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Old 29th May 2007, 05:22 PM   #1
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Default Squeaky Noise in ARC Dual 75

My Dual 75 has a constant low level noise on one channel. The best way I can describe it is a squeaking noise, like air escaping from a container. I brought this amp back from a rectifier meltdown and have replaced all the electrolytics including the power supply caps.

Both channels have noise when warming up, but one channel gets quiet after a few minutes.

The film caps are all orange drop types and are original (early '70s). I replaced 4-5 resistors on the main board, but most of them are the original carbon comps.

I'm going to clean the tube sockets again and swap around the tubes. Any other suggestions on running this down? I'd really rather not start replacing resistors wholesale.

Thanks,
Neddyboy
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Old 29th May 2007, 07:13 PM   #2
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You've probably got an oscillation. Yes, see if swapping tubes between channels changes which channel squeals.

What part of the Bay Area are you in?
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Old 29th May 2007, 10:07 PM   #3
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Hi Sy,

I'm in southern Marin. Geez, I just realized I posted this in the Digital board. D'oh. Any way to move it?

Neddyboy
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Old 29th May 2007, 10:35 PM   #4
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Of course there is. Ask a moderator. Oh, wait a minute, that's me...

In any case, try the tube swap and report back. If we can't get you off and running by remote control, I'll give you the option of a 45 minute drive to my lab.
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Old 29th May 2007, 11:54 PM   #5
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Default Tube Swap to Come

Cool. I'll do that this evening. I've put so much energy into this amp it's become a third child. It still has the original US 6550s in place and it sounds spectacular...except for this little annoying noise.

Thanks,
Neddyboy
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Old 30th May 2007, 03:19 PM   #6
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I once owned an ARC 75A. It developed a squeal too, although your squeal might be from some other cause (these are rather complicated beasts). SY's suggestion to hunt down oscillation is the best first step. However, in my case, it was a passive part failure. I can't remember what part it was, but I replaced at least one passive part - I vaguely recall a noisy resistor. Sometimes Rs and Cs can become leaky or "arc-y" and you will hear a tell-tale squeal. The passive parts get awfully hot in these old ARC designs, IMO. That's not good for longevity. If you have access to a can of spray coolant, you can often locate the bad part by cooling it, thereby altering its noise production. I've even used a plastic tool to gently tap parts (watch out for the high voltages and high temperatures within). A noisy resistor or capacitor will often reveal itself when tapped, cooled or otherwise provoked.
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Old 30th May 2007, 03:22 PM   #7
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BTW, I was so alarmed at the high temperatures inside my ARC that I attached two fans onto a pair of aluminum angle rails and set the fan assembly on top of the amp's cage above the 6550s to pull air up and out. That seemed to help.
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Old 6th June 2007, 04:58 PM   #8
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Default Next Stop Resistors

Well, I traded around tubes and cleaned the sockets. Part of the noise seems to have abated, but it's still noisy. Some decreases as the amp warms up (maybe 15 minutes). I get a lot of noise when I shift the driver tubes around in their sockets, so I think a good pin and socket cleaning is in order.

After that, resistors. I'm well armed with spray coolant.
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Old 6th June 2007, 05:18 PM   #9
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Noise still in the same channel?
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Old 6th June 2007, 05:52 PM   #10
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Default Oops, Forgot

Some of the noise moved with the tube swap and some stayed. I guess it's a little more complicated than I thought. The whistling seems to have moved, but has diminished and seems to go away with time. Before it was constant no matter how long the amp was on. I'm beginning to suspect dirty pins on the tubes, possibly the sockets, possibly a tube.

The amp had a rectifier meltdown at some point in its history and I've had to replace a lot of components due to heat/high current damage. I think there are probably some screwed up resistors still lurking.
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