Replacing the LDMOS Devices on an RF-Kit RF2KS Linear Amplifier

Disclaimer: These notes serve as an aide-memoire to myself. I do not hold any responsibility for their validity or outcomes associated with their use by others. Reinhard at RF-Kit provides an excellent support service, as does the community on Groups.io - always refer to them if you have any doubt.

Safety: Always take great care around high voltages and currents.

Preparation

Switch the amplifier off and remove the power connector.

Remove the lid and store the screws safely.

Remove the RF input from the PA board – this is the SMA connector towards the front of the board. This should be disconnected and can be left off throughout the entire process but don’t forget to reconnect it prior to putting the lid back on.

It’s probably not entirely necessary when there’s no RF input to the PA board but I recommend a dummy load on the amplifier’s output.

Connect a transceiver to the RF input socket on the rear of the amplifier.

Connect a PTT lead between transceiver and the amplifier.

A CAT connection is a ‘nice to have’ but not totally necessary.

LDMOS Info

The devices being used are BLF189XRB – there are two of them. Each is installed with an iridium strip that sits underneath the device to help conduct heat.

Data sheet: https://www.ampleon.com/documents/data-sheet/BLF189XRB_BLF189XRBS.PDF

Each physical LDMOS device has four tabs (see the Simplified Outline’ below). Two are the Drain connections (1,2 on the diagram), two Gates (3,4) and one Source (5). The Drain connections (with the missing corners on the tabs) should be connected to the 53V DC supply on the amplifier.

BIAS current measurements

At several stages in the testing and setup process, it is necessary to measure the bias current on the LDMOS gates. This is done as follows:

If there is no CAT connection, send a dit at low power (0.2 Watts) to the amplifier while in STANDBY mode – it should set itself to the correct frequency.

Put the ATU into bypass mode.

Press the Operate switch on the amplifier.

Press the PTT button on the transceiver.

Send a CW dit (again, low power) – the amplifier will display ‘PTT’ and ‘BIAS’ on the display. Keep the PTT engaged while measurements are being made. No further RF input is required.

At this point, you can measure the voltages being applied to the LDMOS.

There should be +53V DC on the LDMOS Drain connections (these are the innermost of the tabs on the devices).

There are four Gates (two Gate tabs on each of the two LDMOS devices) – there should be 1.8V on each of these.

If no 1.8V is present, then the device is probably broken. In my experience a broken Gate has around 0.3V on it.

(Note that each physical device has two devices inside it – it’s possible that only one has blown. If one of the tabs has a lower voltage than the other (on the same physical device) then lifting the lower voltage tab may result in 1.8V appearing on the other, good tab. In practice, once a single tab has been lifted the entire device will probably have to be replaced, so this is just for info.)

Remove PTT once the voltages have been measured.

Removing a blown LDMOS device

This is the trickiest stage but is OK with patience and care.

Make sure the amplifier is unplugged from the mains.

Remove the aluminium brace over the LDMOS device by unscrewing the hex screws.

Use a 60-70 Watts soldering iron to apply heat to the LDMOS tabs. Use a solder-sucker to remove as much solder as possible.

Use a sharp bladed object (e.g. craft knife, small screwdriver) to lift each of the tabs on the device.

The tabs will bend upwards as they are released from the board. When all four tabs are disconnected, slowly prize up the LMOS device from the board with a small flat-bladed screwdriver. Below it is a copper base; it will have a sticky mess on it – this is remains of the iridium strip.

Once the LDMOS device is removed it is necessary to clean up the board.

Smooth out any remaining solder and remove as much as possible with a solder-sucker or braid.

The remnants of the iridium strip will need to be removed so that bare, shiny copper is visible. I used ‘Brasso’ (a metal polish) and a sharp wooden or plastic edge (e.g. an old cut-up credit card) to scrape up the mess. Take care not to scratch the copper base – do not use metal objects to clean it. Once the bulk of the mess is removed, I used Iso-Propyl Alcohol (IPA) and cotton-buds to remove the remaining mess. This part needs patience – it takes a while for the mess to be removed.

Once the board and copper base are cleaned up, the new LDMOS can be installed. If both LMOS are being replaced, I recommend removing both the old devices and cleaning both sides of the board before the new devices are installed together.

Inserting a new LDMOS device

Place the iridium strip in place on the copper base.

Place the LDMOS device over the iridium strip. Make sure the Drain tabs (with the corners missing) are facing inwards, towards the other device. These are fed with 53V.

Use a matchstick or similar to get everything lined up. Make sure the outer tabs on the LDMOS are centred on the PCB board where they will be soldered.

Continue to watch the placement and adjust as necessary while the aluminium brace is placed over the LDMOS device. Insert the hex-screws, make sure everything is in place, then tighten the screws by hand initially, making sure contact between device and board is even.

When the screws are finger-tight and the device evenly screwed down, use a hex screwdriver to tighten them a bit further.

Then solder the tabs.

Repeat for both devices if necessary.

Now power up the amp, apply PTT and a 0.2W dit as above and re-check the voltages. There should be +53V on the Drain tabs and +1.8V on each of the four Gate tabs. Remove the PTT after checking.

Each device needs to be set up individually.

Remove power from the amplifier and make sure the SMA connector to the PA board is still disconnected.

Remove the +50v power lead from the back of the PA board.

Place a DC ammeter in series with the +50v supply and the board. The ammeter should be capable of at least 5 Amps (preferably 10 A) – make sure the joints between ammeter and supply are well protected against accidental short-circuits on the amplifier’s chassis - I used PVC tape.

In front of each LDMOS device is a PCB jumper – normally these are positioned forward (LDMOS in circuit). For the purposes of testing each LDMOS device individually, the other one needs to be taken out of circuit.

Set one Bias Jumper at a time to the front and the other to the back. Make sure power is OFF while the jumpers are changed. This way you can adjust the Bias current for each LDMOS without the other one effecting it. Jumper to the front is active.

To trigger the bias voltage, send a CW dit at low power (0.2W) and retain the PTT until finished reading the current.

Bias current is measured on the ammeter in the supply lead. It is adjusted (one LDMOS at a time) using the two pots with red blobs on them - a very small screwdriver is required. Each pot corresponds to one LDMOS device.

It takes a few seconds for the current to settle - adjust the pot to 2A bias, then disconnect PTT.

Repeat for each LDMOS device.

Switch off the amp before changing jumpers!

When bias current has been set.

Remove power.

Remove the ammeter and reconnect the +53V supply to the PA board.

Ensure both LDMOS jumpers are set forward.

Reconnect the SMA connector to the PA board input.

Replace the lid.

Test the amplifier.

The amplifier should now be back to normal and will run at full power.

Important - after using the amplifier for a while, check the hex-screws and tighten them. They work loose after the iridium has heated and bedded in.