I have a Technics SA-929 stereo receiver that is in good working order. However, when I was inside it replacing a burnt out light bulb and repairing the protection relay, I noticed that there is a wire coming from the printboard that holds both the AC power input and accessory outlet connections going to the "speaker -" connector for the "B" speaker terminals.
There is a resistor inline with the wire coming from the AC wiring board.
I've never noticed anything like this in a stereo receiver before. As the previous owner seems to have been very meticulous about packing it up, I believe the non-polarized power cord set is original to the unit.
It wouldn't matter so far as I can tell, the line cord is *not* polarized. So it could be either one, depending upon which way the plug is inserted into the outlet.
I would not dare to call myself an expert, but I think it is important to do what I can to point out that things can usually be fixed instead of just thrown away. I'm definitely learning...
I have the complete service manual for the unit. However, it doesn't seem to show any connection between speaker - and the AC line.
That's not to say it isn't there. I have yet to spend a lot of time looking at it, as right now I'm dealing with a bat that is somehow in the wall of my house.
There's no need to kill it. Simply leave all the doors and windows open at night (if possible), and it will likely find its way out, hopefully before biting everyone in your family and infecting them with incurable rabies that will cause them to die in unspeakable agony.
I'd prefer not to kill the poor thing if I can help it.
The trick is to find out where it got in, because this is the second one. (The first either came out or died in a different wall. I think it got out, as I never smelled anything bad.)
Best not kill it. I've just - 10 minutes ago - finished reading an article in my Sunday newspaper, about a mystery disease that is sweeping through the bat population of North America, and decimating whole colonies. They think that it might not be a disease as such, but a fungal infection which leaves a white deposit on the noses of the bats. Apparently, bats are responsible for consuming millions of tons of insects which do harm, and if the bat situation becomes serious enough in terms of loss of numbers, it could have a devastating effect on the balance of the ecosystem.
Back to the resistor question. Such resistors, mostly with no immediately discernable purpose, are often to be found strung around the insides of amplifiers and hifis. You sometimes find things like a metal bracket that for whatever reason, the manufacturers didn't want hard grounded, connected to hard ground via such a resistor. Could it be something like this, and the connection point on the speaker connector board, was just a physically convenient point to hook it to ?
As to finding it on the schematic, it might well not be shown where you expect it to be. I have seen these resistors shown as a completely isolated entity, within dotted lines, hidden up a corner of the diagram somewhere. You might find it shown on the layout diagrams, or the internal wiring diagram or block diagram though.
Actually, I adore bats, but couldn't resist the temptation to play on people's prejudices.
Bats will die fairly quickly if they don't have food, so you want to get the little critter outta there promptly. In theory, opening up everything might work, as the bat might have come in tru de window.
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People have been aware of this problem for decades. Though bats do have a higher percentage of rabies than other mammals, it hardly ever transferred to humans unless they touch a sick animals.
Bats are our friends. They kill tons and tons of inset pests. Love 'em.
According to this article, the current white nose problem, is a new one, not a decades old one. The same fungus has also been found on bats in Europe, but it doesn't appear to kill them, so they are thinking that maybe the problem has existed over here before, and the bat population is immune to the effects. They think that the white powder acts as an irritant, and keeps waking the bats during their hibernation period, and that this results in premature useage of the fat deposits that they have built up to see them through, and thus, they die of starvation.
On Mon, 24 May 2010 07:33:52 -0700 (PDT), "William R. Walsh" 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 voltage that could build up in the circuit. That would probably happen if you were connecting a speaker up, and you were charged up (could be anywhere from a few volts to tens of thousands of volts). Walking on a nylon carpet in a dry house (less than 20% humidity) can generate enough static electicity to jump a gap of more than a centimeter. In northern Canada during winter, I've generated 2-3cm sparks this way. The DC voltage or the transient current may stress or damage components. Touching the case or metal knobs of your amp when you are charged, may also stress components in your amp, especially if the case isn't grounded (has a 2-wire cord). The resistor allows for a discharge path to the AC line, so that an excessive high voltage is not impressed across the insulation between primary and secondary of the power transformer/switcher. The interwinding capacitance also allows transient current flow, that reduces the instantaneous voltage difference between primary and secondary. Without the wierd little component, your power transformer/switcher would have to withstand up to 10-50kv. That's $$$$$! Typical transformers are rated 2-4kv. The DC currents that are generated while you build up a charge (scuffing your feet across a rug) are quite small, in the nano- or micro-amp range. However, once you are charged (humans have about
150-300pf self capacitance) to many kilovolts, you can make a fairly nasty zap. If I remember correctly, the human body model for eltrostatic discharge has about 1500 ohms effectively in series with a few hundred picofarads. For testing equipment, that capacitance is charged to 3-5 kv. The transient current is quite significant! The risetime is quite small, and transient currents can induce voltages in places you wouldn't expect. Sometimes you also find a ceramic capacitor in addition to (or often instead) of the resistor. I've seen values of around .001uf, rated at several kv. Be very careful replacing these components.... should that capacitor or resistor fail, you could die. You should be very concerned about the reliability of these parts. When you discharge a current into the electronic circuit (it ultimately heads for ground) you create a circuit between your body capacitance (200pf), body series resistance (1500 ohms), and the power transformer insulation in parallel with the protective resistor/capacitor. You can calculate the voltages applied to the power transformer or switcher if you know winding and stray capacitances. Colour TV's and monitors are a problem, since you can get discharges off the front of the screen. That introduces a similiar current to ground that must be dealt with. Switched power supplies will have similiar issues plus dealing with protecting their control circuits. Of course the "protective" resistor or capacitor is chosen to limit the "leakage current", should you touch the unit and be at ground potential. That current depends on your electrical code.... values are from a few microamps to a few hundred microamps. For me, I can sense tens of microamps, and it starts to tingle around half of a milliamp (AC 60Hz). I can't feel DC until quite a few ma., then I sense it as a muscle pulling. The protective resistor/capacitor is to both save your amp from you, and you from your amp. It protects you in case you over-stressed some part because of your charge, and that overstressed part breaks down and applies AC to the circuit and you. It protects your wallet from other non-lethal circuit failures. It appears to give you some risk in the form of a small leakage current. If you doubt the component, measure the AC current (not voltage) between amp and ground, for both polarities of the plug (if you have a choice of polarity). Put a resistor (say about 10kohm) in series with your meter so you don't blow your meter up should the case or amp circuit be live to ground. Any current greater than a few hundred microamps, and I'd retire your amp.
** Note that the unit is a * receiver * therefore it has antenna sockets on the back.
A rooftop mounted antenna can acquire very high DC voltages on a dry, windy day - more then 10kV.
The PROBLEM is with the insulation of the transformer in the PSU of the receiver - the primary side is linked to the AC supply so does no "float" up with the rest of the unit when the antenna becomes charged.
If no leakage path exists to drain away the charge generated by wind on the antenna, the resulting 10kV plus voltage difference will stress the insulation in the AC tranny until is flashes over inside, making a small spark and associated EMI which may be audible during FM or AM reception.
More importantly however, repeated sparking will cause the AC transformer's insulation to eventually fail and the whole unit becomes live !!!
A resistor of about 4.7 Mohms to 10 Mohms is sufficient to keep DC voltage build up on the antenna connection within safe limits and poses no shock hazard to uses despite one end connecting to the live AC conductor.
BTW:
Such resistors are of a special high voltage type approved for this purpose and should never be replaced with general purpose ones.
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