Question about the function of a capacitor in a circuit - Page 1
Forum section: electronics / beginners

author: fedaalis84
author_rank: Contributor
timestamp: March 01, 2019, 11:02:55 pm
content: I'm a newb here & I know some basic things about capacitors. But wondering specifically what this guy here is doing in this part of the circuit?This is a schematic for the main board on the Sony STR-DH130.Pretty sure this part of the circuit is part of the Protect system, to detect a voltage overload from the output transistors on the right audio channel.Long story short, those two output transistors on the right channel shorted out (bringing the emitter resistor and an adjacent 100ohm resistor down with it). Anyway, when desoldering the emitter resistor, my dumb self also desoldered C761, a teeny... tiny SMD 220PF capacitor.I also jacked up the trace from that capacitor to Q761 (the current detector).After desoldering the output transistors, and even my SMD capacitor/trace mishap, the system will still start - and was not going into protect mode.Fast forward to today, I installed a new emitter resistor, and also placed a jumper between R761 and Q761 EDIT: From the bottom of R761 to the base of Q761 (eventually once my SMD 220PF capacitors come in the mail, I'll get one of those bad boys in, too.)...But afterwards (and checking my work with the multi-meter for any kind of weird solder bridges anywhere, everything checked out OK)But now, the system will go into protect again after powering up.So I'm wondering, is it because I still need to get this capacitor back in?Thanks for the help!

author: xavier60
author_rank: Super Contributor
timestamp: March 02, 2019, 01:49:06 am
content: It's unlikely that the missing capacitor is the cause. To be certain, solder in something else, no matter if it's larger than 220pF.Is there DC on either channel's output?What is the voltage on the collectors of Q762 and Q712?. Normally they should be almost the same as the negative rail.

author: fedaalis84
author_rank: Contributor
timestamp: March 02, 2019, 02:26:56 am
content: Quote from: xavier60 on March 02, 2019, 01:49:06 amThanks xavier60,I'll try adding some random cap.There should be no DC on the right channel's output, I have the output transistors out of circuit at the moment. I'll check the collectors on Q762 and Q712, but the left channel was not having issues before - so I think the protect has to be from my tinkering today.Before, when it was going into protect when I had the right channel's transistors in, it would hit protect much faster. Now it takes some time, and almost seems like it won't. Then bam, Protect.I might try desoldering the jumper if a random cap doesn't resolve the issue.Thanks for the input!

author: fedaalis84
author_rank: Contributor
timestamp: March 02, 2019, 05:04:48 am
content: Update:Tried an unknown value SMD ceramic cap. Still goes into protect.Tried unsoldering the jumper, still goes into protect.BUT something that I hadn't considered:I replaced the bad emitter resistor (which was previously behaving like an open circuit). So that could be causing protect to surface via some means that I have not identified.Didn't have time tonight, but next I will try removing that & see what happens.Couldn't be something else in the protect circuit that, because I have the right channel transistors out, is now being wonky because there's a connection via the emitter resistor that wasn't there previously?

author: fedaalis84
author_rank: Contributor
timestamp: March 02, 2019, 03:49:46 pm
content: I don't have a DC power supply, so I can really only measure voltages on top of the board, because I have to use the units power supply.Anyway, on J469, which connects to the emitters of Q762, and Q712 (with a 68k resistor between either emitter and the junction) I'm reading -53V DC, and drops down to almost nothing once it goes into protect.Because the right side is open (transistors are out), this voltage must be coming from the left channel, right?Stumped at why it's going into protect now, but didn't before I messed up C761 and the trace to the base of Q762.. if the excess voltage is coming from the left channel, which seemed fine before.

author: fedaalis84
author_rank: Contributor
timestamp: March 02, 2019, 04:27:10 pm
content: I am reading 60 some-odd volts after the bridge rectifier. It takes in about 45 volt AC. Now, that should be normal. But I'm pretty sure the filter capacitors should smooth it down to 42 some odd volts like the main rails.. right? But I am still getting +-62 some odd volts DC after the filter caps.Does that point at those being bad?Am I drawing completely incorrect conclusions here due to inexperience?I do see -62 and 62 volts on pins 15 and 14 of IC700 which according to the schematic should be reading something like -43.6 and 43.6 respectively..

