Repairing a Monoprice 8 inch powered subwoofer - Page 1
Forum section: electronics / repair

author: edgargarcia1949
author_rank: Contributor
timestamp: February 16, 2025, 03:15:33 am
content: I am trying to repair a Monoprice 8 inch powered subwoofer that had 4 leaking 4400 microfarad capacitors which I have replaced.I powered it up and it blew the 1 amp fuse.I removed, checked and replaced all diodes.I them removed all transistors including a power transistor and found one npn 12 volt voltage regulator transistor to be shortedbetween ground and output.Waiting for that part to come in as that may fix the unit.My question is where can I find a possible schematic diagram of the power supply circuit in this subwoofer.It has no model number printed anywhere, only says "Made in China"If anyone knows where to find a schematic, I would be grateful.

author: MathWizard
author_rank: Super Contributor
timestamp: February 17, 2025, 01:13:48 am
content: Seems like it's a modern product, so you probably won't find any official schematics. Sometimes I'll make my own. There shouldn't be too much in it, compared to say a full stereo.I just finished mapping out most of my current stereo. It wasn't setup to drive a SW directly, for some reason. So I need to mod the signal paths, and add on some circuits for the SW.I also want to add some more circuits to trigger and control the emergency shutdown, and not have it attempt restarting over and over, bringing down the household power lines every time. It's way overpowered for my needs, so I'll add some inrush NTC thermistors too.

author: edgargarcia1949
author_rank: Contributor
timestamp: February 17, 2025, 02:31:56 am
content: The pcb is one sided so I could produce a drawing of all components with their values and connections but I have never tried to produce a schematic using that information. Maybe some one could use my drawing to produce a schematic for me. That may be a viable option for me.

author: MathWizard
author_rank: Super Contributor
timestamp: February 17, 2025, 01:59:16 pm
content: You can get circuit simulators, and place all your parts , and even test it. I use LTSpice, but there's easier ones to use than that, but IDK which ones to recommend.

author: edgargarcia1949
author_rank: Contributor
timestamp: February 17, 2025, 08:03:06 pm
content: Thanks for the reply. I am attaching a drawing I made plus 2 pictures of front and back of thepower supply pcb for this subwoofer. Interestingly, it says "7100 Amp with auto on off switch pcb ver 1.0 2011.05.03"on the pcb which gives us a year when this was made.Horizontal lines on back of pcb represent capacitors and vertical lines represent diodes.These marks helped me remove, check and then replace these items.If you could show me how to make a schematic from this information, that would be great.

author: edavid
author_rank: Super Contributor
timestamp: February 17, 2025, 08:35:48 pm
content: Quote from: edgargarcia1949 on February 16, 2025, 03:15:33 amThis is confusing. Was it an NPN transistor or a 12V regulator? If it was a regulator (7812?), I would question whether you could test it with an ohmmeter.When you say "all transistors", what did you remove? I don't see any transistors in the photo.It looks like it has a simple linear power supply. If the regulator is not shorted, check the bridge rectifier, then the audio amp IC.

author: edgargarcia1949
author_rank: Contributor
timestamp: February 18, 2025, 01:28:29 am
content: There is a 2n3904, a TIP122, a 9013 5v1 and a 78L12 which I found to be a 12v voltage regulator.I watched a video on how to check the 78L12 and it showed that I should get a resistance reading between theground pin (middle) and the output pin (left) and my meter showed a short.All the other transistors I have referenced checked fine with a component checker.The drawing I included in my previous post shows where these 4 components are located.All bridge rectifier diodes checked OK.Not sure how to check audio amp IC.

author: edavid
author_rank: Super Contributor
timestamp: February 18, 2025, 04:43:51 am
content: Quote from: edgargarcia1949 on February 18, 2025, 01:28:29 amSorry, somehow I missed that.QuoteI guess you would have to lift the power supply pin and see if the short goes away.I assume you checked the voice coils?

author: edgargarcia1949
author_rank: Contributor
timestamp: February 18, 2025, 06:59:33 pm
content: I unsoldered one wire from each voice coil today and measured 3.7 ohms on one and 3.8 ohms on the other.If the speaker is a 4 ohm speaker, and there is a good chance it is, then these resistances are okay.

