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Old 26th September 2007, 08:28 AM   #1
kyrie48 is offline kyrie48  United States
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Default HEAT; transformer vs. transistors???

I just picked up a HK PA2000 100 wpc amp, and it seems to be putting out more heat from the transformer than the output transistors. Is this normal? If not, what would be the likely cause?

TTYL, kyrie48
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Old 26th September 2007, 02:41 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
some options:
drawing too much current from the transformer.
shorted turn around the core.
shorted turns in some of the windings.

Disconnect the output from the bridge rectifier to isolate the PSU and amplifier from the transformer.
Check if the heat is better or worse.

Disconnect the rectifier from the transformer and check the AC voltages on the various secondary windings.

Inside a dummy fuse insert a 10r resistor.
Insert the dummy fuse into the mains fuse holder.
Measure the AC voltage across the dummy fuse. CAREFUL you are working at mains voltage now!!!!
Calculate the no load current in the primary.
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Old 26th September 2007, 03:09 PM   #3
Nordic is offline Nordic  South Africa
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My transformers run at between 50 and 60 degrees after a day of playing (amp gets switch on in the morning, and off when I go to bed) is there any reason to get concerned over this?

My heatsinks are pretty large considering a single output pair, and slowly build up heat throughout the day, reaching maybe 5 to 10C above ambient.... transistors are quite cool to the touch though.
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Old 26th September 2007, 03:20 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Nordic
My transformers run at between 50 and 60 degrees after a day of playing (amp gets switch on in the morning, and off when I go to bed) is there any reason to get concerned over this?
I suspect this is well inside the manufacturer's spec, but that still seems very hot, too hot to rest your fingers on. What ambient is inside the amplifier?
If the external/surface temperature is 50degC to 60degC then what are the internal temperatures like?
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Old 26th September 2007, 04:14 PM   #5
Nordic is offline Nordic  South Africa
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I used thumb thermometer.... i.e. it got hot, but not so that I have to remove my finger from it, maybe I got the scale a little wrong then. Amp is biased at 1.5A and powered off +&- 35V.
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Old 26th September 2007, 11:28 PM   #6
gni is offline gni  United States
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Downloaded the User Guide.

1.5A through each output resistor? Wow!. . . is that total from all
outputs or just one?

4 x 45 watts into 8 ohms
2x100 watts bridged into 8 ohms

Make sure your load isn't too low if using bridged mode.

Maybe the previous owner bumped the bias up. . . the unit is rated
at 260W/120VAC - that's about 2.2A at the primary side. . .


Transformers do seem to build in temperature over the course of
a good day use. . . big thermal mass.
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Old 27th September 2007, 12:50 AM   #7
kyrie48 is offline kyrie48  United States
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Thanks all!!
And to AndrewT for the test method. I'd mess up something if I tried that, so I'll have to take it to a Tech.
I thought it had something to do with too much current, but don't know why the heat sinks are cool even with no volume/signal. Without the current going out the outputs, how can the transformer be getting so hot?

gni, it does this even with no speakers hooked up.

TTYL, kyrie48
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Old 27th September 2007, 07:26 AM   #8
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If the transformer is being run at too high a flux density due to design issues or high line voltage it can actually run with a high primary excitation current with no load. The power company gave my neighborhood a new pole transformer (right at the edge of my property), and till they adjusted things a bit, my line voltage was up around 125 VAC. This may be enough to push marginally designed transformers into soft saturation.
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Old 27th September 2007, 08:52 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by wrenchone
If the transformer is being run at too high a flux density due to design issues or high line voltage it can actually run with a high primary excitation current with no load. The power company gave my neighborhood a new pole transformer (right at the edge of my property), and till they adjusted things a bit, my line voltage was up around 125 VAC. This may be enough to push marginally designed transformers into soft saturation.
It would be nice to know the primary current. But we cannot afford to lose a member.
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Old 27th September 2007, 01:06 PM   #10
gni is offline gni  United States
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Something is very wrong then. . . .they have current meters that
clip around wires. . . give you an idea. . . light bulb test can give
an indication. . . bright bulb = max current for bulb. . . 70W / 120VAC
bulb = just over 500mA

Swap transformers. . . might be cheaper than taking it to a tech.

Would love to see some pictures of the inside!
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