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Old 20th July 2004, 09:54 PM   #1
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Default Mains Transformer Question

I bought a broken HK AVR-65 off of ebay for like 35 bucks US and cannibalized some parts off of it for other projects.

But I still have the mains transformer and was thinking that, even though it's an E+I, I could save myself some major cash by using it in an amp. The problem is, I don't know what the specs on the transformer are. The amp it came out of was a 65w/channel @8ohm, 5-channel amp. So I'm sure the VA is pretty high, but I don't know what the secondaries are.

I've searched online for the specs, but I can't find them. Does anyone happen to know the specs on this trans? If not, what's the easiest way to test it? I'd rather not build an entire power supply to find out what the secondaries are.

PS. I've tried searching the net using every number code on the transformer and came up with nothing.
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Old 22nd July 2004, 09:23 AM   #2
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Just plug it in the mains and measure the secondary AC voltage! Bear in mind it will drop a few volts when fully loaded.
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Old 22nd July 2004, 10:07 AM   #3
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Then multiply the ac voltage by 1.41 to figure out what the dc voltage will be after the power supply.
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Old 25th July 2004, 03:00 AM   #4
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Quote:
Just plug it in the mains and measure the secondary AC voltage!
I thought about that, but I've never done it before and I wanted to make sure I wasn't going to blow it up or anything. I'll try that... which brings me to stupid question number 2:

The transformer has no markings indicating which side is the primary or the secondary. On one side there are 6 wires (two red, two blue and two black). The other side has 5 wires (two yellow, two black and one blue). So how do I know which side is primary? Will it hurt the trans if I put the mains on the wrong side temporarily? I would think that it would... but then again, what do I know?
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Old 26th July 2004, 06:46 AM   #5
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Mains wire is usually the one with the most resistance, therefor the thinnest wire.


If your going to do testing, just be sure to hook up a ~3A or so fuse in series with the wire your testing. Not a glass fuse or anything, but a heat triggered one or something, same as in your house fuse box.
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Old 27th July 2004, 06:18 PM   #6
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Okay, I'll give it shot. Thanks to everyone for all your help.
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Old 27th July 2004, 06:38 PM   #7
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If they follow convention, the black leads are the primary. And it may be a split primary, i.e., one that can be wired in parallel for 220 or in series for 110.

Before hooking up any voltages, use an ohmmeter and trace out what's connected to what and at what resistance. Draw out a little map. Then pick the highest resistance winding, connect a signal generator across it (set the gen for 50-60 Hz and a few volts output), and measure the voltages on the other windings so you can determine ratios. The ratios will scale to mains voltage.
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Old 30th July 2004, 08:05 AM   #8
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Actually, I just emailed Harmon Kardon and they sent me a complete service manual in .pdf. Go figure... It obviously didn't tell me the voltages or anything, but it mapped out which wires were which. It seems that it has 3 sets of secondaries for the various circuits in it. I must say, I was kind of surprised that they so readily sent it to me though.

Anyways, I'm still going to practice what you all have suggested, so I know what to do if I run into this problem again. Thanks again.
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Old 30th July 2004, 05:10 PM   #9
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Default Voltages

If they gave you the DC voltages after the rectifiers, the AC voltage is likely .707 x the final DC voltage + a volt or so for rectifier loss. Just apply some power supply theory in reverse.
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Old 3rd August 2004, 02:54 AM   #10
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Well, I've looked thoroughly through the 89 page manual and found the voltages for all three secondaries. #1 is +/- 50.3v after the regulators and #2 is +/-47.6v after the regulators. I don't really care about #3 because it's low voltage for the digital components.

However, it doesn't give a current rating or VA for the trans. The service manual shows that a 6 amp fuse is used on the mains line. So am I correct in assuming that 6 amps is the current rating for the trans?
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