author: richnormand
author_rank: Unknown
timestamp: February 18, 2025, 10:58:20 pm
content: Quote from: edgargarcia1949 on February 17, 2025, 02:31:56 amIf I remember correctly Dave once did a video on this and other techniques to reverse engineer and trace a pcb to generate a schematic on the forum.You might want to look it up. A simple psu is not too bad but for more complicated situations I found it very useful.Good luck with your repair.Cheers.

author: edgargarcia1949
author_rank: Contributor
timestamp: February 19, 2025, 01:26:45 am
content: Thanks for the suggestion.I will look into it.I thought of trying to reverse engineer this pcb.I am also adding to my pdf pcb drawing of the bottom side of the pcb where all the traces go.That should help me a lot to figure which components connect to each other.I will post that when I am done.

author: themadhippy
author_rank: Super Contributor
timestamp: February 19, 2025, 01:52:02 am
content: get hold of the main amp chip data sheet,often the circuit is very similar to manufacturers suggested design

author: reodor
author_rank: Contributor
timestamp: February 19, 2025, 08:36:02 am
content: Quote from: edgargarcia1949 on February 19, 2025, 01:26:45 amI have been experimenting with a method for reverse engineering using KiCad. I have not tried it a lot, but it seems to be workable. I have only tried it with single sided boards for now. If you want to have a go, here's what I do:Take a high resolution picture of the board and place it in the schematic editor in KiCad. You may want to adjust brightness and contrast so that the things you draw on top of it will be easier to see.Place components on top of the image right where they are located in the picture.Draw wires (still on top of the picture) where there are copper traces.Delete the picture. You are now left with a schematic that is horrifically unreadable.Rearrange the schematic so it becomes readable. This can be tedious and time consuming, and requires great care.KiCad has some strange quirks that you need to be aware of. The one trap that has caught me out several times is when I drag a component and a rubberband wire accidently crosses a corner of another wire, KiCad will automatically add a junction at that intersection.

author: edgargarcia1949
author_rank: Contributor
timestamp: February 19, 2025, 05:33:22 pm
content: I will look into KiCad and try to make a schematic but in the mean time, I have taken my pdf drawingand added the traces. I tried to be as careful as possible.The drawing was made using a CAD program called TurboCad which I have been using for 20 years or moreand interestingly, I can draw anything I want in 3D which I find to be quite fun.

author: MathWizard
author_rank: Super Contributor
timestamp: February 20, 2025, 02:10:36 pm
content: I was trying to make some of the circuit in LTSpice, but yeah I was getting fooled by the S9013 placement. Looking at the PCB I thought the 3 pins in a row were the BJT, for the TIP biasing.Sometimes when mapping a circuit in LTSpice, I'll place the parts as they are laid out from top-view, and then fill in the connections when probing the bottom side. And yeah it sure makes for a tangled mess sometimes.IDK if LTSpice can take a tangled messy circuit, and lay it out in a more presentable, efficient way or not. I'm sure some programs can tho.

author: edgargarcia1949
author_rank: Contributor
timestamp: February 20, 2025, 08:45:14 pm
content: I have improved my pcb drawing by adding component numbers and nets.I came up with 49 components and 29 nets.This information looks like enough to draw a schematic.I will be working on that while waiting for a new 78L12 to arrive whichmay or may not be the problem.Attached is new pcb pdf file and txt file of component numbers and net numberswhich are also on the drawing.

author: dietert1
author_rank: Super Contributor
timestamp: February 20, 2025, 10:35:56 pm
content: That pdf document is a good step towards a schematic. It shows the bridge rectifier with its 2x 4700 uF cap, then the TIP122 that routes power towards the other 2x 4700 uF and the integrated amplifier.That 12 V regulator LM7812 is used for the 555 auto on/off timer and the TL072 dual opamp. Probably it gets hot and may need a heatsink to work reliably. One can solder a copper sheet of some 5x5 cm to it's cooling tab.For initial testing the board after repair i would insert a fuse between transformer output and bridge rectifier, maybe a resettable fuse (PTC) or a 24 V light bulb of some size.Regards, Dieter

author: edgargarcia1949
author_rank: Contributor
timestamp: February 20, 2025, 11:01:52 pm
content: Thanks for all your feedback. Believe me, it is very much appreciated.I am very new to this and it shows but I have a long career of troubleshootingelectrical and electronic equipment with Xerox corporation servicing high-end copiers.The power transistor's cooling tab is screwed onto a metal plate with a mylar spacer and thermal paste.The 78L12 is a small device that looks just like a regular transistor and is rated for 100 milliamps and hasno cooling tab so I don't think it gets that hot.I could be wrong.I have a bunch of fuses now so if I try powering this thing up after replacing the 78L12and it blows a fuse, then I will have to look for a short elsewhere.This information I have accumulated will definitely help with that.

