Would appreciate a little help with small amplifier board diagnoses. - Page 1
Forum section: electronics / beginners

author: guitchess
author_rank: Regular Contributor
timestamp: November 03, 2013, 05:18:13 pm
content: Hello,I'm trying to fix the amplifier in a set of small computer speakers for my sister-in-law. I thought I had enough experience to accomplish this, but the solution continues to evade me. So I thought I would reach out.This amplifier board uses a 12v supply uses a Toshiba IC to drive the small speakers and an NJM IC to drive headphones, if I'm reading the board correctly that is. Board tophttp://myconco.com/kboardtop.jpg. Board bottomhttp://myconco.com/kboardbottom.jpg. There was a small heat sink on the DIP but I removed it to identify the ICProblem: When voltage is supplied, the board goes into uncontrolled oscillation, regardless of whether the power button is off/on. Furthermore, it doesn't matter if I let the signal go to the speakers, or if I send it to the headphones. It sounds like a short to ground, but I can't seem to find it.What I've done:Checked all the resistors, all read within spec. FYI, I removed the resistor at the end of the Toshiba IC cause it looked like it had a dark spot in the center, but it read within spec in/out of circuit,Checked all the caps with my homemade ESR meter, albeit a rudimentary onehttp://www.ludens.cl/Electron/esr/esr.html.It only reliably reads caps above 1uf, and it really only gives a pass/fail reading, which leads to my first question.After not finding any bad passives, I removed the headphone IC. The fault continued. Reinstalled that IC and removed the Toshiba IC and the noise stopped, making me think that it was the chip. I ordered another and after installation the noise returned. So, I have apparently missed something.I went back over all the passives and the only thing that I can't confirm to spec are the caps that are small enough that my ESR meter can't read them. I did check them for short and found them fine. Don't really know how to check them for open though.I would like to check voltages but the Toshiba IC and the cap at its end gets very hot very quickly, and I don't want to burn them. If you notice the swelled top on that cap, it wasn't like that until I tried to check voltages the first time.Questions:Do you think that the simple nature of my ESR meter is causing my inability to find the issue? This is the first time I've used it to troubleshoot, although I spent lots of time testing and experimenting with it.Since none of the smaller caps seem to be shorted, is there a safe way to check to see if they are openOne thing that puzzled me is that while looking for shorts, I noticed that the jumper wires that go the headphone amp section are shorted until a headphone plug is inserted. At first, I thought this may have something to do with the issue but if so why would the problem persist even with headphones?Thanks in advance for your input. I realize that for less money/time I could just build a new amp to power the speakers, but at this point I just want to use this failure as a learning experience and I will learn much more if I find what is causing the issue.<>

author: Alana
author_rank: Frequent Contributor
timestamp: November 03, 2013, 09:17:10 pm
content: What are those ICs? Maybe datasheet for them contains schematics that could be usefull - such boards sometimes use reference designs.

author: HalfSpace
author_rank: Regular Contributor
timestamp: November 03, 2013, 11:03:26 pm
content: Hi Guitchess,Looking at the photos the cap with the swelled top is connected across the yellow connector which I think is the power adapter input. For the cap to heat up like that is ether leaky (drawing a lot of current), has had reversed voltage or over voltage across it. Check to see if you have the correct power adapter connected. This cap will need replacing anyway.The Toshiba IC is the power amp driving both speakers and headphones which is switched in when the headphones are inserted. The smaller IC I think is a preamp. If the cap is open circuit the power amp may self oscillate because it is the supply rail decoupling cap for the power amp IC.Your ESR meter may not measure the cap correctly in circuit, so best to remove the component for test.Hope this helps, best regardsHalfSpace

author: Unknown
author_rank: Unknown
timestamp: November 03, 2013, 11:54:02 pm
content: Quote from: guitchess on November 03, 2013, 05:18:13 pmLow frequency oscillation / 'motorboating'?It's a common fault/problem with cheap amplified speakers, usually due to the power supply not having enough grunt to handle the amp's over-enhanced bass response, resulting in low-frequency oscillation of the supply. Check the power supply voltage while it's happening; if it's varying along with the oscillation, try using a supply capable of higher current. Also, as HalfSpace mentions C101 (which appears to be across the power socket?) looks a bit sus, as does C1A alongside the SIL amp package.

author: guitchess
author_rank: Regular Contributor
timestamp: November 04, 2013, 06:14:06 pm
content: Quote from: Alana on November 03, 2013, 09:17:10 pmta8027 dip - I removed this one and used the datasheet specs to test output voltages and got some variances, which is what made me think it was the IC. Put a new one in and the same noise persists. Don't know what the chances are that I bought a bad one, but I'm starting to wonder.njm4558 sip - removed this one and the noise continued.

author: guitchess
author_rank: Regular Contributor
timestamp: November 04, 2013, 06:17:02 pm
content: Quote from: HalfSpace on November 03, 2013, 11:03:26 pmHave taken several of the caps out to confirm that they are good. The cap that shows swelling has been replaced, even though it tested fine, and the fault persists.

author: guitchess
author_rank: Regular Contributor
timestamp: November 04, 2013, 06:20:46 pm
content: Quote from: Tac Eht Xilef on November 03, 2013, 11:54:02 pmThis is a set of Sony Vio Desktop speakers. They normally draw their power directly from the back of the computer(speciallized jack) so for testing purposes I am using bench power supply.

