Steve Deckert
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UPDATE - FATHER MURPHY
Murphy's Laws were drafted after Murphy died. It's a list. There are several versions of it on the Internet. The one I grew up with said for example; Rule # 7 - Any tool dropped while working on a car in your garage will automatically roll to the exact dead center of the vehicle so as to become out of reach from all four sides.
The set up - it takes one. It's what pages Father Murphy who is now an evil spirit who especially likes to mess with audiophiles. So the set up is this...
I made an assembly video of the amplifier being built for the newer techs on our team. This of course means I have to make it without mistakes... this was the first ring. Naturally long before it was even finished, a mistake was made and filmed. I put in a 47K resistor where a 100K resistor goes. I decided to leave it in the video as a teaching tool. At the same time I bragged in the video about how I measure everything as I go, so I found it right away. This was the second ring. Then I realized if they watch the video without listening to my important commentary they would build the amp with the wrong resistor at which point I would know they didn't listen to the video. This is playing chess with Father Murphy. Score Father Murphy zero, Steve one. This was the third ring.
Not to foreshadow or anything but never play chess with evil spirits.
When firing up the amp for the first time, I checked all the voltages and they were perfect. I smiled. This was ring number four, and Father Murphy one, Steve one or so I was about to find out.
I put the amplifier on the scope and couldn't get any kind of substantial output from it. Like a half a watt, yet everything was working perfect. This is how I know the ringing had been answered. He's awake, he's here now, and this is where the punishment starts.
I noticed that I forgot to connect the input jacks to ground. That was of course because I had already posted a picture in the forum before the amp actually worked. Father Murphy two, Steve one.
I fixed the jack and took new pictures so I can fix the video, and it makes perfect sense why I couldn't get a signal out of it since there wasn't really a signal going into it.
Lesson one, Murphy NEVER works in single shots. If he has to show up to your gig, you can bet it's going to be worth his time as pure entertainment. I imagine it is the same satisfying feeling that I used to get when I was five and used to tease my younger brother about pooping his pants. It was so much fun.
I hook everything back up and virtually nothing has changed. This seemed impossible by I remember he works in threes, so I just started looking for problems. Damn, this means real troubleshooting. I quickly discover that the signal going through the stages is but a fraction of what is should be and find out it's a partial short to ground at the input of the amplifier, which is in mono due to the scope and signal generator being hooked up to both channels, so the problem was on both sides. This whole thing was impossible. Literally I had to start taking things apart... could it be a bad input switch, or cable, or something I can't see in one of the jacks?
After taking everything apart I determined it was actually the cable, the brand new perfectly installed cable had an 80 ohm short. I completely removed it and measured it again, and it was fine. Murphy three, Steve one.
I re-installed it to see if I was loosing my mind and measured it again. 80 ohms to ground. I should have known, Cables is one of Muphy's favorite tools of torment. When a cable starts to go bad it either opens or shorts, and does it intermittently. Since the cable is flexible, it depends on how it is bent as to if it will work right or not. Murphy four, Steve one.
The cable was something different I found laying on my bench. It looks like the excellent cable we normally use on the outside, but has only one conductor and is built more like a coax. I couldn't see any reason why it wouldn't work as well or better so I used it.
Now with the one bad cable removed there was only the other side to investigate and it had a similar issue of about 150 ohms to ground. I removed it and replaced it with our regular cable to match the first side. Murphy five, Steve one.
The odds of two cables doing this just from being soldered into place (by someone who can solder) is pretty high. Murphy six, Steve one.
And for the check mate, I wired the 12AU7 as a 6922 so that only half of the heater worked. So that's on the video too. Murphy seven, Steve one.
This is another excellent testimony of why big manufactures use circuit boards with cable connectors. Make it so a monkey can't screw it up and they'll all probably work the first time.
Anyway, I wrote this true story to make myself look bad so he would leave. That's how it works. Now it is time to test the amp and see if it works.
Steve
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