Speaker - terminal connected to the AC line?
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From: Paul G. on 25 May 2010 11:47 On Tue, 25 May 2010 07:33:01 -0700 (PDT), "William R. Walsh" <wm_walsh(a)hotmail.com> wrote: .......[snip!}......... > >So that's got me thinking... > >Perhaps this serves to protect the hybrid module from such things? >It's my belief that the hybrid power amplifier is already something of >a delicate part. > >I've noticed already that this receiver also contains additional relay/ >outboard circuit protection against failure of the hybrid possibly >putting a power supply rail on a speaker. Earlier and other models did >not do that, instead relying on ineffective protection measures built >into the hybrid module. > >Maybe the Technics/Panasonic engineers were trying to improve the >reliability of their design? I've seen a lot of these receivers that >were dead, usually as a result of hybrid module failure. > Although I repaired a number of amps with discrete and module outputs, I don't have enough experience to say which are worse. The early discrete circuits were pretty vulnerable.... I don't remember any customers that that suggested electrostatic discharge as a cause for the amp's failure. Usually it was power on/off, or load abuse (shorting speakers, disconnecting while at full volume, etc.). I believe that ESD (electrostatic discharge) into the speaker line is not likely to bother the output circuit. The output devices are designed to withstand short bursts of current, usually in excess of 10-20 amps. The half microsecond or so 2-3 amp transient generated by the human ESD model is not likely to bother the output devices, since there is sufficient capacitance in the devices themselves and the associated circuitry to absorb significant charge. The output circuit is not likely to be in a high impedance state. The tiny DC current generated by scuffing across a rug, even though driven by a very high voltage would not affect the module, unless the module circuitry was in excess of hundreds or thousands of megohms. The components that are likely to suffer from static voltages are the power supply where it isolates the input AC line. The thousands of megohms insulation will build up a high voltage as the static current tries to get to ground. That's where the weird little resistor allows a path for the current to seek ground. As has been stated earlier, if your amp has a receiver in it, you can also get some very impressive static voltages (driven by very tiny currents) from external antennae . As you say, the newer amps have protection circuitry that senses overcurrent, etc, and shuts off the output terminals. I remember looking at the schematic of a cheapy 5.1 ch. Yamaha amplifier - it monitored about 10-15 voltages and currents, and the microcontroller would only energize the output if all the parameters were in the correct range. There were hybrid modules throughout this amp. I suspect the paranoia was not so much for the amp, but to protect the customers speakers. (There is a very negative reputation for an amp that blows a customers speaker!) I was quite impressed with the degree protection and control of this amp... you could even run diagnostic routines on it to check its performance, yet its price was around $200. The fault I found was someone hadn't tightened the screw that held one of the output modules to the heatsink, and its overheating would shutdown the amp at high output levels. ......[snip !]......... > >Fortunately, I do not think that this will be a problem. The unit >seems to operate safely in every regard. If anything, I will probably >be replacing the main filter capacitors as a precautionary measure. >The power supply section of this receiver runs hot and the original >filters are ~23 years old. They work fine now, but I do not want a >problem. > >William The little electrolytics throughout the amp are just as likely to fail as the main ones (maybe even more so), so if the main caps are cool, and your amp doesn't have 120 Hz hum in it, why bother? I would only do a total recap if the amp was something very exceptional. Your chances of screwing something up doing a complete recap are quite high. If the thing dies badly, you can always go scrounging on "big garbage pickup day" and get another Japanese amp that has been tossed out and will probably remain useful for many more years. Paul G.
