I have been using an Elecraft KPA-500 linear amplifier and a KAT-500 Antenna tuner with my Icom radios (mostly the IC-7600 and briefly with the IC-7300) After much research and discussion, I figured out what I thought was the best way to interconnect the IC-7600 with the KAT/KPA-500 so that the Amp would be protected by the Automatic Tuner, in the case of antenna problems. The Tuner was configured to “know that it was being used with a KPA-500.” According to the Manuals, that means that in the event of any antenna issue, the KPA-500 would be immediately disabled from transmitting.
I thought the key to this protection was to use the INHIBIT line from the Tuner to “unkey” the Amp in the event of a Fault in the antenna or a bad match on the Tuner for any reason. I connected it all, and began operating with this setup. I worked a few contests using a Vertical, a ZS6BKW, and a 160m Dipole fed with Ladder Line. All seemed to be going well until…
One night I was working a Contest and the Tuner started having problems on 80m CW using the 160 Dipole. I had used this before, so I kept retuning as I moved up the Band, but something was wrong. The Tuner kept retuning when I keyed the rig. Suddenly I smelled that awful well-known odor of burnt electronics. I shut down the Amp and disconnected it, for further investigation.
Damage to the Elecraft KPA-500 Amp
Testing showed that the AMP could be used on all Bands except 80 meters. On 80 meters, there was no power output that I could measure. I started looking online and quickly determined that I had probably damaged components in the 80 meter Low Pass Filter (LPF) on the RF/LPF module. I began conversing with Elecraft about the possible damage. I must say that my interaction with Elecraft was outstanding. They were very concerned about my problem and helpful with the troubleshooting. We exchanged several emails and they even sent me step-by-step instructions for making numerous measurements on the RF/LPF module. This led to the discovery that the 270-volt line was dead, disabling the Power section when 80 meters was selected.
I decided to remove the RF module to see if I could make a repair (some had reported success in similar situations). Further online research uncovered a detailed set of photos for disassembly of the AMP, and removal of the RF/LPF module. This was incredibly helpful and saved me from several potential mistakes. I successfully removed the module and could immediately see the damage. There were a couple components that were obviously fried, and a couple PC Board traces that did not look good.
Return the KPA-500 RF module to Elecraft for Repair
Elecraft offered to help me through the repair if I chose to do it myself. We discussed several other options, including sending just the RF/LPF module to them for repair. Since the damage was in a fairly dense section of surface mount components and toriods, and I had no surface mount rework equipment, I decided to let them do the repair.
I shipped the module to Elecraft. They had told me that the lead time might be a couple weeks for the repair. After approximately that time period, I received a phone call from the Technician at Elecraft that was doing the repair. That was a shock! He said the repair was in progress, and he just wanted to offer to discuss it. As I had a number of questions, I called him and we discussed it for something like 20 or 30 minutes. This was extremely valuable to me and again demonstrates how Elecraft goes above-and-beyond in dealing with their customers, and I had bought this used, not directly from Elecraft.
Reassembling and Testing the KPA-500 – NOT all Good News.
I received the repaired RF/LPF module and I could easily see the repairs. They had replaced some Caps and repaired a couple traces on the LPF Board. I installed the module and connected all of the wiring. When I connected all of the equipment and keyed the Amp – NOTHING happened. NO OUTPUT POWER! Well this was disappointing. I thought I had probably failed in my reassembly. I knew they had run a several-hour test and burn-in after completing the repair. It turns out that there was still no 270-volts present on the RF/LPF module.
Back to talking with Elecraft and searching the Web for answers. Apparently there is a capacitor on this module that is known to fail in a mode that it shorts when voltage is applied. This appears to be a common failure. It is possible that it has been remedied on newer Amps. My Amp was used and about 4-5 years old when I got it. Just my luck that it would fail now, just after other repairs were done. I guess it could have happened in shipping. Following Elecraft’s instructions, I confirmed this was the failure.
Elecraft was as disappointed as I was. They offered to send me replacement capacitors if I could make the repair. They also offered to fix it quickly, at no cost, if I shipped it back to them. While I appreciated the offer of the no-cost repair, I decided it was time to learn to do some SMT repair work. We identified 2 capacitors that should be replaced. One was obviously physically damaged and they recommended replacing the other one as well. Both of the caps were fairly easy to access, so I was going to give it a try. I got several of each; Elecraft had one value in stock and sent several of them, and I bought the other one online at Digikey, opting not to wait for Elecraft to get new stock.
Repairing the RF/LPF Module at Home
I found a very inexpensive SMT rework station online which is sold by a number of Vendors. It got pretty good reviews, so I thought I would give it a try. I had some old Computer boards that could be used to see if the Rework Station and the Operator (me) were up to the task. The hot-air worked fine for removing and attaching SMT capacitors. I did blow a couple off the PC Board in the process from the moving air.
One of the first caps I had ready to put on the board was shot across the room by my tweezers, never to be seen again. Fortunately both I and Elecraft had provided multiple pieces of each capacitor. Finally I was able to replace the small caps. I reassembled the Amp and measurements show that all the necessary voltages are now present on the RF/LPF module.
Preparing to Use the Repaired Amp – WHAT HAD HAPPENED??
This story is not over yet. The Amp is ready but I want to have a better understanding of what happened, and must make sure there is no repeat performance. There are several possibilities for the failure of the RF/LPF module:
- An antenna problem caused high and varying SWR that the Tuner could not handle
- RF was getting into the Tuner control circuitry and relays
- There was simply a component failure – it is known to happen
Due to the retuning that I saw just before the failure, I think there is a good chance that my antennas and/or grounding were the culprit. I did find a splice in my feedline that was near-failure and might have been arcing when this happened. I also found that my station grounding is not good, after reading several good articles on Grounding. Some antennas are very close to my shack, and I will be taking some EMI/RFI mitigation steps in the wiring.
Why did the Tuner NOT Protect the Amp?
This is the question that I still cannot answer. Reading the manuals, I would have thought the Tuner would have quickly FAULTED and UNKEYED the Amp. This damage looked to be caused by high voltages which I thought were a result of high SWR between the Amp and the Tuner. Maybe my expectations were too high. I will be much more careful in the future when using the Amp and Tuner.
Final Outcome – updated 10/30/2019 –
I have done a bit of work improving grounding, and adding more ferrites to feedlines. I also rebuilt my ZS6BKW and installed a new 80 meter dipole that is capable of handling well over the maximum 500 watts delivered by the KPA-500 Amplifier.I also used a dipole design that “broadbands” the antenna, so the SWR with no Tuner is less than 3:1 over the full 80 meter Band.
I have started using the Amp again, but for now I am only running up to around 200 watts. That seems to be working fine, including operation in the 80 meter Band. I hope to get back to full-power soon, but want to do a little more testing of the antennas and grounding before trying that.
There are some potential warning signs of danger to your Tuner or Amp. When you first tune on a frequency, the next time the Tuner sees RF energy it will Tune. Depending upon the power applied, this may or may not be the ultimate solution for a match. When you put full power on the Tuner, it may briefly tune again. I think that is normal. I see it often, and it is usually just a “blip” for a fraction of a second. If you get more tuning activity than a quick “blip” when you key-up for the first time on a frequency, I think that is your warning that something is in need of attention. In the worst case, you may see the Tuner active every time you apply energy. These extra tuning cycles should tell you that there is either a problem with your antenna or feedline, or that RF is getting into your setup. My theory is that this is what happened to me and I did not see the warning signs soon enough. ….. or it still may have just been that unlucky component failure… I will never really know.