, 10:50:49 pmDidn't know such a thing existed. I'll see what I can come up with.Quote from: JustMeHere on September 08, 2023, 10:50:49 pmI had brushed away the accumulated cobwebs but now will hit it with the air compressor. But I did identify a small area of foreign material on the control board, between R57 and one of the U7 legs. It looks like some stray spillage of the liberally applied thermally conductive paste between U7 and the heatsink. Can I clean it up with some 90% alcohol?

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 12:08:27 am
content: Quote from: Kim Christensen on September 08, 2023, 11:39:41 pmDo these photos help? The LED wires are the red black yellow.

author: bdunham7
author_rank: Super Contributor
timestamp: September 09, 2023, 12:43:49 am
content: Extremely clear and detailed closeups of the circuit board area shown in the middle photo would be helpful. I'd like to be able to read the lettering on the ICs.

author: CaptDon
author_rank: Super Contributor
timestamp: September 09, 2023, 12:51:15 am
content: Is the power switch 3 position?? Off, On, On Stdby? Most of those amps had a standby defeat position or a separate switch to disable standby.You may try looking up a schematic for the unit online. Perhaps an internal jumper is available. I have not personally found a subwoofer amp that didn't have a standby defeat switch included.Any RCA terminated cable of reasonable quality should work. Subwoofer specific cable sounds like bullshit. The lower the frequency easier it is to send a signal through a cable. And at line level there would be no current over a few milliamps so almost the shittiest cable would still work fine.

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 01:00:44 am
content: Quote from: bdunham7 on September 09, 2023, 12:43:49 amWorking on the photos. In the meantime, the ICs are all the same 4558d jrc 6120bPower switch is two position off/auto.

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 01:08:17 am
content: Hoping these are getting close.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 09, 2023, 02:28:27 am
content: I wonder if this amplifier really goes into standby, or whether the circuitry just mutes the audio amp. What's the part number on U7? (The one bolted to the heatsink)

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 02:57:24 am
content: U7: 67af Lm3886tf

author: bdunham7
author_rank: Super Contributor
timestamp: September 09, 2023, 03:05:47 am
content: Quote from: Kim Christensen on September 09, 2023, 02:28:27 amQuote from: 2222 on September 09, 2023, 02:57:24 amI'm starting to see where this might be going. The JRC4558D ICs are generic dual-op amps commonly used in audio circuits. The LM3886 is a well known all-in-one amplifier module and it has a mute input. I'm going to guess they use one of those dual op amps as a comparator and latch using one of those electrolytic capacitors and the same signal that goes to the LED also goes to MUTE pin of the LM3886.OP, can you tell us what sort of test equipment you might have and how comfortable you are taking some measurements?

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 03:20:11 am
content: Quote from: bdunham7 on September 09, 2023, 03:05:47 amMy most sophisticated test eqpt is a version of this $50-ish DMM:https://shop.mastech-group.com/as/en/product-list/content/MY64I am fine with taking measurements (good fine motor skills for holding the test probes still, plus assortment of jumper clip cables, etc.), given clear procedures, and am grateful for any education I can absorb along the way.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 09, 2023, 03:44:39 am
content: Yup, that's where I was going...See what others say first, but if you can find and remove the equivalent resistor connected pin 8 to on the LM3886as per the datasheet, you should be able to disable the muting.EDIT: I think I got my logic backwards here.First though, you'll want to verify that the IC is still actually powered when the unit is "Off" by measuring the DC voltage across pins 1 & 4. If that stays powered, next you could check to see how the DC voltage across pins 7 & 8 changes when the amp turns on/off when detecting input audio.According to the datasheet you may be able to disable muting by just shorting pins 7 & 8 together, but that makes me a bit nervous not knowing exactly what's connected to pin 8 on your PCB.