author: edgargarcia1949
author_rank: Contributor
timestamp: March 01, 2025, 02:25:27 pm
content: Update on trying to repair Monoprice 8 inch powered subwoofer:I have created a schematic for the power pcb on my subwoofer.Attached is 2 pdf files showing my results.The short that was blowing the fuse was an error on my part.I unintentionally had the collector of the power transistor shorted to ground.The correct placement of a nylon spacer fixed that problem.I now have a subwoofer that comes on with a power led on but outputs no sound.I have a portable oscilloscope coming in so I can check signals on the pcb.I especially want to see if signals are getting into the amplifier chip and make sureall voltages look OK.I don't know how the timer and op amps work so any advice on those 2 would help.

author: dietert1
author_rank: Super Contributor
timestamp: March 02, 2025, 06:42:46 pm
content: There seem to be minor mistakes in the schematic, but obviously the 555 timer controls the standby pin of the power amplifier chip. The timer gets its signal from Q3. D9 and D10 appear to be a rectifier for the audio signal at the output of IC3A. IC3A appears to be wired as a 100x amplifier. I'd guess it gets the audio signal if present and turns on the amplifier chip.I`d guess two of the four amplifier inputs are wired to a DC signal. The amplifier has two channels that should drive two subwoofers or two subwoofer coils.Regards, Dieter

author: edgargarcia1949
author_rank: Contributor
timestamp: March 02, 2025, 10:28:58 pm
content: Thanks for the input. I am currently reverse engineering the input pcb and will post that schematic when done.I just purchased an inexpensive hand-help oscilloscope to help me find out where the audio signal is missing because Iam getting no noise from the speaker. I will be able to see where the audio signal goes into the power pcb and what isstopping it.Are my minor mistakes on my schematic worth noting?This is my first attempt at drawing a schematic and drew what is called a ladder schematic of whichI am familiar.

author: MathWizard
author_rank: Super Contributor
timestamp: March 03, 2025, 04:42:44 am
content: I tried it in LTSpice, D5 is a 5V1 zener diode, they are used backwards compared to regular diodes, with the reverse breakdown voltage. I get about 16.5V unloaded from the TIP122.The op-amp buffer with the inverting tied to the output, where else does that goes? Maybe to R14 instead of R14 being tied to the divider? But there's no caps there either so IDK. But it's just some AC GND basically, 1/2 way between GND and 12V rails. Then the other half op-amp, amplifies the audio in. Or at least a sample of it, IDK if this is also the output audio amp.But then that op-amp also feeds in to the 555-timer shutdown/stby/overload. If the audio in (or whatever that vc on the PCB is) if that AC signal is too big, it will feed through C13 and D9, and turn on that BJT, and pull down it's collector, and then the lets D8 drain the 100uF cap as soon as there's any amount of voltage on it, like 500mV, and that makes the 555-output go high.Also if that BJT is off, and on startup it takes a long time to charge up that 100uF cap, the 555 output stays high, and D6 will be off, and the STBY pin should be in STBY/mute/overload/protection mode or whatever they call it, until the cap voltage gets to (2/3)*Vcc