author: xavier60
author_rank: Super Contributor
timestamp: March 02, 2019, 04:29:34 pm
content: Quote from: fedaalis84 on March 02, 2019, 03:49:46 pmThat is a bit difficult to follow. If the voltage on the collectors of Q762 and Q712 are the same at about -53V, it means that the overload detect circuitry isn't causing the shutdown. Voltage readings are valid only while the power relay is on.An overload protect detection will cause a voltage across R326, about 0.6V.A DC protect detection will cause a voltage across R323, about 0.6V.

author: xavier60
author_rank: Super Contributor
timestamp: March 02, 2019, 04:34:27 pm
content: Some amplifiers have a 4Ω/8Ω switch on the back that changes the main rail voltages.

author: xavier60
author_rank: Super Contributor
timestamp: March 02, 2019, 04:40:56 pm
content: Rectified and filtered 45 VAC will be close to 62VDC

author: fedaalis84
author_rank: Contributor
timestamp: March 02, 2019, 04:44:21 pm
content: Quote from: xavier60 on March 02, 2019, 04:40:56 pmThanks, OK - so that's not abnormal?I'll look for a switch. I suppose it could be set to something odd that would cause the main rail voltages to be about ~20V higher than they should be?I think I'm in a bit over my head knowledge wise. Will have to keep learning.

author: fedaalis84
author_rank: Contributor
timestamp: March 02, 2019, 05:56:12 pm
content: So I think I'm going to wait until I get the right-channel transistors in before worrying too much.The leads on the emitter resistor on the left channel read at about -20V, whereas on the right they read -4V (because the transistors are out, and not pumping any voltage).The busted emitter resistor from before was reading a few mega ohms between the left and center pins. Now it's a functional resistor there. So those -4 volts are getting over to Q762. Could be what's leading to the protect, and explain why a random cap, and desoldering the jumper did nothing. But that protect did not start until I put in the new emitter resistor...

author: fedaalis84
author_rank: Contributor
timestamp: March 03, 2019, 02:40:06 pm
content: Definitely the protector circuit causing the shutdown. JW463 runs to the protect pin on IC702 and should be reading 3.3V, but instead is giving -150mV. Slowly rises back to 3.3V once in protect due to a big cap in the area

author: fedaalis84
author_rank: Contributor
timestamp: March 03, 2019, 03:13:05 pm
content: Quote from: xavier60 on March 02, 2019, 04:29:34 pmSo, xavier60, if you're talking about only a 0.6V drop across R323/R326, and I'm looking at -150mV on JW463/Pin 87 on IC702 (Protect pin), should I be suspecting a short in one of Q323/Q324/Q325?

author: fedaalis84
author_rank: Contributor
timestamp: March 03, 2019, 04:00:09 pm
content: So I'm still seeing something strange with the main power rails.On R545, the resistor for the positive power rail, I'm reading about -20VDC. On R546, the negative power rail, I'm reading -62VDC.R545 should be giving me positive 62VDC, correct?Also, something weird... The positive leg of the bridge rectifier diode reads a steady 62VDC even after the AC is turned off by the relay in protect mode. It doesn't drop, it just sticks at 62VDC... Weird, no?

author: xavier60
author_rank: Super Contributor
timestamp: March 03, 2019, 05:05:42 pm
content: Quote from: fedaalis84 on March 03, 2019, 04:00:09 pmR545 must be open. The positive rail capacitor is staying charged because no current is being drawn from the rail. R545 and R546 function as fuses.Don't bridge out R545. replace it with the same value or something close and hope that there is no permanent overload present.

author: fedaalis84
author_rank: Contributor
timestamp: March 03, 2019, 05:10:32 pm
content: Quote from: xavier60 on March 03, 2019, 05:05:42 pmThanks!!R545 is reading at 0.00 ohms when I check resistance, is tha4 because it's still in circuit?Also, R546 is registering at 0.7 ohms, when its rated at 0.47, that seems pretty far out of spec?I'll try maybe replacing them both.

author: xavier60
author_rank: Super Contributor
timestamp: March 03, 2019, 05:18:33 pm
content: The charged up rail capacitor is likely to be causing a false reading, It will need to be discharged before you start handling the board to avoid further damage. Don't directly short out large capacitors. use a low value power resistor or anything with suitable resistance.

