As an aside, you may notice a little bias ‘bleed’ superimposed on top of the audio on playback head 1. This is due to its physical proximity to the record head. The bias signal is effectively a LF (low-frequency) radio signal and the wires to the record head act as an antenna launching (transmitting) the bias signal, which is then the picked-up (received) by the playback head connecting wires. Although this bias contamination can be seen on the ‘scope it cannot be heard as it it is outside human audible range and outside the upper frequency limit of a guitar amplifier.
In principle the head alignment procedure is fairly straightforward. First, position all five heads by eye so they are centered on the drum face as described earlier. Next, inject a 100mV 1KHz sine wave from the signal generator, use the ‘scope to monitor the output and power the Echorec up. Hopefully the scope will show at least some output signal from the heads. Now begins the game of carefully adjusting the heads to maximise the signal level. The game is easy to play at low frequencies up to about 1 or 2KHz, however becomes progressively more difficult, and frustrating, at higher frequencies above 8KHz where correct gap alignment is super-critical; in some cases the game is impossible to win.
For instance, the open-frame heads fitted to Binson’s earlier Echorec models are only good up to around 5KHz, and if the gap has widened due to wear then their upper frequency break point is even lower—they’ll have significantly reduced or no measurable output above 8KHz. The newer shielded heads Binson introduced in the 1960s have a better HF response, topping out at around 10 to 12KHz. However, if you’re seeking clarity and definition, then I suggest getting hold of the new Photovox brass heads. These are manufactured to exacting tolerances, possessing an extremely narrow gap, which yields an upper frequency response above 20KHz. Further, they’re well shielded, keeping mains hum to an absolute minimum.
Anyway I think that just about covers everything. Can I just finish by saying, whichever type of heads are installed on your machine, be assured you will have your work cut out for you; the entire alignment process can easily take the best part of day. So be warned. It’s skilled work requiring expertise and the patience of an angel, and there is a proverb about that—”Fools rush in where angels fear to tread.”
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