Amplifier problem, 1.5A Bias current ?! - diyAudio
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Old 21st March 2012, 02:44 PM   #1
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Default Amplifier problem, 1.5A Bias current ?!

Okay so I recently bought an old 70's amp made here in New Zealand by National/Panasonic. Quite a decent looking amp, large transformer, steel chassis, nice wooden cabinet and aluminium front panel, typical styling of the era etc. Turned it on and apart from a slight mains buzz (yet to be investigated, related problem maybe?) it seemed to work fine and sounded nice.

Then I noticed its temperature. Just being turned on and idle, the output devices started to get very hot, too hot to touch. I measured the voltage over one of the 0.45 ohm emitter resistors, and measured .67V, that works out at almost 1.5A idle current! I turned it off immediately, and started to examine the board for a bias trimpot. I find where one would normally go, but instead had two fixed resistors installed in place of a pot. Funnily enough, the pcb was designed to use two discrete resistors or a trimpot, each having their own dedicated through holes, so it was made like this at the factory.

I found a 5k trim pot that fitted in the space, and replaced the resistors and installed the pot. Using the pot I was able to bring the idle current down to around 1A but no lower.

Both channels were set up the same and exhibited the same behaviour, would there likely be some fault in this amp or is it possibly a class A amplifier? I can't find any info or schematics on the net at all, so am not really sure where to start looking other than removing and testing each component etc. Its quite obvious that its producing much more heat than it was designed to. I have looked and could not find a bias diode, so its probably using a transistor. If the bias transistor blew, would it maybe cause similar symptoms? Any help on this would be greatly appreciated.
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Old 21st March 2012, 03:32 PM   #2
Bone is offline Bone  United Kingdom
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Class A bias!
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Old 21st March 2012, 05:09 PM   #3
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With both channels the same I would suspect it's class A or A/B and biased well into A. So why do you say "Its quite obvious that its producing much more heat than it was designed to."? Does it not have sufficient heatsink? If it is class A, I would expect it to have a lot of heatsink. Another clue might be high wattage or current (fuse?) input rating compared to power output watts rating.
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Old 21st March 2012, 05:13 PM   #4
RJM1 is offline RJM1  United States
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Does it have a model number?
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Old 21st March 2012, 05:44 PM   #5
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Quote:
Originally Posted by cyclecamper View Post
So why do you say "Its quite obvious that its producing much more heat than it was designed to."?


Have Panasonic ever designed an amplifier in real class A?
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Old 21st March 2012, 05:48 PM   #6
AndrewT is offline AndrewT  Scotland
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Is it possible you have 230Vac mains and the amplifier is set to receive 115Vac?
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Old 22nd March 2012, 04:19 PM   #7
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Thanks for your replies. It is definitely made for 240 VAC, as it was made in New Zealand. The heatsinks consist of a flat piece of aluminium sheet bent into a large U shape with the two output devices mounted on the bottom, and to one flat side of the U there are two more similar heatsinks mounted lengthwise. Hard to explain clearly, so have drawn a diagram that hopefully better explains it.
Pretty simple, 3 bits of sheet bent in a U, the ends of two of them joined to the side of one. In the diagram, the dotted lines show the edges etc, and the two red circles indicate the position of the two output devices. There is one such heatsink assembly per channel.
Model number is National NM27-114. Its a nice looking amp, and I was going to start the process of replacing the DIN connectors with RCA's and binding posts, but then I noticed how hot it was getting. Compared to the likes of a Musical Fidelity A100 which has a similar output arrangement designed to be biased into class A and uses the entire case as a heatsink, the heatsinking on this amp is pretty minimal.
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Last edited by flyingtele; 22nd March 2012 at 04:22 PM.
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Old 22nd March 2012, 04:39 PM   #8
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place pictures of your amplifier ...true documentation is hard to find for this machine

To my understanding here was never a Class A made amplifier from National but i might be wrong ....

There is a number of reasons for this to happen but first i dlike to see some pictures

kind regards sakis
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Old 22nd March 2012, 04:54 PM   #9
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sure, will take some photos once I get a chance and upload them.
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Old 22nd March 2012, 05:04 PM   #10
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good ... i ll keep an eye
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