‘Technical Shorts’ by Gerry O’Hara, G8GUH
|‘Technical Shorts’ is a series of (fairly) short articles written by Gerry O'Hara, a CVRS Director and vintage radio enthusiast, each focussing on a technical issue of relevance in repairing, restoring or using Eddystone valve radios. However, much of the content is also applicable to non-Eddystone valve receivers. The articles are the author’s personal opinion, based on his experience and are meant to be of interest or help to the novice or hobbyist – they are not meant to be a definitive or exhaustive treatise on the topic under discussion….
References are provided for those wishing to explore the subjects discussed in more depth.
Alignment using a Wobbulator
My Technical Short on ‘Receiver Alignment’ covers the basics of why a receiver needs to be aligned, how to check alignment and, if necessary, how to re-align a receiver. In addition, several of my other articles posted on the EUG website cover other aspects of receiver alignment, eg. the Technical Short on ‘Detectors and Discriminators’ provides a basic grounding in detector principles and operation, together with details on aligning a Foster-Seeley type FM detector as found in Eddystone S770U and S770R VHF/UHF receivers. In addition, many of my restoration articles include some detail on aligning particular receiver models, so I will not repeat that level of detail here. Instead, I will describe what a ‘wobbulator’ is and how it can be set-up and used to assist in the alignment of Eddystone receivers, including some examples.
Some Alignment Basics
I would encourage you to read the two Technical Shorts referred to above for starters. Once you have digested the information in them, you will understand the benefits of some form of visual method of checking a receivers alignment or for actually undertaking the re-aligning. Basically the ways of aligning a receiver (‘worst’ to ‘best’) are to use a:
- Received broadcast signal and tune ‘by ear’;
- Signal generator and ‘tune by ear’;
- Signal generator and an output meter;
- Wobbulator and oscilloscope (‘scope); and
- Wobbulator, ‘scope and accurate frequency source (eg. crystal oscillator or frequency synthesizer) as a marker;
- Modern spectrum analyzer.