Mercedes W123: Fixing that non-working or jumpy tachometer

Now, a common problem with many Mercedes from the late 1970's to mid 1980's is a jumpy or non-operational tachometer, the gauge that tells you the RPMs. Most cars show this number x1000, but Mercedes makes it interesting... x100. The common source of the issue is the tachometer amplifier (tach amp), a very hard part to actually find. A working used tach amp (or if you are lucky to find a NEW one) can cost upwards of $300-400, but what if I were to tell you that this repair could cost you a few pennies? This serves as a diagnostic but it also is a fix, especially for those that might not be experienced or comfortable soldering. After being sick for a few weeks with a horrible combination of the flu plus bronchitis, sinusitis, and several other itises, I am starting to make more videos as I regain my voice. But I'm back.