[Elecraft] RS232 problems with KPA100

[Jack Brindle]

On Jun 25, 2005, at 5:02 PM, Matt Osborn wrote:

> The high voltage bias supply on the KIO2 is between -6v to -12, so
> your 19.4 is probably sufficient, but the KIO2 Vss, U4 pin 8 is -9v on
> my KIO2. The MAX1406 wants from -13.2 to -10.8, but the absolute max
> ratings are from +0.3 to -14.0, so you're probably OK with the -5.87.

I would expect the supply to be a bit more robust in the KPA100 than  
in the KIO2 because of differences in the circuit. Still, the supply  
is in the range I have seen in many KRC2 units, and should work just  
fine. Note that the KIO2 and KRC2 use the same FET-based circuit for  
the negative supply, while the KPA uses a bipolar transistor circuit  
which also develops a larger bias supply for the amplifier. The KPA  
circuit has much greater current capacity. Having said that, for the  
KPA tthose voltages are relatively low, and may be a clue. Does the  
amplifier work properly? If so, then this is probably not the  
problem. But... (see below).

> The real suspect is that U4 pin 12 should be reading +5V from the
> AUXBUS on the control board.  The only other use of AUXBUS on the
> KPA100 is U1 pin 28 (RB7 Serial Programming Data).

> It would be worth checking out AUXBUX line for a short (C25) or an
> open P1 pin 1.

If the Auxbus line were being shorted, the entire radio would not  
work, but would show an IO error. Unless this is being seen, the  
Auxbus is just fine. Also, just as a hint, the Auxbus is not used for  
device programming anywhere in the K2/100. It _is_ a bussed signal,  
and if it is shorted in any connected module (like the KPA100) it  
will cause the entire Auxbus to go silent (and the K2 to show an  
error message).


> On Sat, 25 Jun 2005 16:06:46 -0400, "Frank Van Cleef W1WCG"
> <w1wcg at snet.net> wrote:

>> 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.

But did you verify continuity between the computer end and the  
MAX1406? The RF chokes are the primary items to worry about in case  
of "blown" interface. This, of course, would indicate the use of the  
wrong cable from the K2/100 to the computer. RS-232 voltages on  
anything but pins 2 and 3 of the DE9 connector are definitely bad  
news for the interface.

>> 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.

This is definitely low. What about the +90-150 supply? It appears  
that something is pulling the supply down. You should go through the  
diode voltage chart, comparing your measurements against those in the  
chart (page 61 of the KPA100 Rev C manual). I would not expect the  
MAX1406 to be dragging this down unless there is a short on a TX line  
coming out of the chip (or the chip is blown - a VERY rare event).  
Note that the 1406 does not need a -12V supply for the Rx side, so  
even without -12V, it should be passing the RxD through to the MCU.

>> Resistance on the TXD/RXD leads at the chip is about 100 ohms to  
>> ground with the power removed.

That seems a bit low by about a magnitude of order. In any case, the  
RxD input goes to an internal transistor base, while the output is a  
bipolar emitter/collector pair. they should not have the same low  
resistance to ground.

>> 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???

Keep looking. Those diode measurements should be telling. If you  
don't find it by Monday, drop tech support an email. gary is always  
very good at helping solve problems like these.

Good luck!


-Jack Brindle, W6FB
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