author: HalfSpace
author_rank: Regular Contributor
timestamp: November 04, 2013, 11:16:01 pm
content: Quote from: guitchess on November 04, 2013, 06:14:06 pmHi guitchess,TA 8027 is that the correct number as I can not find any datasheet on the web for it?Does the new cap get hot?Best RegardsHalfSpace

author: Unknown
author_rank: Unknown
timestamp: November 05, 2013, 12:29:35 am
content: Quote from: guitchess on November 04, 2013, 06:20:46 pmSo, what kind of oscillation? Low frequency motorboating (distinct thumpthumpthump or purring sound)? Low or high pitched squeal (if so, roughly what frequency/pitch)? Does it sound good and solid like it could go on forever, or thin and reedy like it could stop at any time (but doesn't)?They're big clues as to where the fault may lie (edit: apart from obviously in the power amp stage).

author: guitchess
author_rank: Regular Contributor
timestamp: November 05, 2013, 03:59:58 am
content: Sorry about that HalfSpace,That was a typo. The IC is a TA8227. The datasheet I have been referencing is here http://www.bucek.name/pdf/ta8227.pdf.A question I had on this subject is: In the datasheet a "test circuit" is given. Does that mean that the IC needs to be placed in that circuit to test output voltages also given in the sheet, or can I apply the Vcc then test bare IC? This was the proceedure that made me think the orig chip was bad. In that attempt, most of the voltages were spec, but there were a couple that were off by a volt or two.

author: guitchess
author_rank: Regular Contributor
timestamp: November 05, 2013, 04:10:08 am
content: Quote from: Tac Eht Xilef on November 05, 2013, 12:29:35 amTac Eht Xilef,The oscillation is definitely a low frequency "motorboating." It is present even when the power switch is off. When the switch is turned on, the low frequency oscillation continues but a higher frequency harmonic is added. This oscillation is present in the loudspeaker or the headphones, which ever is active. Also, when feeding an audio signal to the amp, none is allowed to pass through.Thanks

author: guitchess
author_rank: Regular Contributor
timestamp: November 05, 2013, 04:19:37 am
content: Quote from: HalfSpace on November 04, 2013, 11:16:01 pmThe new cap does not get hot because I now do most of the testing with headphones plugged in. The oscillation is the same whether speaker or phones, and I was afraid of damage to the speakers.Just to be clear, I was the cause of the original cap swelling. I was testing voltages with full voltage, no speakers attached, and not monitoring temps close enough. There is a significant reason I am posting to the beginners section.

author: Unknown
author_rank: Unknown
timestamp: November 05, 2013, 06:24:12 am
content: Cool, knowing that about the oscillation really does helpYou've seemed to rule out power... wow, reading the datasheet for the TA8227, it seems to be an unstable little bugger. Honestly, if it were mine I'd shotgun replace all the electros around it just on spec; every single pin with an electro anywherenearit seems to come with an associated warning about oscillation!But since you want to find out what's going on rather than just fix it, and assuming the board in your hand approximates the datasheet reference/test circuit, I'd start with the caps associated with the gain-setting resistors on pins 5 & 8 (C1 & C2 on page 5). I wouldn't even bother testing them first, just replace them with low ESR caps - the whole thing looks so unstable that any slight decrease in capacitance or increase in ESR is enough to make it oscillate. If that doesn't solve it, I'd move on & replace the ones between pins 3 & 2 (C4) , and 10 & 11 (C5). The one on pin 9 (C3) should be replaced too.(I usually deplore the generic default "blame the electros for everything!" unless you know they're shit - brand, obvious damage, etc - but given the symptoms I think it's pretty well justified in this case...)

author: HalfSpace
author_rank: Regular Contributor
timestamp: November 05, 2013, 09:30:21 pm
content: Quote from: guitchess on November 05, 2013, 04:10:08 amHi guitchess,If the TA8227 is still oscillating with the power switch off, the standby pin1 (Vcc2) must still have voltage on it with the power switch in the off position. We need to get the TA8227 to go into standby mode. De-solder one side of R102 and lift that side out of the PCB. Power the board up and see if it still oscillates with power switch on or off. Check the voltage on pin 1 and the voltage going to R102 from the power switch in both on/off positions.Best RegardsHalfSpace

author: trackman44
author_rank: Regular Contributor
timestamp: November 06, 2013, 01:19:40 am
content: By the looks of those traces there is also the posibility that you might have a micro fracture between trace and solder pad, enough to induce a resistance causing the oscillation. Bend the PCB slightly to see if the oscillation goes away. If it does go way momentarely then check continuity on all the traces between solder pads.Will

author: guitchess
author_rank: Regular Contributor
timestamp: December 22, 2013, 05:10:37 pm
content: I just wanted to give a quick update for those that took their time to try to help and any that may have been interested.It sends as though I was my own worst enemy. My power supply I was using was outputting ac. So, hours of testing were wasted. Not to mention the time you nice folks gave trying to help me.After correcting the power supply issue, and several more hours of testing, the fault simply lay in a couple bad capacitors. These components were hard for me to find because the first, being an electrolitic, only barely reading into the marginal according to my esr meter. The second being a 102 ceramic, didn't test open or shorted. The only reason I suspected it was because a small amount of voltage was getting past it.The unit is now working fine, despite the beat up appearance due to the inordinate amount of handling/testing. Unfortunately I have $6 in fixing it when it only required $.49, but I learned a lot.Thanks for the help. Any other advice for a rookie is always appreciated.