The Electronics Curse

Many years ago I believe I coined the phrase; "The Electronics Curse." This curse like "Murphy's Law" was used to describe the situation where having designed and constructed some circuit, it was tested. Usually with good results and some application for the time. But with some circumstances beyond my/our control, the circuit would go off into the wild blue yonder.

 

The Curse

"Your amplifiers will oscillate, your oscillators won't!" Often described a new circuit on test that did not come close to working like it should.

Sometimes it was a construction mistake. Carrying the output too close to the input. The short but inductive earth wire was making the circuit 'whistle' at VHF. Or the electrolytic capacitor being connected in parallel with a ceramic capacitor. The designer having forgotten the parallel tuned circuit this creates. Or that the audio amplifier was 'tweeting' (Not the new version) and making a sound like a bird repeating at a regular interval.

This last has triggered my writing this. I have for a long time wanted to re-construct the Direct Conversion receiver from 1968 QST magazine. Using modern 'second used' components. That is a polite way of saying scrounged from junked electronics.

The circuit is this one:-

[You may remember this circuit from The Anode March 2021

Using modern transistors I constructed the circuit of the audio amplifier. Using scrounged capacitors and new(ish) resistors, I built the circuit on Vero like board. This even in the 60's was the quick and easy way of putting a circuit together. Also disliked by R.F. engineers it would provide a path for R.F. feedback of the bad variety. Producing a PC board is out of the question. Cost alone makes this out of the reach or of no interest to most people round here.

What the three transistor circuit was doing, was doing, was oscillating at several MHz. Which was charging/discharging the electrolytic capacitors at audio. 

The resulting 'squawk' was disturbing the cat! I had the output connected to one of the small PC amplified speakers that people threw away.

When I got to the back of the board I found the trace that I had forgotten to cut, was running past the input. So I cut the trace along with a few others for good measure. Switching on, the circuit was quiet. No longer disturbing the cat. And a gentle background hiss said the circuit was sensitive enough to use.

This all is not surprising as modern (1990's) transistors have a much higher transition frequency (FT) than the 1960's ones. So much more gain at VHF and H.F. So a small amount of capacitance and lots of gain add up to instability and oscillation.

So the next step is to place it in a screened enclosure.

The next step in the project is to try out some cores for the mixer. These will have to be 'cheap'! Lets see how the Amidon and Micro-Metals perform vs the old Mullard/Philips types.