author: fedaalis84
author_rank: Contributor
timestamp: March 03, 2019, 11:35:27 pm
content: Quote from: xavier60 on March 03, 2019, 05:18:33 pmDefinitely the case. Reading high voltage on the capacitor side of the resistor & very little on the other.Replacements on their way.I think it might have been my fault. When I was measuring voltages on IC700 I think I may have briefly shorted pins 14 & 15 together...probably why R545 is toast.So I don't think that's what's causing the protect.

author: fedaalis84
author_rank: Contributor
timestamp: March 04, 2019, 09:22:38 pm
content: Found the source of the protect.Whenever I have the system set to output *just* to speakers A, or just speakers B, it will short out & go into protect - and I get DC on both output channels. If the system is set to output to both speakers A and B, it will not go into protect - output on left & right look good with +- ~50ish mV; and the relays also are closing the circuit successfully to the speaker outputs. Also does not go into protect when not outputting to any speakers.Something any electronics repair guy/gal worth his or her salt would've caught a LOT earlier... But - hey, this is myFIRSTever attempt at electronics repair . So if I can get this old hunk of junk up and running, I'm happy - and it was a success.Got some new relays on order. New 0.47 ohm resistors for between the filter caps and IC700... And still waiting on heat paste for the right-channel transistors, and a 100 ohm resistor to replace one that went bad on the right channel, and she should be up and running...Wish me luck!

author: xavier60
author_rank: Super Contributor
timestamp: March 04, 2019, 09:44:27 pm
content: I'm not sure about all of that. With one of the 0.47Ω open, that is a reason to have a DC protect detection.Some thought needs to be given to the possible reason for the output transistors failing. Like being overloaded by driving too low impedance or shorted speaker lead.The circuitry(2 transistors and resistors) that sits between IC700 and the output transistors has the function of controlling the idle bias current through the output transistors. This circuitry needs to be carefully checked on both channels for cracked joints and damaged transistors.A problem with this circuitry can destroy the output stage again.

author: fedaalis84
author_rank: Contributor
timestamp: March 04, 2019, 09:57:53 pm
content: Good call. I'll give that circuitry a check before firing her up again once R545 & R546 are back.I don't think it's the biasing transistors/resistors, because -VOUT2, +VOUT2, -VOUT1, and +VOUT1 on IC700 would read some ridiculous voltages when the relays were in a "shorted" state (i.e. just A, or just B). But I'm getting good voltage at the base (and emitter) on the left channel, and all of the VOUTs look good when the relays were in A&B.I'm fairly certain the original cause of the output transistors shorting out was a speaker wire short. When I re-did the speaker wires on the right speaker, they were very frayed and little strings of wire could have easily been touching.But I'm still weirded out by how the relays can be causing a short when just in A, or just in B though?? I popped them both out, and already ordered new ones, so I gutted one of them, and from the way the relay is setup, I'm not getting how it could cause a short...Learning as I go here.

author: fedaalis84
author_rank: Contributor
timestamp: March 06, 2019, 03:16:26 pm
content: Quote from: xavier60 on March 04, 2019, 09:44:27 pmI checked out the joints on Q701, Q702, Q751 and Q752 - and that looks OK.Q752 and Q702 both check out as they should for NPN transistors.Q701 and Q751 are giving an interesting reading. I get a diode in either direction between the base and emitter (on both 701 & 751), but only getting a diode between base and collector when the positive lead is on the base, and the negative lead is on the collector.Data sheet for Q701/751:https://toshiba.semicon-storage.com/info/docget.jsp?did=19351&prodName=2SA1162They're both *identical* on each side, so I'm going to go ahead and wager that everything is OK - also, I'm getting a resistance between base and emitter on 701 & 751 that is identical to R708/R758 respectively.So I think that all checks out good thereEdit -Lol, this would be why:

