if your KPA500 has LPF board rev. D9 or earlier then you will probably need to make LPF T/R Switch Rework Rev.E modification,
Elecraft parts kit: E850607.
The modification can be done as prevent action in order to avoid the loosing the full power accompanied by burning smell or it can be the solution for your already ruined amplifier.
As I cured several of the KPA500 amps for friends around me using this modification and I still getting emails regarding this kind of rework, manual, parts needed etc. I decided to post short article over here in order to help to others.
The typical KPA500 misbehavior sounds like it is arcing inside and smells the classic something is burning smell. The SWR LED's are dancing upwards as it arcs. If you have connected the KAT500 tuner then it has the SWR around 1.3:1... and power is lost. Try to read first then decide...
If you want and you have SMD tools as same as ESD protection tools then you can try to make this mods first and test if it was the root-cause of your problem. If not then you can send to Elecraft service.
It is not complicated just a bit tight on the board and there is not so much space around the parts to remove the old one.
Step 1. Check if it is applicable for your KPA500.
Your Low Pass Filter board the Elecraft part number E850406 is old Rev.D9 or earlier.
Step 2. Remove the parts and replace with new one.
KPA500 LPF module Rev.D
1. Remove R9 and replace with new 6.8Ohms/1.5W size 2512 resistor
Elecraft part - E500567
Digikey part - RPC2512JT6R80CT-ND
2. Remove R24 and replace with new one 1kOhm size 0805 resistor
Elecraft part - E500230
Digikey part - 311-1.00KCRCT-ND
3. Remove Q5 and replace with new BU931T transistor
Elecraft part - E580068
Digikey part - 497-2797-5-ND
4. Remove R5 resistor (no replacement)
5. Check R3 and verify that in this position of R3 is soldered Zener diode. many people asked me how to determine that it is diode instead of resistor. If it is diode the band on the component is visible and this band will point to Q4 when installed.
Elecraft part - E560074
Digikey part - CD1005-Z7V5-ND
6. Check C43 and verify by C-meter that in this position C43 is soldered 82pF. It is in 12m band LPF part. If you are not sure or you can not measure it to double check then replace it with new capacitor 82pF value.
Elecraft part - E530494
Digikey part - 490-11627-1-ND
7. Check C44 and verify by C-meter that in this position C44 is soldered 68pF. It is in 12m band LPF part. If you are not sure or you can not measure it to double check then replace it with new capacitor 68pF value.
Elecraft part - E530489
Digikey part - 1206GA680JAT1A-ND
Digikey part - GRM31A7U3D680JW31D-ND
Mouser part - 81-GRM31A7U3D680JW1D
8. You should mark your modified board with Rev.D10 label in order to identify it later if needed.
KPA500 LPF module Rev.B
All work must be done very carefully. Do not overheat the PCB and check that you are not damaging the nearest parts with your hot soldering tip while you are removing the desired parts.
I made this modification on KPA500 for several friends already and in all cases their amplifiers worked well after that.
The datasheet for Darlington BU931T is here.
Drop me an email if you need the complete manual for the KPA500 LPF T/R Switch Rework Rev.E modification in pdf format. The parts can be purchased from Digikey or other suppliers. If you would like to buy the parts in kit directly from Elecraft then the ordering number is E850607.
Caution! If you are not moderately experienced with SMD or if you have not the SMD tools like soldering machine, hot-air machine, fine tweezers or if your hands and eyes are no longer reliable... or you do not know what is the SMD then I am recommending to ask someone close to you with these capabilities to help. In the opposite case I am strongly recommending to contact Elecraft service directly.
Hope it helps.
73 - Petr, OK1RP