author: bdunham7
author_rank: Super Contributor
timestamp: September 09, 2023, 03:44:43 am
content: OK, so first look at the datasheet for the LM3886 and make sure you are confident you can identify pins 1,4 and 8. Then set your DMM to the 200VDC range and connect the negative lead semi-permanently (alligator clip or anything that is secure and you don't have to hold) to the negative side of the speaker driver. If it isn't obvious which is which, post a photo. It won't blow anything up if you choose wrong, though. Then very carefully (don't short anything out with the probe) use the positive test lead to measure the voltage at pins 1, 4 and 8 of the LM3886 two times--once with it in the 'active' mode and one in the 'standby' mode. What I'm looking for is what V+ and V- are, if they drop off in standby and if the voltage on pin 8 changes with the change in color of the LED. Again, be super careful--I don't think you'll hurt yourself, but you can turn your subwoofer into scrap in a flash. Literally!https://www.ti.com/lit/ds/symlink/lm3886.pdf?ts=1694178223256&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM3886

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 05:06:11 am
content: I am finding 3 modes:Off (when pwr switch is set to off). No power to the pins, no LED.Standby (when pwr switch is set to Auto). Pins 1-4 measure 65.5 VDC, Pins 7-8 measure 0.3 VDC. LED red (indicates standby).Fully On (when pwr switch is set to Auto). Pins 1-4 measure 65.5 VDC, Pins 7-8 measure 18.3 VDC. LED green (indicates on), subwoofer is working.Additionally I found a section of (3) resistors (R29, 12, 28) near the IC that get unreasonably hot (cannot hold finger to them), with excessive heat appearance, and burnt-looking PCB directly under them. There apparently is a trace going somewhere under these three from pin 1 of the IC.(Shining a light under the board shows burnt area under the 3 overheated resistors (second photo) IC is to extreme left (first photo)

author: bdunham7
author_rank: Super Contributor
timestamp: September 09, 2023, 05:51:13 pm
content: It helps if you explicitly specify polarity when you post measurements like this, and the reason I wanted you to take the measurements the way I did was to verify that this was a bipolar-supply setup. Single-supply would be idiotic for a subwoofer, but you never know. So I'll assume that if you put the negative lead on pin 7 (or any ground point, which should include one side of the speaker) you woud see about -33V on pin 4, +33V on pin 1, about 0V on pin 8 in standby and -18V on pin 8 when active. This verifies that the system just mutes the LM3886 for the 'standby' mode.There are two ways I can think of to go at this. The first would be to put a 22K resistor from pin 8 to pin 4--this should unmute the amplifier without putting any undue burden on the original switching circuit. This will probably work, but there are no guarantees since we don't have a schematic and we haven't fully reverse-engineered the system. The second would be to replace all of the small electrolytic capacitors that you see nearby. I don't normally advocate wholesale recapping, but this device is just the right age to be among a fairly large group of devices that suffered widespread failures of small random capacitors throughout their control and signal circuitry. If my general theory about how the sensing circuit works is right, a bad capacitor could definitely cause the issue.Edit: Of course your third option is to figure out exactly how it works.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 09, 2023, 06:01:13 pm
content: Quote from: 2222 on September 09, 2023, 05:06:11 amThose look like they are 1001/2 watt resistors. (brown black brown gold) Though the first "brown" band almost looks black, but that's not logical and it's probably discoloration from heat. You could measure their resistance with the power off to get a better idea. Measure the voltage across them when powered up and enabled. 7.1V across one would mean it's dissipating 0.5 watts. (volts x volts / resistance = watts) I would expect it should be much less voltage because they should not be run at their max rating.

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 06:38:13 pm
content: Quote from: bdunham7 on September 09, 2023, 05:51:13 pmMy results very close to your prediction.Standby mode:(-) lead on pin 7, (+) on pin 4 = -32.7v(-) lead on pin 7, (+) on pin 1 = -+33v(-) lead on pin 7, (+) on pin 8 = 0vActive mode:(-) lead on pin 7, (+) on pin 4 = -32.8v(-) lead on pin 7, (+) on pin 1 = -+33v(-) lead on pin 7, (+) on pin 8 = -18.3v

