Tag: kit

R2 receiver update: Time for new electrolytic capacitors

Crunch time at work has been limiting my basement tinkering, but I recently found time to work on my R2 receiver a bit more. It is pulling excessive bias current, which had me scratching my head. The audio power amplifier bias is supposed to be adjusted so that the whole board pulls 100 mA, but it’s taking 120 mA even with the bias pot set to its minimum. After double-checking all of the component values and verifying all of the bias voltages on the board, I was left scratching my head. Then I remembered the age of the kit. Even while building it, I had doubts about the electrolytic capacitors…

This R2 kit includes 15 Panasonic Z-series electrolytic capacitors. Aluminum electrolytic caps are good at one thing: Lots of capacitance at a low cost. In nearly every regard, they have inferior performance, with high equivalent series resistance (ESR), inductance (ESL), and, yes, a short lifetime. I’m not sure exactly how to translate the lifetime specifications for electrolytics to room temperature storage, but the rule of thumb tends to be that they can tolerate 5 years on the shelf, after which they require a “reforming” procedure. Reforming them at that point can get them to last another 5 years or so. After that, figure that they are shot. Old electrolytic capacitors can have excessive leakage current. In extreme cases, this current is enough to heat them up excessively and they go bang! The capacitors on this board are old enough to vote. Maybe one or more of them are to blame for the extra milliamps.

I had a little trouble finding the perfect capacitors, but then I was looking for performance at least as good as the original. This was perhaps foolish, because I couldn’t find data on the Z-series caps. Panasonic discontinued them in 2000. In lieu of further information, I arbitrarily went for high-quality models from a favorite manufacturer. In some cases, these were nearly double the price of the cheapest options, a breathtaking 23 cents versus 12 cents, quantity one. Seriously, replacements for all of the caps cost all of $2.85, including a few spares for good measure.

My one mistake so far is to forget about the T2 transmitter. If the R2’s capacitors are bad, the T2’s surely are as well. Maybe the spares I bought will end up there.

Troubleshooting is half the fun of this stuff. Who needs sudoku when you can puzzle out a misbehaving circuit?

Building a KK7B R2 phasing receiver

Hi, I’m back! It was a rough week, with a death in the family. As it ended, I found some time for some “solder therapy”. There is something good for the soul in putting together electronics. I don’t know if it’s the scent of the rosin or the satisfaction of seeing a project come together. All I know is it’s good for me.

KK7B R2 receiver, top side

The target of my soldering was my KK7B R2 receiver kit. This is the companion to the T2 I wrote about a few weeks ago. To my surprise, the R2 was easier to put together despite the higher part count. The components are larger, forcing a less dense pin matrix, and the board is separated into sections: mixers and diplexers, audio phase shift, low-pass filter, and audio amplifier. Conveniently, the board uses jumper wires (not yet installed on mine) to connect the sections. The intent appears to have been to make it easier to extend the board by swapping in a different phase shift network or adding alternate filters for CW or contesting, but it will make the board easier to debug as well, since I can bring it up a section at a time.

Keep in mind that this is a vintage kit that is no longer available, but updated versions of the design are still produced by Kanga US.

The board surprised me with its heft. I work mostly with surface-mount digital boards where the fiberglass PCB is the heaviest component. This board, though, weighs 134 g (4.7 oz), which is a lot for 100 cm2 (16 in2). The shielded inductors are the culprits. Each black cylinder in the photo is the ferrite shield of an inductor. Each inductor is noticeably heavy for its size, and this board has 10 of them.

KK7B R2 receiver board, bottom side

Once I wash the flux off (it’s not as much fun as soldering), the next piece of the puzzle is to build or buy a VFO. Rick, KK7B, designed a companion VFO for these boards, but it is out of production at the moment. I’m thinking about one of the Si570 VFOs out there. This little chip offers a very low phase noise synthesizer with tuning in steps of a fraction of a Hertz. Having this on a single chip was a pipe dream when the R2 and T2 were designed.

After that, I’ll need to build a power amplifier to boost the T2’s milliwatts up to a reasonable level — at least a watt, maybe as much as 50 W. Finally, I will have to integrate all the pieces into a working rig. Professionally, I do a lot of microcontroller work, so it’s tempting to build a fancy digital control panel with circuitry to integrate every R2 and T2 option imaginable. Instead, I’m trying to keep myself to something simple: one band, SSB only, and a digital frequency readout as the only frill.

I still have to pick the initial band. I’m torn between 20m, my favorite for PSK31, and 40m, which I like for SSB. I’m after whatever DX I can land in either mode. What band would you recommend?