Q Acoustics s1000 subwoofer - no auto power on - Page 1
Forum section: electronics / repair

author: KernelFergus
author_rank: Newbie
timestamp: April 25, 2025, 04:45:27 pm
content: Hello wonderful people.My dad has this old Q Acoustics s1000 active subwoofer that he would like to use.However, it's does not turn on when a signal is received as it is supposed to.I already managed a sort-of repair on the input jack (the RCA connector broke off/worked loose and damaged the traces on the input board (removed for images).But this refusal of the amplifier circuit to power on has me lost. As much as I have tried (at least a few hours) I was unable to find a schematic and I lack sufficient knowledge of the circuit design and principles to understand how such a power circuit operates.Any help with this would be greatly appreciated since this is otherwise a perfectly decent subwoofer.Edit: I have another image of the top side of the input board, but I've hit the file size limit and I can't upload it.

author: thm_w
author_rank: Super Contributor
timestamp: April 25, 2025, 10:10:22 pm
content: You can resize the photo and add it, preferably, or make another post here and then add the photo to that.Is it auto power on only? There is now manual power control.Did you trace the RCA signal from the input, maybe play a 100hz sine wave and follow it with oscilloscope or multimeter?

author: MathWizard
author_rank: Super Contributor
timestamp: April 25, 2025, 10:31:29 pm
content: So looking around, it seems to be powered right off 120/240V. I can see the rectifier on the 2nd PCB, right off the mains input, but I can't say I see any step down transformer like for a switch-mode power supply. So yeah I wonder what voltages it uses, and if there are any transformers or coupled inductors.Please get a smaller picture of the topside of the PSU. There's probably not that much to it. Did you actually check any voltages ? If your not sure where to check anyways, yeah don't bother. But once you know a few places to check, it's well worth the effort, to solder on a few wires, like a common GND wire, and then use alligator clips with your DMM. DMM probes slip and can be hard to hold in place, and flux residue can mess up your readings, and then if you slip and touch the wrong things, you can blow stuff up.You can test a lot of parts unpowered, with a DMM, in circuit. But I'd remove most or all of the electrolytic capacitors, the things that look like tin cans, with 2 leads. And I might remove some other capacitors too. Then your DMM readings will do a lot better, testing resistors, diodes, and transistors.That would be the safest way to rule out a lot of stuff, but yeah it's slower.

author: KernelFergus
author_rank: Newbie
timestamp: April 26, 2025, 12:21:22 pm
content: Quote from: thm_w on April 25, 2025, 10:10:22 pmUnfortunately it is auto-on only. The red light for the standby status comes on, so there is power being generated and supplied to the input board.the only signal generator/scope I have access to is a "scoppy" that I made from a raspberry pi Pico, but that has allowed me to trace the signal as far as a resistor network located nesxt to jp4, close to the remains of the input jack in the top right of the image. From there either it goes somewhere this device cannot follow, or the network is toasted (it's is visually fine).Edit: it's a low noise stereo pre-amplifier. Njm4558lI've attached another photo of the top of the power board, if the attachment isn't allowed, here is a link to the photo:https://1drv.ms/i/c/cca8eb9b7eb744d9/EXIk62DOkqZPg6HJryAnOBUB9s1SyUXtubR4_vsU1PBYSgHosted by OneDrive.The transformer lists it's output at 2x26v ac, makes 2x28v, and the DC from the rectifier is 77v.The njm4558l is receiving approx -4v across most of its pins, except I believe pin 4 which is getting -14v.

author: MathWizard
author_rank: Super Contributor
timestamp: April 27, 2025, 10:04:18 am
content: Ok so there's a transform in the case, and not on the PCB.-4V on most of the op-amp pins sounds a bit funny.IC201 is surely the audio amp IC, and it reminds of a STA540 or TDAxxxx. They have a STBY pin, which is pretty much just the base of a transistor, and the chip stays powered down, until some small voltage/current is applied.So I'm guessing when audio is applied to the input, some little BJT circuit on the PCB, gets turned on, and that signal turns on IC201, if it's a similar chip that is. So what IC is it ? If it has that STBY pin, track back what it's connected to.

author: KernelFergus
author_rank: Newbie
timestamp: April 29, 2025, 03:49:46 pm
content: The amplifier chip is a TDA7294. The standby pin is pin 9, which goes across to the input board over the green wire and into one of the outputs from one of the pre-amp chips.The positive voltage input for all of the pre amps (3 total, all NJM4558L) is tied together and traces back across the red wire to the power board, and is sitting at -4.4v. this traces over into the emitter of a transistor (Q151). From here the collector goes through a big resistor straight to one of the DC filtering caps.I'm guessing, but I think the voltage supply might be the issue.

