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And Farnell for ?28.03

Transistor MRFE6S9060NR1 are still available through Mouser for ?29,69 and Digi-Key for ?30.54 so if you blow one up you can still get good ones.

LDMOS transistors are expensive and can be easily blown up!

Best practice methods to avoid costly failures.

Most of this is learned from bitter experience of building 100’s of PA’s.

Power supply cables should be kept as short as possible and well decoupled at both ends. I would recommend several ceramic SMD capacitors with at least one ATC of about 30pF on a PCB across the terminals of the PSU and the input connection of the PA. A good quality feedthrough capacitor of 100pF – 1000pF should be used on the tinplate box.

Solder all earth tracks to the tinplate box top and underneath and make a continuous bead of solder with as few gaps as possible.

Keep the input and output connection tails (if used) to less than 1.5-2mm (with a 3mm heat spreader the centre pin of the SMA connector can be cut short and rested on the PCB surface and the nut then just clears the heatsink when fitting the plug)

If you are using a Lime Mini/Portsdown for DATV then an RF relay on the lime output , operated by a transistor switch on the Portsdown PTT output is essential as the calibration spike can be 15dB higher than the required signal and WILL blow the gate of the PA transistor (see BATC Wiki pages). The software is supposed to stop this but Dave Crump G8GKQ (the writer of the software) will back me up in saying that software is not perfect and does not always behave as expected!

Although I have never had an LDMOS power transistor fail due to static discharge, precautions should always be taken to avoid it happening, such as earthed wrist strap, earthed soldering iron and an ?earthed metal bench surface. The bench surface can even be a sheet of cooking foil.

Cleanliness! Make sure to check for swarf and metallic specks before assembly. Use a damp cloth and a magnifying glass to examine and clean, especially where the power transistor and coupling capacitors are fitted.

Don’t use screw heads that are too large – RF can arc over a gap and then burn the PCB.

Once the board is assembled check for swarf again and carefully clean any away (a small paintbrush is ideal for this). Do this also every time you adjust the tuning stubs.

If you use a different assembly method like my prototype pallets then use a heat spreader the same size as the pcb and solder the pcb down onto the heat spreader. Fit the SMA connector mounting blocks snugly against the ends of the heat spreader – gaps WILL cause instability.

Before testing, turn the bias potentiometer until the gate to ground resistance is minimum (about 820 ohms).

Never apply supply volts unless the output and input are terminated at 50 ohms. This can be a dummy load or antenna on the output and a driver amplifier, signal generator or 50 ohm termination on the input.

Bias can be applied at the same time as or after the supply voltage but not before. Best practice is to obtain the bias volts from the supply volts in the PA assembly itself and relay or transistor switch it. For Datv in its simplest form, a dropper resistor ?can connect the two so that they switch on together.

The recommended supply voltage for this transistor is 26 volts although I would add that all my testing has been carried out at 28 volts with no problems. The difference in power output I have measured at around 0.3 – 0.5dB so it is perhaps best to stick to 26 volts.

A large heatsink should be used with fan cooling for DATV. This is to avoid shortening the life of the device with excessive temperature. A maximum temperature rise of 20-30 degrees Centigrade should be aimed for.

Don’t use too much heat sink compound between the heat spreader and the heatsink. The surfaces should be smooth and flat with a fairly light smear of compound (Dow Corning 340 is my preferred compound).

Correct sequencing of turn on and turn off events is also important, as in the earlier paragraph, the correct sequence would be;

1.Connect load and input source

2.connect 26 volt power

3.turn on bias volts

4.turn on signal drive

The reverse would be;

5.turn off signal drive

6.turn off bias volts

7.disconnect 26 volt power

8. disconnect input source and load

Transistors that will work with the G4BAO PCB

Plastic transistors that will fit with no modifications;

MRF6S9070(NR1,MR1)

MRF5S9070(N, NR1,MR1)

M59070N

MRF9045(NR1,MR1,LR1)

CERAMIC CASE TRANSISTORS THAT WILL NEED A SPECIAL HEAT SPREADER AND/OR LARGER HOLE IN PCB.

SRF9060(L,LS)

MRF9060(L,LS)

MRF9045(L,LS,LR,LR1)

MRF9030(L,N) – (NOT TESTED YET BUT SHOULD BE OK)

MRF9010 -- (NOT TESTED YET BUT SHOULD BE OK)

There may be more but I will leave that to others.

?

The PCB manufacturer is saying dispatch by end of next week - 17th July
It's getting closer!

I found this Polish site on Ebay?
this transistor should be good for 70 watts

Try not to blow the device as I only managed to buy enough transistors for the kits. I will not be carrying any spares. They are available but good ones can be expensive and some of the Chinese ones are a bit suspect.

TRhe amplifier should also work with SRF9060, MRF9060, MRF9045,MRF9030 and maybe 9010 although I haven’t tested the 9010 or 9030 on this PCB.

Bias current for this transistor is 600mA

Note that the PTT voltage can be in the range 8 - 28 volts and can be tied to the main supply for DATV

The power transistor os an LDMOS so the gate is static sensitive so use proper handling precautions when handling it.
Once it is fitted to the populated PCB it is pretty safe - that is why I prefer to populate the board first , then fit the transistor.

I have just come across a case where PA's were being blown when driven by a Lime Mini on a Portsdown transmitter at 23 cms
This one was the latest software and gave out a spike of signal a few MHz away from 1255MHz at about 1258MHz at 15dB more signal than the eventual DVB-S2 signal.
This got through the driver amp OK but blew the gate on the PA.
I ALWAYS use an SMA relay on the output of the Lime Mini to prevent this happening and I have 3 Portsdowns with lime minis all with relays.
The relay signal is obtained from the GPIO pcb and is used to drive a bipolar transistor switch (the 3volts is not enough to fully turn on a mosfet switch) to power the 28 volt relay.
The 28 volts is from an Ebay step up power supply just to run the relay.
I see the calibration can be turned off in the software but I don't 100% trust this!
A relay is a lot cheaper than a blown PA!

I prefer to use a 10k? 10 turn pot for this and a suitable type is Bourns 3224x sold by RS components as 743-2306?
for values of the preset lower than 10k then R4 should be reduced in value or even shorted out to prevent the bias voltage for the device not being achieved.

This project would not be possible in its present form if it were not for all the hard work of designing the matching that was done by John Worsnop G4BAO.
He mentions this in a few modest words in his article but I know there is much more to it!

I am afraid the first batch of PCB/transistor/ATC partial kits is now sold out for the 70 watt version.

There may be one or two people drop out of the pre orders – if so the next on the new list will be contacted

I have just purchased a batch of SRF9060 transistors which give slightly less power and need a different heat spreader arrangement.

The rest of the information is similar.

..

?

G7NTG updated G4BAO amplifier for 23 cms now 60 watts

Here is where to find information;

[email protected] this site is read only and has all the information I have plus some articles on construction.

[email protected]? look in Files/G4BAO poweramp 23cm files

And also look in G4BAO posts there are member discussions here where people post ideas and suggestions.

Or ,look at;

Github.com/g4bao/23cm-45W-PA

Please do not contact John G4BAO at his request but contact me with any queries

Look on his website for a 2.5 watt driver amp although this is not enough to drive the 70 watt amp to full power.

If you want you can go on the new list and I will be in touch when I have all the parts and am ready to ship sometime in late July 2020.

Jim G7NTG thebigclunk@...