author: fedaalis84
author_rank: Contributor
timestamp: March 06, 2019, 11:05:07 pm
content: Well it wasn't the relays - turns out, when the system is in just speaker set A, or just speaker set B, it operates on the upper tap (~45VAC which is then rectified to ->>> ~60VDC). When it's in speaker sets A&B, or no speaker output, it operates on the lower tap (~33VAC which is then rectified to ->>> ~43.9VDC).When either (or both) of R545, or R546 are burned out, it causes a wacky voltage output from the VOUTs on IC700, which - when on the lower tap, does not cause enough voltage issues for the protect to trigger, but when on the upper tap - it does pass enough voltage through the protect circuitry to trigger a protect.Whew.OK.I went against your advice and jumpered R545 and R546 just for the heck of it... And I don't get protect regardless of speaker output chosen (WHEW).HOWEVER; I get good voltage on the VOUTs on the left channel on IC700 - but I'm getting the positive & negative rail on the VOUTs on the right channel. I'm going to GUESS that this is because I don't have transistors in circuit on the right channel (nor do I have R759 in) and I'm getting the positive & negative rail because the biasing transistors on the right channel are not functioning due to an open circuit???Any thoughts on that one?I don't want to toast a set of transistors to test (I already toasted an "extra" set today playing around with various things).Anything I could do to perform a test WITHOUT transistors in circuit on the right channel?All of the other circuitry there looks good.THANKS!

author: xavier60
author_rank: Super Contributor
timestamp: March 06, 2019, 11:57:40 pm
content: Quote from: fedaalis84 on March 06, 2019, 11:05:07 pmThat makes sense with the speaker A/B switching changing the rail voltages. It automatically functions as the 4Ω/8Ω switch I mentioned earlier.The VOUT pins are actually the Collectors of the NPN and PNP transistors which are the output stage of a rail to rail transcondctance amplifier that can be seen in most discrete audio power amplifiers designs.They source/sink a few mA and will pull to full +/- rail voltages if left open circuited.The function of the idle current bias circuitry is to clamp the difference between the VOUTs to a precise temperature compensated voltage that slightly turns on the output transistors, idle current.There must be a problem in this area.If both VOUTs happened to be both at about the same voltage either full + rail or Full - rail, this would be normal because of the missing output components causing the feedback loop to be open.Think of it as a very large op-amp.

author: fedaalis84
author_rank: Contributor
timestamp: March 07, 2019, 03:04:14 am
content: Quote from: xavier60 on March 06, 2019, 11:57:40 pmThanks xavier60!!Yes, the VOUT- and VOUT+ pins on the right channel are equal to the - rail and + rail respectively.Just to be sure, before toasting another set of transistors, I'll try pulling the left channel transistors out of circuit and take a look at the VOUT pins on that side. And if we're looking at normal voltage, I'll probably grab a new IC700. And if i see the positive and negative rails, I'll try tossing in all of the transistors.But starting to look like:A speaker wire short toasted R545, which in turn wonked up the biasing on the right channel, frying the transistors there.When I took out the right channel transistors, it didn't go into protect because it was set to output to speaker sets A and B, operating on the lower tap without enough power to trigger a protect on the left channel - looking as if it was simply the transistors that caused the problem...All of that doesn't explain why the right channel transistors fried out, but the left have successfully been protected by protect mode even throughout all my dinking around with it.

author: xavier60
author_rank: Super Contributor
timestamp: March 07, 2019, 06:48:46 am
content: Removing output transistors will open the feedback loop. Tack in a resistor from Base to Emitter of one of the missing transistor's pads to keep the loop closed. Use a 1K or else anything from 470Ω to 2.2K.

author: fedaalis84
author_rank: Contributor
timestamp: March 07, 2019, 01:49:33 pm
content: Quote from: xavier60 on March 07, 2019, 06:48:46 amVictory! Or at least a partial one.I don't have any spare resistors around, so I took a 2.2KOhm SMD resistor out of an old broken phone handset & jumpered it with some wires between the base and emitter of the PNP transistor on the right channel.Boom, I'm getting 2V on the VOUT- pin on that channel. It should be a small negative voltage. But I'm assuming because the VOUT+ circuit is open on that side, it's leaning toward the positive.In any case... I *THINK* I'm safe to put the transistors in! Wooo.Will give that a try next.

author: xavier60
author_rank: Super Contributor
timestamp: March 07, 2019, 01:53:15 pm
content: Did you find a reason for the large voltage difference between the VOUTs on the right channel?

author: xavier60
author_rank: Super Contributor
timestamp: March 07, 2019, 02:12:00 pm
content: If there really was a large voltage difference between the VOUTs on the right channel, a reason needs to be found even if the problem has seemed to have fixed itself.It is possibly the reason for the original failure of the output transistors.

