by 5B4AIY » 08 Jul 2012 07:49
For those following this post, after a number of email exchanges between myself and Bob, he discovered that the open-circuit DC voltage on the TX DATA line was only -8V rather than the -9V that it should be. He measured tha DC voltage on pin 16 of IC10 and found it was only about 4V, rather than the 5V it should have been. This indicated that either IC10 was drawing an excessive amount of current, or that R22 was no longer 10 ohm. It turned out to be the latter, R22 was very high. He had originally soldered IC10 backwards, and had subsequently removed the damaged chip and replaced with a new one. R22 was probably damaged as a result of mounting IC10 backwards as in this configuration the IC substrate would have been forward biased and would have been essentialy no more than a forward biased diode junction, effectively shorting the power bus and overloading R22. He is awaiting some replacement resistors, and when R22 is replaced, this should solve this problem.