The following article is for publication in the Chiltern DX Club Digest.

 

DXpedition Amplifiers
In the months before going on my recent XR0YG DXpedition, I started to look at a light-weight amplifiers to add a little extra punch to the signals. I had my eye on the Acom 800, which has been doing the rounds on the internet as “Acom’s new DXpedition amplifier” for nearly a year, but when I spoke directly to the factory in Bulgaria it became obvious that it wouldn’t be ready on time.

I was aware that two other team members had light-weight amplifiers: Nigel, the Tokyo High Power HL-550FX, a switch-mode linear rated at 600 watts and 10.2kg; Michael the Tokyo High Power HL-1.2KFX – a heavier linear PSU predecessor of the HL-550FX – rated at 750 watts output and weighing 15kg. The Elecraft puts out around 500 watts (more if driven harder) and weighs 12kg – I was offered a great deal, so opted for the KPA500.

In between getting the amplifier and departing for Easter Island, I started to hear about a couple of DXpeditions having KPA500 failures. A Finnish trip to Lord Howe Island late last year lost one, apparently because the mains voltage went above 250v. Additionally, our own Jim/G3RTE and Phil/G3SWH each lost one (i.e. TWO amp failures!) whilst away on the Solomon Islands earlier this year.  However, prior to going away I didn’t have any more details about these failures.
When we arrived on Easter Island, Michael and I were very careful to measure the mains voltage before plugging our amps into the wall. The KPA500 and Michael’s HL-1.2KFX both have linear power supplies and the manuals give clear instructions about the transformer taps that need changing, depending on supply voltage. Although high voltages can damage equipment, low voltages can also be bad for high-power FET amplifiers and the linearity of the RF produced, so care is in order.

Our supply averaged around 200v in the day and evenings but rose to 225V at ‘quiet’ times (when the islanders were asleep) and dropped to 190v on peak demand. Therefore both amps needed appropriate transformer tap changes to align with the average voltage, which is lower than that usually found in the UK. It was important for us both to monitor our amplifier HV voltages during the week when operating to make sure they were within acceptable tolerance.

Why did Nigel’s amplifier not need a transformer tap change? Well, Nigel’s HL-550FX amplifier has a switched mode power supply designed to detect and accommodate fluctuations in supply voltage.

We had a couple of scary moments where we saw the amplifier HV voltages rise steadily upwards, but these quickly reversed so we didn’t need to switch the amps off. Of course, we were also very careful to switch the amps off when not in use. Thankfully, all the amps got through the DXpedition with no issues whatsoever. Of course, after the DXpedition the first thing to do is put the taps back – or fall at the last by damaging your amplifier in your own shack!
On returning home I had a chat with Jim/G3RTE and Phil/G3SWH about their KPA500 failures. Their amplifiers both failed with the same problem but in unrelated incidents, 24 hours apart. Elecraft analysed their PA modules and fault logs and found the root cause of both failures to be the Transmit/Receive (T/R) switching circuits “correlating with high SWR”; they have committed to reviewing the firmware and PA circuitry design.

The jury is still out over Elecraft’s T/R switching issues but in the meantime, if you travel with an amplifier that has a linear power supply – make sure you check the voltage taps!

John Warburton G4IRN.

 

May 2013 Update: Apparently Elecraft identified a batch of faulty FETs; Jim and Phil were unfortunate to be on the receiving end of them. Elecraft identified all the units and fixed them.

 

(With thanks to, Jim/G3RTE, Phil/G3SWH and Michael/G7VJR for providing input).