author: fedaalis84
author_rank: Contributor
timestamp: March 07, 2019, 02:59:39 pm
content: Quote from: xavier60 on March 07, 2019, 02:12:00 pmNope. Not finding the reason. But there is definitely still a problem, I toasted another set of transistorsI am missing R759 (should be 100ohm) - I put in a ~300 something ohm guy. So not sure of that is the reason.Voltages were great - everything was great, no protect... Tried out some music on headphones - worked (but the music was scratchy and low volume).Pulled out the headphones, went into protect. Fried right channel transistors.Not sure what's up...

author: fedaalis84
author_rank: Contributor
timestamp: March 07, 2019, 03:12:26 pm
content: You must be right and there's a problem with the biasing circuitry on the right channel. But everything had checked out good on my basic tests.. At least, the transistors were functioning as expected (diodes in the correct direction, etc.), resistors were the correct values...I think I identified a starting point for an investigation:R778 is reading ~19kohm resistance(EDIT: resistance to ground)on the emitter resistor side, but anywhere from 200kohm-3mega ohm on the other side.On the left channel, R728 is reading ~47kohm on both sides, reliably similar on either side.These are "4.7 ohm" resistors, but with an inductor on the other side of the board it looks like.It's almost as if the inductor is connected on only one side of R778..Do you think that could jazz things up?

author: fedaalis84
author_rank: Contributor
timestamp: March 07, 2019, 04:26:49 pm
content: So I made a few more observations.Firstly - I think the weird resistance measurements on R778 were just because the positive rail capacitor had some charge in it. So I don't think that's an issue.I tossed a potentiometer in between the base and emitter of Q754, and have it sitting at around 1.8kohms. Both VOUTs are reading OK voltages.HOWEVER...I removed a toasted replacement of R759, so that part of the circuit is open... And the emitter on Q753 is reading +15-20VDC... Its only connection anywhere in circuit is to the collector via C768.So now I'm thinking C768 is leaking DC from the collector...?

author: fedaalis84
author_rank: Contributor
timestamp: March 07, 2019, 06:08:54 pm
content: Well I'm done for a while... I let some magic smoke out of IC700 today.Once all parts come in - I'll replace everything as it should be, and hope for the best.I think R759 is why the right channel continued to short out - I had put a ~380ish ohm TINY SMD replacement, probably below the 1/4W rating of the original 100ohm there. And that was fried afterIf it wasn't that, then it was IC700, which I'll be replacing anyway.I did my due diligence on the biasing circuitry prior to the output transistors, and after double checking, triple checking, that all looks OK.If it still doesn't work I think I just need to let it go, maybe keep it on a shelf until I've learned some more (and am willing to spend more money trying to fix it).

author: fedaalis84
author_rank: Contributor
timestamp: March 08, 2019, 09:27:30 pm
content: Looking like Q751 is not functioning properly actually. The diode readings from base to emitter are reading about twice as high as the left channel equivalent.Will try replacing & see what happens.

author: fedaalis84
author_rank: Contributor
timestamp: March 08, 2019, 09:40:02 pm
content: IC700 is out, but everything else is back in, except instead of transistors, I have 1K Ohm resistors from the base to emitter.I'm getting about 0.5V DC on the left channel, but up in the area of 3-4V DC on the right.

author: xavier60
author_rank: Super Contributor
timestamp: March 08, 2019, 09:57:34 pm
content: Quote from: fedaalis84 on March 08, 2019, 09:40:02 pmI would expect to see some negative voltage coming from the protection circuit. It should be the same on both channels. Did the Emitter resistors get damaged on the right channel?Do you mount the power transistors well?Thermal paste must be used. The transistors have to sit tight and flat against the mounting surface. The leads must be soldered last of all.

author: xavier60
author_rank: Super Contributor
timestamp: March 08, 2019, 10:02:25 pm
content: On second thoughts, I think it's stray leakage. Are you using extra soldering flux?