From: GregS on 25 May 2010 12:17 In article <pan.2010.05.25.15.36.54(a)gmail.com>, Meat Plow <mhywatt(a)yahoo.com> wrote: >On Tue, 25 May 2010 15:05:59 +0000, GregS wrote: > >> In article <pan.2010.05.25.13.50.51(a)gmail.com>, Meat Plow >> <mhywatt(a)yahoo.com> wrote: >>>On Tue, 25 May 2010 02:30:29 +0000, Paul G. wrote: >>> >>>> Probably a guess, but I suspect the mystery part is a high value >>>> resistor as Phil stated. Its purpose is for "leaking" any DC static >>> >>>And Paul G wins the contest! >>> >>>Go claim your prize, a date with Phil Allison. >>> >>>The resistor is there to protect the amplifier from static discharge. >>>Said static could build up while wiring up the speakers or even after >>>they are wired scooting them around on the carpet. >> >> I think I have added resistors like that to my circuits. It can help but >> is not foolproof. A static charge to the amplifier metal will occur >> regardless of resistance to AC ground. It can help prevent transformer >> arcover. >> >> greg > >Nothing is foolproof. My reference was strictly static discharge via >speaker wires. Since the resistor is not in the schematic from what I >understand, it must be a factory fitting after a trend was spotted in >warranty repairs. Maybe the output device is sensitive to ESD augmented >by the output circuit design. I would go along with saying, it has nothing to do with output devices. it will discharge a DC differential in the amps circuit vs AC ground. It could do this and that, but could provide power off protection and transformer arc protection. Pretty much guessing beyond this. HV arc noise ? I had problems once with an 8 track capstan, had to add a static drain wire. greg
From: William R. Walsh on 25 May 2010 12:31 Hi! > Although I repaired a number of amps with discrete and module > outputs, I don't have enough experience to say which are > worse. I don't know how much experience it takes, probably more than I have. In any case, the majority of burnt amps/receivers that I've seen were hybrid-based. I've also heard others talk badly of the hybrids in terms of reliability. > As you say, the newer amps have protection circuitry that senses > overcurrent, etc, and shuts off the output terminals. I remember > looking at the schematic of a cheapy 5.1 ch. Yamaha amplifier - > it monitored about 10-15 voltages and currents, and the > microcontroller would only energize the output if all the > parameters were in the correct range. There were hybrid modules > throughout this amp. That does sound impressive. I haven't seen an amplifier with that much protection in place, at least not yet. Of course, the work I do is generally limited to stuff that I find in the trash or somewhere else and don't feel that I can pass up. What I find is usually stuff that's a few or many years removed from the current state of the art. I unintentionally tested the protection circuit in a Sherwood RX-4109 receiver the other day. I was moving a door and I snagged some speaker wires, pulling them out of the speaker and shorting the wiring. Its protection circuit worked exactly as intended. The sudden power off startled me (to say the least) but there was no lasting harm. I reconnected the wiring, moved it out of harm's way and turned the set right back on. > I was quite impressed with the degree protection and control of > this amp... you could even run diagnostic routines on it to check > its performance, yet its price was around $200. I credit the ever-falling price of microcontrollers for this sort of thing. (There was an interesting article about that on the Dan's Data web site recently.) > The little electrolytics throughout the amp are just as likely to > fail as the main ones (maybe even more so), so if the main caps > are cool, and your amp doesn't have 120 Hz hum in it, why > bother? Maybe as a pre-emptive strike? It had been my understanding that electrolytic capacitors operating in less than wonderful conditions were more likely to fail than those running in a cooler location or under less electrical stress. Here's the thing...I had wanted to find one of these little Technics receivers for a long time. The trouble is, every one I could put my hands on had been badly mistreated, blown up or both. I spent years looking without any luck. I'm not going to even try to justify my desire to have one. I just wanted one in nice working condition and that's that. Something about the overall design appealed to me. When I saw this one sitting at a public television and radio fundraiser sale, I just had to have it once I saw that it was described as "working". I didn't even argue much over the price. Now that I've got one in good working order, I guess you could say that I'm just a little...protective (?) ... of it. (Yes, I realize how that may sound dumb. It is, after all, just a "thing".) William
