Tag: capacitor

KK7B R2 receiver: lifted pads, a scorched board, and it works anyway

I’m slowly making progress on my KK7B R2/T2 transceiver project. At my last report, I was waiting for replacement capacitors to arrive. They did, and I pulled out my ancient solder-removal iron, a Radio Shack unit from who knows how long ago.

Either my unsoldering skills are rusty or I was too impatient, or both. I managed to lift four pads, completely demolishing one. The other three were salvageable. I’d like the board to be perfect, but having it work is an acceptable substitute.

The new caps went in easily, with only a little fiddling to wire around the ruined pad. Better yet, the excessive bias current I saw before the replacement is gone! The board is supposed to be adjusted to 100 mA current. It used to start at 120 mA, with the bias adjustment set to its minimum, and drift its way up to more and more current from there (going as high as 200 mA before I’ve lost my nerve and switched it off). Now it starts at 84 mA and… drifts its way up from there to 130 mA and more.

OK, so one problem was solved. I can always increase current with the bias adjustment potentiometer.

After scratching my head a bit, I finally noticed one small sentence in KK7B’s articles on the R1 and R2. The audio output transistors need a heat sink, do they? Oops! I dug around a bit in my junk box but couldn’t find anything that would fit. The articles say the audio amp will drive a pair of headphones fine without the output transistors, so I decided to take them out.

I recently got a Sparkfun hot-air soldering station (a Sparkfun Free Day prize!) and thought I’d give it a shot. Sure, hot air is usually for surface-mount parts, but solder is solder, right?  Not being sure how to set the airflow and temperature settings, I set both on the high side, put some flux on the output transistors’ pads, and went for it. The result wasn’t pretty:

Oops... A scorched R2 PCB

Yes, I scorched the board. Oops. Between this and the lifted pads, I think I need to work on my unsoldering skills.

Since the transistors are 50 cent parts (TIP29/TIP30), I unsoldered them the easy way: I cut the leads, removed the leads from the board with my conventional iron, then cleaned out the holes with my solder sucker. I didn’t damage any pads this time!  (The picture above was taken after all of these steps.)

A little more soldering to hook up a BNC and some other goodies, and the board was alive!

R2 on the bench, surrounded by test equipment

That’s a Tek 191 signal generator as the VFO (variable frequency oscillator), with a frequency counter as the readout. That’s an MFJ QRP antenna tuner in the foreground.

I didn’t build a phasing network yet, so I drove only one VFO input. That causes the R2 to function like a conventional direct-conversion receiver,receiving both sidebands simultaneously. That said, it works. I was able to hear signals, though frankly many were quite hard to copy. I’m not sure what else might be wrong.

The frequency counter spits out a lot of digital noise. I learned to flip it on only to spot-check my frequency. It’s a lot quieter in standby mode.

Did I mention that IT WORKS?

Despite the success, I’ve been struck by a bit of paralysis in moving forward. There are so many choices to make for integrating the radio.

  • What kind of VFO should I use? Should I design my own or buy a kit?
  • Which modes should I include?
  • How much power output do I want?
  • What power amp should I use? Should I design my own or buy a kit?
  • Which case should all of this go in?
  • What microphone, and microphone connector, should I plan for?

Keep in mind that this is supposed to me my fast route to getting on the air, so I’m thinking kits for both the VFO and amp. Besides, with as busy as it has been at work lately, it is nice to sit back and just build something.

It’s not a pretty project any more, but it works! Hooray!

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 the AK5388 ADC breakout board

“Honey, the package you’ve been waiting for from Hong Kong is by your computer,” said my dear wife shortly after I got home from work on Friday. Even better, a few minutes later she suggested that I spend the evening in the basement, building up one of my new boards. I have a wonderful wife!

The AK5388 ADC breakout boards finally arrived!  I had a hard time waiting for them. First, Itead Studio didn’t ask me to correct the design until the day the finished boards were supposedly going to ship. (They did apologize for the delay — it sounded like there was a communications snafu between them and the fab.) Then I waited five more days for the board to be fabbed. Shipping from Hong Kong to Ohio took ten days. Looking around on the web, I’ve seen shipping times reported from seven to ten days, sometimes going up to as much as 20 days during the holiday season.

All eight boards look great. Other people who tried Itead reported some over-etching and silkscreen problems, but I don’t see any defects on mine. Since Itead now does 100% electrical testing, I have confidence that the boards will all work. I could spot the tiny dimple in each pad where the flying probes touched down, so it is clear that all eight boards were tested.

I went to the basement and heated up the soldering iron. The board went together easily. The 0.80 mm pitch of the AK5388 was downright easy to solder after the 0.50 mm  A3PN250 FPGA and other fine-pitch parts I’ve been using at work. Besides, I’ve learned some new soldering techniques lately that helped me solder the AK5388 quickly, but I’ll have to share those in another post. I did use a meter to check all of my AK5388 solder joints, though. There were a few bridges, but they cleaned up without a problem.

The pads for the big electrolytic capacitors are larger than necessary. I used PCB’s default EIA7343 footprint. The pads had plenty of room for the soldering iron, but they could have been smaller without sacrificing ease of assembly. (Did I use the wrong footprint once again? 0805 capacitors seem to be the only ones I get along with…)

Lately Digi-Key has been taking much stronger steps to control moisture uptake by the semiconductors they sell. Instead of just shipping some cut tape in an anti-static baggie, they now seal the chips in an airtight bag with a packet of dessicant and a humidity indicator. I opened the bag this one was in about 4 weeks ago. Not bad so far, considering it was in my not-very-dry basement that whole time. Moisture uptake is important for reflow soldering techniques, but as far as I can tell, it is less significant for hand-soldering.

Now I’m left asking myself what comes next. In my original plan, the next step was to couple the ADC to the FPGA, put a USB core on the FPGA, and build a sound card. Once that works, adding a local oscillator and a quadrature mixer will make everything I need for a PC-based software-defined-radio (SDR) receiver, and this long trek will finally result in a radio.

However, I hear that the bands are great these days, and I’m not sure I want to take the time to homebrew an SDR rig just to get on the air. Maybe I should spend some time on a faster route to a radio, then come back to the SDR. I’ll probably have more on that idea next week.

Until then, keep on tinkering, and as always, your comments are welcome!