author: fedaalis84
author_rank: Contributor
timestamp: March 08, 2019, 10:06:40 pm
content: When I did have the right transistors in, I did Mount them with thermal paste and also use the little mica squares. And Soldered them in as the last step.The emitter resistors on both sides still check out good. The protect circuit absolutely does put out some negative voltage, it brings the protect voltage (should be 3.3V) down to -50 or so millivolts.I think the problem on the right channel is the biasing transistor Q751.I had pulled Q752 out of circuit and it tested fine. But it did have some thermal paste between the collector and emitter. I cleaned that off, put some fresh thermal paste on the back and remounted it.

author: fedaalis84
author_rank: Contributor
timestamp: March 08, 2019, 10:10:16 pm
content: Quote from: xavier60 on March 08, 2019, 10:02:25 pmYes, and I haven't cleaned it up in a while. The first few times I made some changes oh, I would brush it off with acetone. But as I started going to the back of the board more often, I started leaving the flux instead of cleaning it.

author: xavier60
author_rank: Super Contributor
timestamp: March 09, 2019, 12:31:25 am
content: When you get the parts, as a precaution, check the bias voltage before fitting the output transistors as before.If I have doubts, I temporarily fit 10 to 47 ohm resistors in place of the emitter resistors when the output transistors are fitted. They act as fuses and hopefully will protect the output transistors if there is a bias problem.Unless IC700 was faulty the whole time, it would be difficult for it to be damaged by other faults. The VOUTs source/sink a limited amount of current which makes them inherently immune to overload.

author: fedaalis84
author_rank: Contributor
timestamp: March 09, 2019, 04:37:10 pm
content: I am going to do a good cleaning of the board and see what I get. Without IC700 in, I'm pretty sure there should be ZERO voltage on any of the bases...Edit:Or maybe some negative voltage from the protect circuit like you say.

author: fedaalis84
author_rank: Contributor
timestamp: March 09, 2019, 04:41:39 pm
content: I am using some cheapo water soluble flux I got cheap on Amazon. Which I've read is maybe the worst type of flux if you don't clean it sufficiently, leaving behind a soapy, conductive residue if incompletely cleaned.

author: fedaalis84
author_rank: Contributor
timestamp: March 10, 2019, 03:45:22 am
content: I tested with a DC6V power supply, and current is leaking, from seemingly anywhere on the board to the bases for both transistors on the right channel. It does not leak to the bases on the left channel.Putting power at the biasing circuitry on either side gives similar voltage and current on both sides.So I suspect you're right, and too much flux and improper cleaning have caused some leaky current.I tried cleaning the board thoroughly with distilled water (my flux is water soluble and I have no deionized water on hand), followed up with 50% ISO (again, not a great option, but all I have on hand).The board is drying, but we'll see where that gets me.

author: fedaalis84
author_rank: Contributor
timestamp: March 10, 2019, 05:09:36 am
content: Still issues on right channel after drying. Same deal.0.6V on left channel, no current.Almost the positive rail on the right, about 6 microamps.Interestingly, one of my 6800uF filter caps is reading -70 to -80 microamps on the top metal part of the cap. The other 6800uF cap (and all other caps on the board) read 0 current. I also read -40V on the top.... that sounds kind of, not good..

author: xavier60
author_rank: Super Contributor
timestamp: March 10, 2019, 06:10:01 am
content: Although there should be no leakage, it is not enough to cause problems with the output stage biasing.Leakage around IC700 might cause DC offset problems, but will not damage the output stage.Leakage could affect the protection circuitry directly.

author: fedaalis84
author_rank: Contributor
timestamp: March 10, 2019, 01:59:30 pm
content: Quote from: xavier60 on March 10, 2019, 06:10:01 amThe voltage drop across base and emitter on Q751 is still out of whack compared to its left channel counterpart (~1V right channel vs. ~0.6V left. So I'm hopeful replacing that may yet resolve the issue.

author: fedaalis84
author_rank: Contributor
timestamp: March 10, 2019, 02:01:54 pm
content: With the filter cap issue, it sounds normal. Supposedly the negative leg of capacitors are generally not insulated from the aluminum can.

author: fedaalis84
author_rank: Contributor
timestamp: March 13, 2019, 09:56:11 pm
content: Still going strong on this bad boy.Just popped IC700 in today.Can't do any testing until I get the display wiring re-done. (I wound up messing up the flat cable, being impatient, and soldering in detatchable jumpers instead... but still need to finish that).Ohhh what a mess this beast is.But learning a bunch in the process.