author: tooki
author_rank: Super Contributor
timestamp: March 03, 2025, 06:45:15 am
content: (I wrote this on 3/1 but got distracted and didn't finish posting...)To understand a circuit, it helps to make visible the signal flow through it. Your reverse engineering is very neat and tidy, but the signal flow is not obvious.I took a stab at rearranging the schematic in LTspice, and at that point, I just went ahead and simulated it, too, which helped me understand what it does.Attached is the schematic of the signal path (power supplies not modeled), rearranged into the signal flow and broken down into functional groups.Since you seem to be good with computers, you can install LTspice (it's free) and open the circuit yourself. Run the simulation (click the running dude in the toolbar), and then you can click any node in the circuit to see the waveform at that point.In the simulation, I use an extremely slow square wave instead of a sine wave, because this stimulus actually makes it possible to see what the circuit is doing in ways that are obscured when using an audio-frequency sine wave.Basically, the incoming signal is AC-coupled into an amplifier with gain of 100, so it easily clips. This clipped signal gets rectified into spikes which turn on Q3. Q3 in turn discharges timing capacitor C7, whose voltage becomes the input to the 555, which is configured as a Schmitt trigger, not a timer.* When this output goes low, it pulls down the STANDBY input of the amplifier (which should be called ENABLE, because the amp is on when the standby pin is high).*I'm not a 555 expert, so I can't rule out that the preceding timer section is, in fact, a "normal" configuration for a 555. But it didn't resemble anything I was familiar with, and the timer section itself functions even if the 555 is disconnected.

author: tooki
author_rank: Super Contributor
timestamp: March 03, 2025, 06:52:03 am
content: Quote from: dietert1 on March 02, 2025, 06:42:46 pmI don't think so? Everything makes sense, once the schematic is redrawn into more familiar forms.

author: dietert1
author_rank: Super Contributor
timestamp: March 03, 2025, 09:19:52 am
content: Certainly the second half of TL072 should drive something. Otherwise it would be a TL071.

author: themadhippy
author_rank: Super Contributor
timestamp: March 03, 2025, 12:08:41 pm
content: Quotenot necessarily so ,especially if a TL071 costs 97p and a TL072 costs 80p

author: tooki
author_rank: Super Contributor
timestamp: March 03, 2025, 07:22:33 pm
content: Quote from: dietert1 on March 03, 2025, 09:19:52 amIt is very, very common to use dual op-amps when only one amp is needed. If the manufacturer uses the dual amp elsewhere in the device (or in other products!), it may be cheaper to buy the dual ones in bulk rather than smaller amounts of two different chips.

author: edgargarcia1949
author_rank: Contributor
timestamp: March 04, 2025, 12:03:52 am
content: Lot to digest. Schematic sent me looks great. I could learn from that.Quick question.Studying the input pcb to draw a schematic, there are 2 signals going out to the power pcbcalled sw out and vc out.Both have a positive biased wire going out on a white wire and the black wire in both casesis connected to a small copper trace on the under side of the pcb with nothing connected to it.Is this normal? It looks like the signal does not need a ground to work.

author: edgargarcia1949
author_rank: Contributor
timestamp: March 04, 2025, 02:53:42 am
content: I loaded and ran your asc file in LTSpice. Naturally I was amazed that such a program exists.I don't know how to use it well yet but when I clicked on a lettered point where the cursor turned into a probeI saw a waveform way off to the right and it was all scrunched up. I don't know how to get it back over to the leftand spread it out a bit. Maybe you can advise me on that.

author: dietert1
author_rank: Super Contributor
timestamp: March 04, 2025, 10:59:14 pm
content: Quote from: tooki on March 03, 2025, 07:22:33 pmThe circuit looks a bit rough though, a bit elektor style. E.g. i could save the transistor and the 555 timer using the second half of the TL072, i mean in case it isn´t used for something else. Easy to make a timer using an opamp as comparator with feedback.Regards, Dieter

author: edgargarcia1949
author_rank: Contributor
timestamp: March 07, 2025, 11:44:01 pm
content: To Tooki,I used LTSpice to help me draw a schematic for my subwoofer input pcb and I found it quite easy and enjoyable to draw that schematic,however, I found it a bit limited and I couldn't do some things I wanted to do. It did not like my op amps when I tried to run it.Anyway, I drew an improved schematic in TurboCad and attached is the result. My next step is to troubleshoot the power pcb until I find the problem.It looks like the vc out from the input pcb refers to volume control and the sw out refers to the frequency pot. I don't know what sw stands for.Many thanks,EdDuson, LA

author: edgargarcia1949
author_rank: Contributor
timestamp: March 08, 2025, 03:41:27 pm
content: I made a mistake on placement of C16 near dual op amp.Attached is corrected drawing.

