[Elecraft] KPA100 - Low Voltage output from High Voltage Bias Supply Circuit

[Don Wilhelm]

Michael,

Congratulations on finding the faulty diodes.  The failed diodes 
indicate that you may have suffered a static surge of some type - 
disconnect those antennas when not in use.

You have also confirmed that the MAX1406 at U8 is damaged - take it out 
and your KPA100 should work for you.

If you do not have an immediate replacement for the MAX1406, remove it 
right now and the KPA100 will function as an amplifier just fine without 
it - of course the computer connection part of the AUX I/O will not 
function, but all else should work.

73,
Don W3FPR

Michael Haygood wrote:
> Don,
>
> I replaced D16 and D17 which fixed the HI CUR, HI REFL messages and 
> lack of power control.
>
> However, I am still reading a low output voltage from the high voltage 
> bias supply.  When receiving, the high bias supply voltage at D1 
> cathode and R12/R11 is about 67 volts.
>
> I've further investigated the low voltage output from the high voltage 
> bias supply as follows:
>
> 1.  De-soldered the high bias voltage side of resistors R12 and R11, 
> then remeasured the high bias output voltage - about 69 volts.  I did 
> this to ensure that nothing in the T/R circuit was draining the high 
> voltage supply; resoldered R11/R12 back into the circuit.
>
> 2.  Desoldered the D7 anode in the high voltage bias circuit, which 
> connects to the VSS supply pin of the MAX1406 (RS232 Interface), then 
> measured the high voltage bias output voltage at D1 cathode - measured 
> 154 volts!  Re-soldered the D7 anode back into the circuit and the 
> high voltage bias supply dropped back down below 70 volts.
>
> The high voltage bias supply output should be between 90 and 150 
> volts.  Is it possible that the MAX1406 is somehow draining this 
> circuit?  Seems strange that it would be because it cuts the output 
> voltage more than 50% when the MAX1406 VSS is connected into the circuit.
>