RS232 problems with KPA100

Posted by Van W1WCG on Jun 25, 2005; 9:06pm
URL: http://elecraft.85.s1.nabble.com/RS232-problems-with-KPA100-tp379774.html

Greetings, all you I/O experts out there -- I need some advice, please....

I'm using K2 #2634 with a  KPA100 and KAT100; therefore the RS232 I/O is done by the MAX1406 chip on the KPA100.  I've done computer control in the past but not for some months, so today I was rather surprised when I went to try out a new CAT program and could not get it to control or read out from the K2/KPA100.  A breakout box showed no signal coming from the KPA100, and the TXD line from the computer indicated signals until I connected the KPA100; then the line seemed to be dragged down almost to 0v by the KPA100.

After verifying continuity between the computer end of the RS232 cable and the RF chokes on the KPA100 board (RFC6, RFC7) I meaured voltages on the MAX1406 chip.  Except for the 5 and 12v supply voltages they were not what the chart in the manual described.  The pin voltages on the MAX1406/U4 chip were:

    1 -    +12.54         2 -    -0.43              3 -   -0.03            4 -    -0.43
    5 -    -0.44            6 -    0.44               7 -   -0.44            8 -    -5.87
    9 -    0.00            10 -   0.00              11-    0.10           12 -    -0.13
  13 -    -0.09          14 -   0.00              15-   -0.09           16 -    4.96

The voltage on pin 8 (-5.87) is at the very low end of the expected -5 to -25v range shown on the schematic, and even the +30v on the HV bias supply is low at 19.54v --almost as though whatever is wrong in the chip is dragging
down the -25v and and the rest of the HV bias.

Resistance on the TXD/RXD leads at the chip is about 100 ohms to ground with the power removed.
All measurements made with a FLUKE 87 DMM.

I don't know how but I'm afraid I've somehow crisped the MAX1406. Is there anything else I should try before replacing the chip???

TIA, all

Van, W1WCG
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