Speaker - terminal connected to the AC line?

On May 24, 7:35=A0am, "William R. Walsh" wrote: > Hi! >

Are you sure of that? I just looked at the schematic of my Technics SA-160 receiver and there it is. R791 3.3Meg 1/2 Watt from the neutral side of the power input to ground (polarized power plug). This _may_ happen physically near the speaker terminals simply because the power and speaker switches are grouped together. On the schematic it's shown on the primary side of the power transformer, not at the speaker connections.

G=B2

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.

Hi!

It's as good as anything to me. It certainly does not show up in the schematic, nor the parts list.

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.

It's not...there is only a two wire, non-polarized cord set.

I don't have any reason to believe the resistor itself is bad. The front of the receiver and all of its controls are plastic.

The service manual for the set explains tests that can be run to determine if the leakage is within specification. It would definitely appear to be well within specifications.

As with you, I can usually feel a tingle from even very low current leakage sources. I much prefer using test equipment to resolve any uncertainty, anything else could result in a nasty surprise at the least. I have not noticed anything from this set, and have no reason to believe there is any problem.

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

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.

On Tue, 25 May 2010 07:33:01 -0700 (PDT), "William R. Walsh" wrote:

......[snip!}.........

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 !].........

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.

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

Hi!

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.

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 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.)

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

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.

....[ snip! ]......

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.

On 5/25/2010 4:39 PM Paul G. spake thus:

Hey, my current amp is one a'them Technics "new class A"s, a SU-V6X. What's yours? Got mine really cheap years ago at a Starvation Army store.

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

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On Tue, 25 May 2010 18:49:19 -0700, David Nebenzahl wrote:

Mine's a SU-V7B. I imagine there's not a lot of difference... I used the schematics for SU-V98 to change some of the circuitry. That was the only close schematic I could find on the internet. It wasn't quite the same, but the reference designators (like R520, C123) were good enough to locate things. The original fan was irritatingly loud... I can't imagine how anyone could put up with it. It uses a hybrid module to drive the output, and I imagine only much earlier models would use discrete transistors. I found it to be a surprisingly powerful amp for class A, but their "new Clas A" just slides the biasing up and down so that at low power it's still class A but biased low enough to stay cool. There is an art to doing that, and keeping the system well behaved and linear. I imagine there could be problems if the amp is biased for quiet music, and a sudden loud passage comes along that requires fast changing the operating point. There are probably a good number of patents that relate to that bias circuitry. I've had no complaints about it, except for having to replace the on-off switch (a low power switch that controls a relay which switches the AC line). Because of the weird shape of the switch, and it being alternate action, I had to jury-rig a small toggle switch with an extension on its handle. I used ductape to channel the air flow over the heatsink, to improve the cooling. Running full output, the heatsink doesn't get overly hot, and the exhaust air is quite warm. The fan I used was a

12v small quiet fan, similiar to those used on smaller CPU's. Typical for much electronics, it's the mechanical stuff that seems to fail first, or isn't properly designed to last.

Paul G.

On Sun, 23 May 2010 23:03:29 -0500, "William R. Walsh" put finger to keyboard and composed:

That gives me an idea for testing subwoofers ...

Connect each speaker terminal to the active and neutral of a mains supply. The frequency is not critical -- either 50Hz or 60Hz is fine. A sinusoidal 240VAC source would be preferred, but 120VAC will do, too.

If you damage your speaker, don't be discouraged. In fact you may need to test quite a few units before you have weeded out all the inferior ones, especially those used in car audio.

- Franc Zabkar

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On 5/25/2010 9:25 PM Franc Zabkar spake thus:

Thank you. I love that! Maybe I can convince my homies around here to implement this new testing methodology. We could set up a free testing station on the main drag around here.

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

Cerwin Vega used to demonstrate one of their professional low frequency audio drivers by plugging it into 120VAC line.

Hi!

Now come on, haven't you watched enough Youtube to know what happens when you do that? ;-)

Interestingly, the very idea of connecting a speaker to the AC line is mentioned early on in my copy of the ARRL's "Now You're Talking" book. They tell you not to do so, and I suppose that there are some people out there who need to be told that...more so because they'd hurt or kill themselves in the process.

William

Hi!

I have thought about doing something like that with this unit, as it does get hot. I don't know if the circuit used is the "new class A" or not. The receiver itself makes no mention of this feature, so I'd guess not. There's not much room to add anything, at least not at a point where I feel it would be close enough to help cool the power supply as well. The last thing I would want to do is add a hum or other annoying problem by way of design "improvements".

On the other hand, it has lasted over 20 years just the way it is. I just don't like it when stuff runs that hot.

As do I. Most recently I had to work hard to force myself not to pick up a microwave oven. I didn't want or need it as a microwave--instead I was hoping to harvest magnets, motors, control board and maybe the case metal. Very little goes to waste around here.

I think they're afraid of liability or something along those lines. I used to know a few recyclers, including one who would let me wander all over the place, pick out what I wanted and pay him some small price for it. Most of the others were at least somewhat receptive to my rambling around when I told them that I knew to be careful and that anything I took home with me was my problem--and the last that they would hear of it. Most of what I picked up was computer equipment, and the education I received was worth far more than the price I paid.

The same with junkyards...although I did not take anything home from there because I certainly didn't have the space or time. I spent a lot of time walking through those, looking at the things people had (for whatever reason) cast away as junk. At least some of those cars would have needed nothing more than a battery and some cleaning to be fine once again. Heh...try doing that now. Most junkyards won't let you anywhere nearby and the "old country junkyard" is a thing of the past for the most part, taken away by tightening environmental regulations.

Exactly. (Of course, there are also the devices that just seem to run and run forever, with little to no maintenance whatsoever.)

Longevity does run in my family, so I might do alright there. The most ready example is that of my grandfather, who at 96 does exactly as he pleases.

William

On 5/25/2010 8:29 PM Paul G. spake thus:

Fan? What fan? Mine has no fan.

Yours must be a higher-powered unit than mine.

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

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On 5/26/2010 6:51 AM William R. Walsh spake thus:

I see nothing gets by you, William.

Now think about *why* Franc made this particular little joke. Ah, now you're getting it.

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

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