author: MathWizard
author_rank: Super Contributor
timestamp: April 30, 2025, 12:07:34 am
content: Ok so it uses split rails, between the (+) of the rectifier, to GND, and (-) of the rectifier to GND. The black wire by the blue inputs, is GND.Here's what the PSU roughly looks like if the rails in were +/-35V and had 13V zeners, to get +/-15V out. Maybe they use 12.5V zener diodes, but it will be pretty similar.So what voltages are on Q151 when the 6-wire connector is disconnected ?

author: thm_w
author_rank: Super Contributor
timestamp: April 30, 2025, 12:33:21 am
content: Definitely some heat around Q151 Q1526 pin connector joints reflowed?

author: MathWizard
author_rank: Super Contributor
timestamp: April 30, 2025, 01:34:06 am
content: Hmm yeah w/ basically no load, I'd have 1.8W on R152/5, and 1.3W on the BJT's. So that's not good. Those 2 resistor's only looked 1/2W or 1W, and OP said the + to - rectifier voltage was 77V, not the 70V I used.At least the zeners only have ~65mW.Ok my mistake, I put the BJT collectors on GND, but they are series pass elements, so there should be next to no current through them, w/ no load, not the 90mA as shunts.So maybe they are 14 or 15V zeners. With a about +/-15mA load, now the BJT's only use ~0.3W each.

author: KernelFergus
author_rank: Newbie
timestamp: May 01, 2025, 02:25:46 pm
content: Quote from: MathWizard on April 30, 2025, 12:07:34 amWhen using the ground of the rectifier:Q152: B and E are 23.5v. the C is 0.Q151: E is 48v but drains the longer I measure it - I stopped at 38v. C is 76v, B is 52v.

author: MathWizard
author_rank: Super Contributor
timestamp: May 02, 2025, 12:17:15 pm
content: Ok and E of Q151, goes to the red wire, I forgot to swap their labels in the 2nd pic.So assuming you unplugged the 6-wires, Q152 sounds like it's off, as it should be with no load. But Q151 or something right around it must have a problem. At a sane current for Q151, the voltage between + on B and - on E should only be about 0.65-0.7V, you say you have 4V, and B sounds low, and E is really low, especially if there's no load on it.If there's still about 77V across - to + of the rectifier, then relative to the common GND node, adding -38.5V to all your voltages, I getE-B-CQ151-NPN 9.5V 13.5 37.5Q152-PNP -15 -15 -38.5So if you have diode mode on your DMM, you can check a few things, but with the caps there, and the diodes and BJT, I'd just remove the zener and /or and BJT and check them. I'd remove and check the caps too, and then check the resistors.

author: tunk
author_rank: Super Contributor
timestamp: May 02, 2025, 03:55:06 pm
content: In the second photo, on the 6-pin connector to the lower left, is the soldering a bit dry?

author: KernelFergus
author_rank: Newbie
timestamp: May 09, 2025, 04:04:08 pm
content: Quote from: tunk on May 02, 2025, 03:55:06 pmI've restripped and rejoined all of the joints on the 6 pin connector and there has been no change.

author: KernelFergus
author_rank: Newbie
timestamp: May 09, 2025, 05:32:51 pm
content: Quote from: MathWizard on May 02, 2025, 12:17:15 pmThe resistors all seem to match their marked values, and the transistors (q151 and q152) SEEM to test ok, and the zeners appear to bo ok as far as I can test (I don't have a variable power supply to test their reverse bias, assuming the guide I found for testing them was accurate).The only thing I have yet to test is the capacitors as I lack the kit to test those.All these test were carried out with a cheap multimeter and some googled instructions for the transistors, so the results may not be entirely accurate

author: MathWizard
author_rank: Super Contributor
timestamp: May 12, 2025, 11:40:59 am
content: To easily check the zener, just remove the BJT, and the V+ and 4.7k will do the work, and you should have close to 15V. 13.5V isn't that bad, if some zeners are only within 10% of their rating, then that would be 1.5V for 15V rating. But most modern zener's should be better than 10% tolerance.But the VBE of 4V is pretty bad and strongly points at the BJT, or a severely leaking 100uF cap. What does the BJT voltage measure with just the 22u and 100u caps removed ?Do you have a breadboard and assorted resistors ? You could solder some leads onto the BJT and that cap, and try some basic BJT test circuit.Just about any NPN BJT could be used in the PSU, so long as it's voltage rating is high enough. Do you have any other BJT's you could swap in ? The pinout might not be the same, so a new BJT with long legs would be nice, but you can just solder on a few leads or wires, (like the ones cut off new resistors).