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We had a Silvertone 1474 come in to us with a couple of problematic noise complaints. One of these problems was an issue in the secondary speaker wiring in one (or both) of the OTs which was fixed by removing the existing wiring connections and replacing them. The second issue was more persistant. It would develop after a while on it's own, but could be exaserbated by hitting the cab/chassis.
It almost sounded like a noisy plate resistor, but It had very specific sound to it which could best be described as the sound of "paper tearing". I originally thought it was a bad tube. But I substituted good bench tubes and still the noise persisted. So, I called my boss over and asked him what he made of it. (This is the best part because this is where 30 years of experience comes into play and can countless hours of troubleshooting.)
He listens and says "You know, I've run into this in the past and have seen this kind of thing happen when there is an intermittent connection from the chassis to Earth."
Had he not mentioned it, I don't know how long it would have taken me to investigate that as the cause, BUT, I remember looking at the How the Mains AC wiring had been done in the past when it was fitted with a 3-conductor cord and did not like the look of how the earth connection was made. It was soldered to the eyelet of one of terminal strips connected to the chassis, but they were heavily oxidized and the solder connection looked bad. So, after he said that, it made a lot of sense. It was a quick and easy thing to check. I grabbed the lead of my multimeter, and slid an alligator clip over the banana plug; clipped the alligator to the aluminum chassis; and touched the lead to the Variac enclosure(which is bonded to Earth), and the output goes silent.
That was it!
Now, I'll often do the mains wiring first if the equipment needs it (ie- in cases where vintage equipment needs to but updated, or done was done poorly). Almost always. This is a matter of safety, and I don't take mine for granted. For the Silvertone in the example above, the entire cable and mains wiring was redone. The materials are inexpensive enough, and I had the oportunity to improve the entire AC wiring. While I'm fairly certain that the problem in this case was the solder connection inside the chassis, a fault like this could develope in a number of different places where there wouldn't have been any visual clues. There could have been a break in the moulded plug end, the wall outlet, or where the cable is strain reliefed at the input to the chassis to name a few examples.
I wanted to share the story of this repair because this was a first for me. I was able to save a lot of troubleshooting time on this job because my boss gave me a heads-up. Hopefully it can help someone out and do the same for them.
It almost sounded like a noisy plate resistor, but It had very specific sound to it which could best be described as the sound of "paper tearing". I originally thought it was a bad tube. But I substituted good bench tubes and still the noise persisted. So, I called my boss over and asked him what he made of it. (This is the best part because this is where 30 years of experience comes into play and can countless hours of troubleshooting.)
He listens and says "You know, I've run into this in the past and have seen this kind of thing happen when there is an intermittent connection from the chassis to Earth."
Had he not mentioned it, I don't know how long it would have taken me to investigate that as the cause, BUT, I remember looking at the How the Mains AC wiring had been done in the past when it was fitted with a 3-conductor cord and did not like the look of how the earth connection was made. It was soldered to the eyelet of one of terminal strips connected to the chassis, but they were heavily oxidized and the solder connection looked bad. So, after he said that, it made a lot of sense. It was a quick and easy thing to check. I grabbed the lead of my multimeter, and slid an alligator clip over the banana plug; clipped the alligator to the aluminum chassis; and touched the lead to the Variac enclosure(which is bonded to Earth), and the output goes silent.
That was it!
Now, I'll often do the mains wiring first if the equipment needs it (ie- in cases where vintage equipment needs to but updated, or done was done poorly). Almost always. This is a matter of safety, and I don't take mine for granted. For the Silvertone in the example above, the entire cable and mains wiring was redone. The materials are inexpensive enough, and I had the oportunity to improve the entire AC wiring. While I'm fairly certain that the problem in this case was the solder connection inside the chassis, a fault like this could develope in a number of different places where there wouldn't have been any visual clues. There could have been a break in the moulded plug end, the wall outlet, or where the cable is strain reliefed at the input to the chassis to name a few examples.
I wanted to share the story of this repair because this was a first for me. I was able to save a lot of troubleshooting time on this job because my boss gave me a heads-up. Hopefully it can help someone out and do the same for them.
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