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 06:43:44 pm
content: Quote from: Kim Christensen on September 09, 2023, 06:01:13 pmWhen powered up, either in standby mode or active mode, same readings:R29 = 17.6VR11 = 15.4VR28 = 17.6VNot sure I measured these correctly. Meter set to 200 VDC, test probes held on each side of the resistor.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 09, 2023, 06:57:37 pm
content: Ouch! 17.6V across a 100resistor would be 3.1 watts! This is assuming they stillare100. What do they measure with your meter on ohms scale with the power off? They sure look like 1/2 watt resistors, but it's hard to gauge their exact size from a photo. What does the color code on them look like to you? Sometimes pictures mess with colors.R11 is 1K, so 15.4V across that would be 0.24 watts, while pushing it for a 1/4 resistor, isn't as bad as R28 & R29.

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 07:09:11 pm
content: Quote from: Kim Christensen on September 09, 2023, 06:57:37 pmR28 and R29 read 0.45 on the 20kscaleTheir bodies (excluding leads) are approx 9mm long.Difficult to get a good color read because of the burning. Green green red gold?

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 09, 2023, 08:07:19 pm
content: Assuming the circuitry they are attached to isn't effecting the ohm reading that much: Combining your ohm meter reading of 0.45K (450 ohms) and the colors you perceive, it's possible the resistors are 510 ohm with the center colors being more misscolored than the ends. (Only way to tell for sure would be to lift one end up, or remove it entirely to avoid cracking it, and measure it again "out" of circuit)510 ohm resistors @ 17.6V would dissipate 0.61 watts. 450 ohms = 0.69 watts. This would explain the blackening and heat without smoke.If they were really coded green green red gold, that would be 5.5K (Not a standard value) then they wouldn't even be warm to the touch with 17.6V across them. (0.056 watts)

author: 2222
author_rank: Contributor
timestamp: September 09, 2023, 08:13:45 pm
content: Possible it could be green green (brown or orange) gold. Even what I perceive as the gold might not be. (It *seems* to be shiny.) Really hard to definitively identify.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 10, 2023, 12:35:00 am
content: Quote from: bdunham7 on September 08, 2023, 11:12:09 pmBetter shielding against 60 Hz noise.

author: themadhippy
author_rank: Super Contributor
timestamp: September 10, 2023, 12:46:47 am
content: QuoteIs there a version available for us in 50Hz land?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 10, 2023, 04:00:26 pm
content: Quote from: themadhippy on September 10, 2023, 12:46:47 amYeah. Same thing should work for 50Hz. Nothing expensive.https://www.monoprice.com/product?p_id=2681

author: themadhippy
author_rank: Super Contributor
timestamp: September 10, 2023, 04:31:06 pm
content: QuoteFancy, a bit of tv coax , some shiny phonos and a dab of snake oil,bet there not even 75 ohm phonos.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: September 11, 2023, 10:29:49 am
content: Quote from: themadhippy on September 10, 2023, 04:31:06 pmPerhaps, but I had a subwoofer doing the same thing with cheap RCA cables. Tired a couple different ones. Tried it with a shielded cable and it worked. It must of had the right snake oil.

author: tooki
author_rank: Super Contributor
timestamp: September 11, 2023, 10:53:20 am
content: Quote from: JustMeHere on September 11, 2023, 10:29:49 amShielding isn’t snake oil.

author: CaptDon
author_rank: Super Contributor
timestamp: September 11, 2023, 12:49:00 pm
content: My recent experience with an amplifier similar to yours with the discolored resistors showed that those resistors were a voltage dropping network each one going to a 15 volt zener diode to create + and - 15vdc power rails for the dual opamp devices. The sad part is that the zeners and those resistors which were flameproof ceramic body style were operating at about 1.5X their design rating and failed. One of the zeners failed shorted and the amp quit completely. I replaced the 1/2 watt zeners with 1 watt versions and the 1 watt flameproof resistors with either 2 or 3 watt flameproof. Not sure which I used as I have both in stock. The owner bought a new amplifier in the mean time and I now own the one I repaired.

