| Electronics > Beginners |
| Question about the function of a capacitor in a circuit |
| << < (5/10) > >> |
| fedaalis84:
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. |
| fedaalis84:
--- Quote from: xavier60 on March 04, 2019, 09:44:27 pm ---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. --- End quote --- I 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=2SA1162 They'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 there Edit - Lol, this would be why: |
| fedaalis84:
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! |
| xavier60:
--- Quote from: fedaalis84 on March 06, 2019, 11:05:07 pm ---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! --- End quote --- That 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. |
| fedaalis84:
--- Quote from: xavier60 on March 06, 2019, 11:57:40 pm --- --- Quote from: fedaalis84 on March 06, 2019, 11:05:07 pm ---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! --- End quote --- That 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. --- End quote --- Thanks 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. :wtf: |
| Navigation |
| Message Index |
| Next page |
| Previous page |