About a week and a half ago, I received an e-mail from Rich W9RNK. After a long period of home-brew inactivity (about 20 years) he decided to pick up the soldering iron and start building again. He said that my post on building N1BYT’s WBR Regen Receiver (the most popular post on this blog by far) had inspired him and I consider that a great compliment. If one of my posts inspires someone to do something they haven’t done before, or haven’t done in a long time then in my mind, it completely justifies having and keeping this blog.
He did get his WBR receiver working after some initial setbacks It seems that his problems were caused by using a core material for the inductor that wasn’t suited for the frequency. He used a toroid with a blue core, which is quoted as not being suitable for frequencies over 3MHz. On substituting the recommended yellow color-coded toroid, the receiver started working. In his write-up, which I link to below, he shows the schematic of 7N3WVM’s version of the tank circuit which includes a 0.22uH inductor from the center-tap to ground. The QRP-Tech Yahoo Group run by Chuck Adams K7QO made the WBR the subject of an informal group-build not too long ago and I noticed that some of the members experienced problems with sensitivity. Steve AA7U found that adding a choke from the center-tap to ground alleviated the sensitivity problem. Based on his experiments, he determined that the optimum value is around 1uH. I had no problems with sensitivity, so my recommendation would be to build the WBR as per the original QST article, and to experiment with adding an inductor if you do experience low sensitivity. However, I do wonder why others have had these problems when I haven’t? In the original article, Dan mentions that the length of the stiff wire connecting the center-tap of the coil to ground should be about 1″. I was careful to make mine about 1″, as well as to connect it to the ground-plane of the PCB, as opposed to connecting it to some other grounded point on one of the potentiometers or the enclosure. That’s all I can think of but hey – if an inductor works for you, that’s great.
The other main issue Rich had with his WBR was drift. I hadn’t measured the drift on mine as it seemed to be quite good. However, prompted by his observations, I decided to take measurements on mine today. From a cold start, it showed by far the biggest drift rate in the first minute (no surprise there) by drifting 120Hz downwards. In the next 14 minutes, it drifted another 190Hz down, for a total drift of 310Hz in the first 15 minutes. In the next hour, it drifted another 240Hz down, and the hour after that, 100Hz. I would have been interested to see what the drift was in the 3rd hour but boredom, and the lure of other tasks to complete prompted me to stop! My WBR (which has an AF preamp stage, unlike N1BYT’s original design) still only draws 13mA so when using it, I used to leave it on all day. I’d find that I could set it on a net frequency, come back an hour or two later and hear little drift, so I’m thinking that had I measured the drift in the 3rd and 4th hours it would have been less still. Not world-class, but not bad at all for a circuit with no attempts made at temperature compensation, and intended just for general listening.
Here is W9RNK’s write-up detailing his odyssey towards a working WBR Regen. It’s a pdf file, so make sure you have a pdf reader on your computer.
Many thanks to W9RNK for writing this up, so that it can be shared with others. Hopefully it will serve as an inspiration to anyone else who hasn’t picked up a soldering iron in a while.
PS – I do, like Rich, think an AF pre-amp is a worthy addition. I took N1BYT’s advice and used the same pre-amp that he used in his OCR II Receiver (Sep 2000 QST). Here’s the schematic of the AF stages of my version of the WBR. If you run yours from 9V, this circuit should be fine. If you’re powering it from a 12V supply (like I do) it might be an idea to include a 100 ohm current limiting resistor in the supply line to pin 6 of the LM386. The small voltage drop across it, combined with the ~0.6V forward drop across the polarity-reversal protection diode should keep you from going over the max voltage of 12V for the little chip -
That’s it for now.