author: 2222
author_rank: Contributor
timestamp: September 11, 2023, 06:57:44 pm
content: I need to decide how to proceed.The workaround suggested by member bdunham7 above seems like it should work for my purposes (turning the amp on [labled "Automatic" on the power switch] forces the amplifier to activate).But that still leaves the overheating resistors, possibly caused by faulty capacitor(s). The board shows a date of 9/96, is that in the range of defective capacitor supplies?

author: David_AVD
author_rank: Super Contributor
timestamp: September 11, 2023, 08:52:11 pm
content: I've seen subwoofers with auto (audio sensing) issues where the culprit was some of the small electro caps on in the input board.The result of the bad caps was low gain in that section causing it to become less sensitive over a long time.Because subs are usually powered up 24x7 it's very common for the electro caps to be shot in 10 years time.A fair proportion ( maybe even > 50% ) of the subs I see for repair only have a single power supply, so the "standby" is sometimes nothing more than muting the output.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 11, 2023, 09:44:54 pm
content: I'd look at carefully removing one of those hot resistors. Measure it's value, and since you know the voltage across it, you can calculate the wattage required. Then order proper replacements.You could get all new electrolytic capacitors and replace them to see if that fixes the issue. At least you'll have all new capacitors to give it a longer lease on life.If the new caps don't fix the issue, I would then try bdunham7's resistor idea to keep the amp alive all the time.

author: bdunham7
author_rank: Super Contributor
timestamp: September 11, 2023, 10:05:44 pm
content: Quote from: 2222 on September 11, 2023, 06:57:44 pmNot really, the capacitor plague was later, starting 1999 or so. But I've seen cases of the small electrolytic caps on various audio products (not a subwoofer guy, so I don't remember any of those) failing in control circuits and things like that from the late-80's onward. So I would not be even slightly surprised if a bulk recap operation were to fix your subwoofer. The capacitors probably don't have anything to do with the resistors burning, that is probably just marginal design, drift and time. You can replace them with better versions and stand them off the board a bit, but I think they are probably a sideshow. Of course, you never know.

author: 2222
author_rank: Contributor
timestamp: September 12, 2023, 06:11:31 am
content: Quote from: David_AVD on September 11, 2023, 08:52:11 pmYes! That seems to accurately describe the behavior! I have to really punch up the bass and volume on the receiver to "wake up" the subwoofer.Quote from: David_AVD on September 11, 2023, 08:52:11 pmThis was a suspicion earlier in the thread. Am I correct in presuming that is a cheap and easy way to do "Automatic?" And possibly that is not as much of a power saver as it could be? (IIRC, it was advertised as a power saving feature. And convenience feature.)

author: 2222
author_rank: Contributor
timestamp: September 12, 2023, 06:17:02 am
content: Anybody have some recommendations on where I can source suitable replacement resistors and caps? Also probably will need some thermally conductive paste to reseat the IC to its heatsink during the reassemble. Radio Shack near here closed decades ago, and Fry's more recently.

author: bdunham7
author_rank: Super Contributor
timestamp: September 12, 2023, 03:25:36 pm
content: Mouser, Newark or Digikey. There's no local sources anymore and even if there were, you'd be picking from their availabe inventory instead of getting exactly what you want.

author: 2222
author_rank: Contributor
timestamp: September 14, 2023, 04:26:23 am
content: Quote from: bdunham7 on September 09, 2023, 05:51:13 pmSomething like this? 22k1/4watt?https://www.digikey.com/en/products/detail/yageo/CFR-25JB-52-22K/1306

author: 2222
author_rank: Contributor
timestamp: September 19, 2023, 03:46:33 am
content: That is indeed what I installed (22k1/4W)-- success!The amp is now always energized when powered on.It sure was finicky and delicate soldering it to the closely spaced interleaved IC legs. Legs 4 and 8 of course are the legs under and behind, greatly limiting access. Required several tries to avoid bridging to the adjacent odd-number "out" leg. Copper braided wick was useful.Thanks for all the helpful guidance and recommendations.I suspect I will need to renew the caps at some point as that resistor section still gets very hot.