Twisted progress

I have made a significant improvement. I will get to that but not without first boring you with some peculiarities from long time back.

You may remember the paper material I got. One reader called it a “time capsule”. Included was the book “Radiobyggeboken” (The radio construction book). Look at what I found in that book:

V2 on the left is last IF amp and detector. V3 to the right is the BFO. But what is the schematic symbol on the connection between the two transformers (top between MF2 and MF3)? Text explains it as a pair of wires twisted together to form a capacitor

May be capacitors were expensive in the 60’s. May be the builder was frugal. Or maybe this was common practice. I don’t know. The idea was new to me. Though I have to admit I once made a variable capacitor using cardboard and Aluminum foil.

It’s fun reading about almost forgotten tricks. This one hit home extra because I have been working on BFO injection to the IF in the HR-10 receiver. See my mostly failed experiments!

Product detector rant and mystery

Of course, the proper thing to do would be to build a product detector into the receiver. An example can be found on this blog post about improving the HR-10.

I can hardly think of the word “product detector” without thinking back to were I was introduced to it. That was in the Danish book “Vejen til sendetiladelsen” which I read in preparation for my license exam. I passed the exam thank you, despite reading a foreign book and never understanding the schematic of that product detector. I didn’t have Internet or search engines at the time, but I have now spent some time trying to find a similar schematic and an explanation. After some effort I found a close match:

From a nice RF Cafe blog where it is fig. 11. That blog seems to be a reprint from February 1972 Popular Electronics.

This is almost identical to what I found in the Danish book. The differences are irrelevant to my lack of understanding it. The similarity undermines one hypothesis; that there was an error in the schematic.

My problem is that I can’t understand how this works, or even how significant level of signal remains on the audio side. As I read the schematic, the point between the two diodes will quickly become positively charged, stopping almost all further signal propagation or mixing. A little will get through because of tiny reverse currents and capacitance in the diodes. But that should be minimal. Nothing in text or drawing suggests the breakdown voltage of the diodes are involved.

Did I finally find the explanation after all these years? Nope. The text in the blog/original article matched the Danish text so closely I am convinced the Danish writer had this article by his side when writing.

I now rely on my readers being smarter than me to explain the workings of that detector once and for all.

Should I modify the HR-10 with a product detector? I hesitate. This is not the best receiver I have. If I wanted to build a better receiver I would not use the HR-10 as a starting point. It has a high value as a really nice vintage receiver. I have fallen in love with this charming box with all its shortcomings. Putting much effort into making it closer to a modern radio seems hard to justify. But I could still pick some of the low hanging fruit. That was the idea behind my attempts at injecting more BFO power via a simple capacitor from the BFO to the screen grid of the IF amp. The lack of dramatic success may be because I tried to inject to the screen grid instead of the control grid. Or because I didn’t use enough capacitance.

What if I (reluctantly) connected the capacitor to the control grid instead? It would have to be a much smaller capacitor. Still, the IF resonant circuit would be detuned a bit and a little power lost to ground. Schematics in this blog post. I decided to try, accepting that I would have to realign the IF again. But I planned to do so anyway once I get to improve the signal generator (SG-402).

I felt like trying the idea from Radiobyggeboken. Meanwhile I had discovered this idea was not unique at all. SM7NDX mentions doing something like that on his HR-10. Unfortunately details are sketchy.

Come on let’s twist again

I just tried randomly. No calculations, no objective measurements.

Twisted enameled wire serves as a tiny capacitor connecting BFO to IF last amp.

I have no idea if this “capacitor” is close to the sweet spot or not. My ears can tell me this was a major improvement.

OK. A little dressing is needed etc. but I have (almost) picked one low hanging fruit. I have a twisted feeling of accomplishment.

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