author: tooki
author_rank: Super Contributor
timestamp: March 08, 2025, 06:52:23 pm
content: Quote from: edgargarcia1949 on March 07, 2025, 11:44:01 pmLTSpice’s schematics are more or less just functional. I don’t think it’s the easiest schematic editor out there. What did you find missing?What did it “not like” about your op-amps? Why not take my amp board schematic and modify it to get started?Quote from: edgargarcia1949 on March 07, 2025, 11:44:01 pmAgain, you haven’t made a clear left-to-right progression of the signal. You should really try doing that, as it is how you understand the circuit.Also, for the love of god, avoid the combined op-amp symbols (the ones that follow the pinout). They make it damned near impossible to quickly identify an op-amp’s configuration.Quote from: edgargarcia1949 on March 07, 2025, 11:44:01 pmUmmm… look at your original schematic of the amp board. It should be blatantly obvious what the sw out signal is.I’m not gonna redraw this schematic, but at first glance, it looks like they simply take the two input pairs (speaker level and line level) and combine them, then the input signal pair splits off and takes two paths:1. L and R inputs are combined and sent to the power board as the vc_out signal used for auto power on. (Ironically, this signal is seemingly named “volume control out”, but in fact is tapped off before the volume control!)2. The input signals go through the volume control, then through a preamp, a summing amp to combine L and R, and then an adjustable low-pass filter*, before leaving as the “subwoofer out” signal to the amp.*I’m half-guessing at the exact configuration of the op-amps, especially the quad op-amp, because I’d have to redraw them to make sense of it.As for troubleshooting: you now have a schematic. What are the symptoms?

author: edgargarcia1949
author_rank: Contributor
timestamp: March 08, 2025, 08:35:02 pm
content: This subwoofer sat in a plastic building in the Louisiana heat for about 2 years.When I first powered it up, it made a loud humming noise and would not amplify the subwoofer out on my stereo.I took it apart recently and saw those leaking caps. Replacing them and a bad 78L12 results in no hum but also nooutput. I am not a signal expert so I do not know how to trace them in my schematic. I tried using an op amp I found inthe LTSpice list that matched the one I have regarding pinout. I also tried making a pot with resistors but that did not work.LTSpice did not like the op amp I chose so I found that to be a dead end.What I do not like about LTSpice is that I can't use the tools that are available to me in TurboCad which I know by heart.For example, I can't use colors as I like, font sizes, circles and many other things.This is my first experience with 555 timers and op amps and reverse engineering a pcb so my inexperience shows.All I am asking for is help.

author: dietert1
author_rank: Super Contributor
timestamp: March 09, 2025, 10:30:39 pm
content: Apparently two audio signals are routed to the power board: One is the sum of left and right audio input, the other one is the processed subwoofer signal with adjustable amplidude and low pass filter. The first signal is used for automatic on/off, the other one goes to the poswer amplfier chip.The description of the failure suggests that low voltage power (rectified from transformer output) got connected to the voice coils. One thing to measure is the actual resistance of the voice coils to know whether something is burnt.Next is the power amplfier. With the speaker voice coils disconnected i would check the DC balance of the two amplfier sections. That integrated circuit has four channels that are wired as two full bridges for the two voice coils. Without audio in and in active mode (not standby) all four outputs should be about half of Vcc.Regards, Dieter

author: edgargarcia1949
author_rank: Contributor
timestamp: March 10, 2025, 02:21:06 pm
content: Please don't judge me too harshly but this morning I set my new hand-held oscilloscope to the signal generator modeand set it to output a 100Hz sine wave at 1.2 volts.I fed that to pin 11 of the amplifier chip TDA7377 and turned on the subwoofer and heard no sound.I checked pin 13 and measured 16.3V DC going into the amplifier chip.If my signal voltage was correct does that mean my amplifier chip may be bad?An earlier check on the subwoofer voice coils measured 3.8 ohms on both voice coils.Opinions?I looked at the data sheet and nowhere does it state the voltage level of the input signal.I guess that this is probably assumed but being a newbie, I don't know that information.

author: themadhippy
author_rank: Super Contributor
timestamp: March 10, 2025, 03:02:10 pm
content: QuoteAll depends on whats happening on pin 7,if that aint correct the amp will be silent

author: edgargarcia1949
author_rank: Contributor
timestamp: March 10, 2025, 07:01:34 pm
content: Thought I might get that question so I checked pin 7 and got 7.84V DC.

