SB1001 Tiny-2 MK-II Capacitor C-23 Reversed

Amateur radio models affected are the TINY-2 MK-II, TNC-NB96, and IPR-NB96 which all incorporate the TINY-2 circuit board.

The TINY-2 pre-MK-II version is not affected. Those models use the MAX-231 RS-232 chip. The miss marked capacitor crept into the design when the MK-II changes were made, which included adding the capability to use the MAX-232 RS-232 chip. (The circuit board accepts either the 232 or 231, but all have been built with the 232).

Another way to identify the boards with the error is to examine the circuit board revision number. Revisions 1.7, 1.8, and 1.81 contain the error. Revisions 1.6 and prior use the MAX-231, and Revisions 1.9, 1.91 and 1.92 have corrected the problem. Some of the final production run of 1.81 boards may have the capacitor correctly installed (the reverse of the board markings) as the problem was identified late in the 1.81 production run as work was done on rev. 1.9. Visual inspection will confirm if this is the case.

This capacitor problem is of the nature of 'you ought to fix it the next time it is in for a major tune-up' and not of the 'park it immediately and tow it to the garage' type. In production we usually install capacitors with a significantly higher working voltage than what is applied to them and they have generally worked just fine in these miss marked boards, even though reversed in polarity. Nothing is going to be damaged if the capacitor is left in backwards, except maybe the capacitor itself. There are no reports of any capacitor explosions or other bad effects.

Repair Instructions
Measure the voltage on pin 6 of U20 (MAX232). It should be in the range of -8 to -10 VDC. Remove C23 and reinstall with the negative lead in the + marked hole. Measure the voltage on pin 6 again. If still within the -8 to -10 VDC range then C23 was not damaged.

If the pin 6 voltage is out of range either before or after reversing C23, the capacitor should be replaced.

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