Tag Archives: homebrew

1kW 144MHz Amp Lives!

Those of you who have been following my 144 MHz 1kW amplifier project (previous posts machining heatsinks, soldering transistor down and building the pallet) will, I’m sure, be delighted to hear that I have had life out of the amplifier. In excess of 1 kW, I hasten to add!

The amplifier was able to maintain in excess of 1000W for over 2 minutes.  At this point, the Bird dummy-load started to get a bit warm, so a longer test was abandoned. The amplifier pallet, however, remained cool enough to touch. As the F1JRD original design notes, the 10-Ohm coax balun does become hot (Lionel suggests around 120C at 1kW with no cooling). I, however, used a small fan running slowly to provide a gentle draft which greatly reduced the balun heat.

The next step is to add the Dallas-Maxim DS18B20 temperature sensor – the idea is to have the sensor buried into the pallet next to the transistor, to measure the copper heat spreader temperature.

BA5SBA RTL-SDR Kit

A few weeks ago I ordered a BA5SBA RTL-SDR direct sampling kit from BangGood (link here). When it arrived, I decided to put it together. The kit includes everything needed, an RTL-SDR dongle, case, PCB, enamelled wire and so on. I worked from numerous build instructions (here, here, here and here), following the clearest description of each stage.

I disassembled the original RTL-SDR dongle, removing the USB plug, IR remote receiver and Belling-Lee socket. This was easy to do. I then soldered the module into the main PCB. The SMT components were easy to solder on. I added the few remaining passives, some larger electrolytic capacitors, etc.

Two wires tack on to various voltage points to add extra smoothing, which were easy enough to connect – I used some medium thickness tinned copper wire, I guess around 0.7mm diameter. That did the trick.

Winding the two inductors was done blindly. I followed instructions to wind 8 turns around a 5mm drill; however, somewhere else said 6-9 turns around 3mm. I noticed after soldering in the coils that 300nH was the suggested inductance. In the future I will remake the coils to the correct value.

Winding the small transformer, T1, was relatively straight forward. I wound 8 turns around the ferrite core. Although I’m not entirely sure my core was ferrite. It was indistinguishable from a 2mm plastic washer. My kit had blue-red-yellow trifiliar wire in, so I followed the colour scheme in the 3rd instruction link above (page 11).

The chip has two pairs of I-Q inputs, pins 1, 2, 4 and 5. The first pair, pins 1 and 2 are connected to the E4000 front end, which mixes the higher frequency signals of VHF and UHF down to an intermediate frequency (IF). The second pair are also used in this kit to take the HF bands (on the Realtek RTL2832U, 0-24 MHz) as a second IF input. A “direct sampling” mode can be selected in the PC software to select this second input, but, there is no default wiring as this has no use inside a TV tuner dongle. By far the hardest part of this build was the soldering of hair-sized wires to the Realtek RTL2832U chip, which then go to the transformer, T1.

After a considerable struggle, these two wires were solder onto the chip. I wish I could offer some useful tips on how I did this, but I cannot – I simply struggled, and faffed around until I made the connections. I would suggest a mobile phone camera placed above the board may help, since you can use the digital zoom to see in some detail. The image above was taken as I was soldering.

Finally, I used some glue to hold the (very) fragile wires in position and soldered the other ends to the transformer. I also added a small amount of glue to the transformer, too, so as to stop it moving. It looks messy, I know, but hopefully it will add some security and stability to those otherwise poor solder connections to the Realtek chip.

My final build looked like this:

Amazingly it also works! The image at the top shows the device inside the supplied box! Excellent!

1kW 144MHz Amp Pallet

Those of you who have been following this project evolve will have seen how I soldered the transistor to the heat-spreader and before that how I machined the heat-spreader & heat-sink after their initial use. Most recently, I have been building the new W6PQL pallet, based on the revision 4d schematic, found here.

This pallet offered several design changes compared to the original F1JRD design. The first is temperature tracked biasing for the FET. The F1JRD pallet didn’t have temperature tracking, but the W6PQL design uses a combination of 10kOhm and 22 kOhm NTC thermistors to track the temperature change of the pallet. A 6V Zener diode is used to clamp the bias supply and to also limit the maximum gate voltage the FET can see. A small 200 Ohm pot allows the bias to be adjusted to get the correct quiescent current. This is the next task.

The story continues with the initial power-up testing! First I need to commission my new General-Electric 50V/40A PSU I brought at the Rosmalen Hamfest back in early March.

Soldering Expensive Transistors

This morning, Royal Mail delivered me a parcel from Jim W6PQL all the way from California, USA. It took a couple of days to clear customs, but it arrived within about 5 days of being ordered. If you followed my previous post on this subject, about machining heatsinks, you’ll know that the last transistor I had failed on the testbench. You’ll also know that the copper heat-spreader was re-machined to suit the new PCB. This is why the heat-spreader has a few extra holes. Seeking advice from veteran microwave DXers & constructor (G4BAO, G4DDK, G8KBV, et al.) I was instructed to solder the device down. I watched a few of Jim W6PQL’s videos on soldering LDMOS parts to the copper heat-spreaders and replicated his instruction as closely as possible. You can see Jim’s instruction video here.

A small length of thin leaded 60/40 solder was made into a wiggle for the length of the transistor and placed in the groove previously machined in the head-spreader. I liberally applied flux to the bottom of the groove and the underside of the transistor and then sandwiched  the solder in between.

The copper heat-spreader was placed on the electric infrared hotplate and heat applied. The black dot is used to allow a laser thermometer to monitor the copper temperature. NB: this method didn’t work well.

The next two images show the solder has melted and the excess squidged out the sides. It’s clear to see when the solder has melted, since the the transistor drops. It is advised to move/slide the transistor in the molten solder to remove any voids and any excess solder. I immediately killed the heat and removed the spreader from the hotplate and placed it on a heatsink. It only took a couple of minutes to cool to a temperature I could handle, and I checked the location of the transistor against the PCB mounting holes.

The PCBs were finally mounted as a test fit. I will populate the boards before mounting them. Unlike the original jrd1 boards, these PCBs do not need to be soldered down. This means the boards can be soldered up and then mounted.

Stay tuned for more updates…