author: themadhippy
author_rank: Super Contributor
timestamp: March 10, 2025, 08:27:09 pm
content: Quoteits above the 3.5v given in the datasheet so it should be on.As a sanity check shove a signal to the main left and right inputs and double check its making its way the input pins on the TDA7377

author: edgargarcia1949
author_rank: Contributor
timestamp: March 11, 2025, 01:20:10 am
content: I will do as you suggest in the morning.However, I read this article at the following url:https://rgelectrics.com/tda7377-amplifier-circuit/and at the end of the article, it says that pin 7 being high puts the amplifier chip in standby modeto save power.Do you think this is correct?

author: themadhippy
author_rank: Super Contributor
timestamp: March 11, 2025, 01:44:46 am
content: Quoteunless im reading the datasheet wrong then no.A voltage on pin 7 below 1.5v brings standby in,and a voltage above 3.5V standbys out,and its sorta confirmed by the line below which mentions the current on pin 7 when in play mode with 5V on pin 7,the relevant bits from the datasheetQuote

author: dietert1
author_rank: Super Contributor
timestamp: March 11, 2025, 11:13:34 am
content: The representative circuit shown in the block diagram indicates that the standby pin needs to be "high" to turn on the amplifiers. They mention that standby pin input current must be limited to 5 mA. The real circuit is different as the threshold of a npn transistor is about 0.6 V, not 1.5 V. So it's more like a logic input to be named "Not Standby" instead of "ST-BY".When the amplifiers are active, pin SVC should show a small voltage (maybe 1 V), and zero otherwise.Regards, Dieter

author: tooki
author_rank: Super Contributor
timestamp: March 11, 2025, 01:53:00 pm
content: Quote from: edgargarcia1949 on March 11, 2025, 01:20:10 amNo. And I addressed this point a week ago in my comments to the schematic:Quote from: tooki on March 03, 2025, 06:45:15 am

author: edgargarcia1949
author_rank: Contributor
timestamp: March 11, 2025, 07:47:04 pm
content: Subwoofer update:I went to measure if an audio signal was getting into the amplifier chip with my cheap oscilloscope and signal generator.I could not detect a signal however when I touched pin 5 with my positive probe, I noticed a detectable hum which I havenever detected before. The subwoofer cone was vibrating.I then connected a sine wave signal at 100% duty cycle and at 3.3V which is as high as my signal generator could be set.I detected a faint hum and the subwoofer cone was vibrating but it was at a very low volume.I don't think that the amplification is taking place that should be and since the Vcc going in is at 16.3vdc and with thestandby pin at 7.84vdc I have decided to try replacing the amplifier chip. I have 2 on order from Amazon for about $10.00 withfree shipping.Thanks for all the terrific input and I will post what happens in a few days.

author: edgargarcia1949
author_rank: Contributor
timestamp: March 15, 2025, 08:02:13 pm
content: Put in new amplifier chip and have a different result.I tried to put my signal generator probe on input pin 5 of the amplifier chipwith the ground clip on chassis ground and got a very loud hum before Ihit the run button. The amplifier chip is now working and amplified the noisethe signal generator was introducing into the circuit before I got a chance to run the signal.I now need to find where the input signal is stopped as themadhippy said.My brother is coming over next week to help me find the problem. He is bringing overhis oscilloscope and he is a retired electronics repair tech.

author: edgargarcia1949
author_rank: Contributor
timestamp: March 19, 2025, 10:17:31 pm
content: Monoprice subwoofer fixed update.My brother came over today with his more expensive oscilloscope and we spent a couple of hours troubleshooting until we found the problem.We lost about a half hour learning to use his oscilloscope and also due to the fact that I had the volume control all the way down. Whoops.Not to bore any of you guys but the final problem was a 10uf capacitor going into the amplifier chip input which had a poor connection to the power pcb.That was C12. I touched it and voila, the subwoofer came on.It is now in position in my workshop and sounds great.I learned quite a bit from this experience and want to thank themadhippy, dietert1, MathWizard and especially tooki for their help.I now have a schematic to help with future problems and I have seen audio signals going into an op amp and coming out amplified whichI have never seen before.Thanks again and if I have future questions, I know who to ask (besides my brother)

author: tooki
author_rank: Super Contributor
timestamp: March 20, 2025, 06:11:05 am
content: Thanks for the update, glad you got it working! And nice to hear that you learned some stuff in the way that you can apply to future projects or repairs.