Mains power puzzle

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** Exactly what I suspected, till I opened the case and took a good look.

... Phil

It uses rectfied mains for the power stage and the inrush current for that is not limited by anything other than two diodes and the fuse. Being a base amplifier it has insanely big capacitors in the PSU.

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Regards, 
Martin Brown

Martin Brown wrote in news:qrqubd $195q$ snipped-for-privacy@gioia.aioe.org:

Did you not mean "Bass" amplifier?

The moment the on switch is thrown, IF the power signature is at or just arriving at its peak, the inrush will be at max and can pop a fast blow 'normal' line fuse.

'Slo-blo' fuses would be spec'd.

The plot thickens...

What about a DBT ? Might tell you something.

What voltages do the meters source?

"Tom Del Rosso" wrote in news:qrr8bc$1c3$ snipped-for-privacy@dont-email.me:

Ohmmeters have their own stimulus voltage for their test.

The meter giving different readings based on polarity must mean that more than a transformer primary winding circuit is in that test loop.

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** Ohm meters are not voltage sources.

The current is much higher with the analogue meter - about 50mA.

..... Phil

Analogue (and digital) Ohm meters do source a reference voltage, how else can they read resistance? Which is why they work well for showing forward voltage drop on semis at the lowest of the resistance ranges...

John :-#)#

Yes, a voltage source in series with a reference resistor. I think he's looking at it as a current source (with a parallel reference resistor). I prefer your analysis but Phil lives upside down.

Hi,

I,ve seen US toroidal transformers saturating on

50 Hz mains, especially on inrush, as it should store 20% more energy on 50 Hz than 60 Hz.

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-TV

Incidentally I was refurbishing a Heathkit capacitor-checker a while back but made a wiring error; the original builder had built it properly and installed the correct slo-blo fuse ~50 years ago and it sat around for that time waiting for the day for me to come along and blow it and have it do its job.

Thanks, original-builder-guy! Be kind to the future, who knows how long your work might last

if the question is about the random resistance readings and not about the inrush current, then:

I would say the Ohm meter is oscillating when connected to the inductive load.

I have seen this odd behavior with DVM's once or twice. Never seen it with the Simpson 260.

m

My guess is more pedestrian, there's a rectifier and filter and some bullshit on the primary side because they were too cheap to get a transformer with an extra low-voltage winding to run some intrinsically-isolated things like a fan or maybe some LEDs

Or the tube heater if it has a tube front end

The 120 A peak current is 24 times fuse nominal rating. It was surprisingly hard to find blow times for various overload conditions. One manufacturer quoted 0.5 to 5 mains cycles for 10 times overcurrent, so at 24 times overcurrent it is most likely closer to a half cycle. After the toroid magnetic peak then the storage capacitor charge current extends to many cycles, so no wonder, if the fuse sometimes blows.

That 120 A peak current depends of the impedance o the feeding network, i.e. in-house wiring, wiring from distribution transformer to house and slightly on the distribution transformer and feeding medium voltage network impedance.

In 230 V countries, the distance to the distribution transformer can be several hundred meters and in a big house the wires can be quite long. The nearly short circuit inrush condition at amplifier turn-on can cause a significant voltage loss in the feeding impedance and hence limits the inrush current. Thus the amplifier fuse may blow frequently when the amplifier is used in one location, while the fuse never blows at an other location with longer mains lines.

Connect an incandescent lamp at the same socket as the amplifier and observe how much the lamp dims momentarily due to the inrush peak. This will give an indication of the feeding network impedance.

Amazon shows the T5A 250V as a fast-blow type, and picture shows infinitesimally-small wire.

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 Thanks, 
    - Win

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** Different wording now.

My analogue meter uses 2 x AA cells for the ohms ranges - so open cct voltage is circa 3V. The meter (clone of a Simpson) reads current flow with a scale calibrated in ohms. 30 ohms corresponding to 1.3V

The DMM used has an open cct voltage of 1.25V on the 2000 ohm range.

AFAIK, DMMs use voltage ratio compared to an internal reference resistor so are very accurate.

..... Phil

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** No - its entirely conventional.

..... Phil

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** Got news for you. Practically ALL iron core supply frequency transformers *saturate* at switch-on and the worst point in the AC wave is near a zero crossing.

That's why engineers invented "soft start" systems for them.

Whole topic on its own.

.... Phil