From: Meat Plow on 25 May 2010 13:36 On Tue, 25 May 2010 16:17:18 +0000, GregS wrote: > In article <pan.2010.05.25.15.36.54(a)gmail.com>, Meat Plow > <mhywatt(a)yahoo.com> wrote: >>On Tue, 25 May 2010 15:05:59 +0000, GregS wrote: >> >>> In article <pan.2010.05.25.13.50.51(a)gmail.com>, Meat Plow >>> <mhywatt(a)yahoo.com> wrote: >>>>On Tue, 25 May 2010 02:30:29 +0000, Paul G. wrote: >>>> >>>>> Probably a guess, but I suspect the mystery part is a high value >>>>> resistor as Phil stated. Its purpose is for "leaking" any DC static >>>> >>>>And Paul G wins the contest! >>>> >>>>Go claim your prize, a date with Phil Allison. >>>> >>>>The resistor is there to protect the amplifier from static discharge. >>>>Said static could build up while wiring up the speakers or even after >>>>they are wired scooting them around on the carpet. >>> >>> I think I have added resistors like that to my circuits. It can help >>> but is not foolproof. A static charge to the amplifier metal will >>> occur regardless of resistance to AC ground. It can help prevent >>> transformer arcover. >>> >>> greg >> >>Nothing is foolproof. My reference was strictly static discharge via >>speaker wires. Since the resistor is not in the schematic from what I >>understand, it must be a factory fitting after a trend was spotted in >>warranty repairs. Maybe the output device is sensitive to ESD augmented >>by the output circuit design. > > I would go along with saying, it has nothing to do with output devices. > it will discharge a DC differential in the amps circuit vs AC ground. It > could do this and that, but could provide power off protection and > transformer arc protection. Pretty much guessing beyond this. HV arc > noise ? I had problems once with an 8 track capstan, had to add a static > drain wire. > Could serve dual purpose. Guessing is good, stimulates the mind. I could go along with the arc noise. Snapping while it was on with a good jolt could cause speaker protection to kick in or pop a midrange or tweeter.
From: Paul G. on 25 May 2010 19:39
On Tue, 25 May 2010 09:31:40 -0700 (PDT), "William R. Walsh" <wm_walsh(a)hotmail.com> wrote: .....[ snip! ]...... > >Here's the thing...I had wanted to find one of these little Technics >receivers for a long time. The trouble is, every one I could put my >hands on had been badly mistreated, blown up or both. I spent years >looking without any luck. > >I'm not going to even try to justify my desire to have one. I just >wanted one in nice working condition and that's that. Something about >the overall design appealed to me. > >When I saw this one sitting at a public television and radio >fundraiser sale, I just had to have it once I saw that it was >described as "working". I didn't even argue much over the price. Now >that I've got one in good working order, I guess you could say that >I'm just a little...protective (?) ... of it. (Yes, I realize how that >may sound dumb. It is, after all, just a "thing".) > That's funny.... when I was visiting my daughter in Ottawa, I spotted a bunch of stereo stuff piled up for garbage pickup. I grabbed a Techics (!) "new class A" amp, and a Technics CD player, and stuffed them in the trunk of the car for the 1200 mile return trip. My wife was really pissed about the lack of space in the trunk, but hey, the stuff was free. The CD player worked just fine (still does), and the amp worked OK, but had a noisy cooling fan. I put a much quieter fan in it, and redesigned the air cooling flow, and modified the circuitry to give it proportional control of the fan. Works great.... you have to have your head right against the amp to hear it. I find it depressing to see the stuff that gets tossed out. Around here (Nova Scotia) you can't throw electronics stuff out, it has to go to a special recycler. Now you can't get your hands on old electronic throw-aways, the recycler refuses to let anyone have any of the stuff so you can repair it (and do the ultimate recycling where you bring it back to life). I have had electronic equipment that I've tried to preserve and gone to extraordinary lengths to keep them running, even though I KNOW it's not worth the effort. It's like you want to cheat the grim reaper of electronics. You kinda wish someone would do the same for you..... Paul G. |