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
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
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.
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.
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.
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.
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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