DESIGN PROCEDURE 



Figure 44.2b may produce a chassis of more convenient shape, but would be 

 extremely liable to oscillate. As a general rule one does not go far wrong 

 by allowing the theoretical circuit diagram to suggest the physical layout. 

 If the panel controls are then arranged to be near the stages they operate on 

 it is usually the case that, since the theoretical circuit is logical, a logical, 

 and therefore conveniently usable, panel layout results automatically. 

 Furthermore the length of wiring runs is minimized. Unfortunately the 

 rule sometimes breaks down with composite valves. For example, what 

 should be done if part of the electrical circuit is as in Figure 44.3a ? It all 

 depends on circumstances, but Figure 44.3b is a possibility. 



HT + 



When the several chassis have been completed and are bolted together 

 on a rack, it may be found that the various units, though they behave 

 properly alone, do not work properly in concert, due to some kind of inter- 

 action, of which the most likely is excessive pick-up by the amplifier circuits 

 of artefacts originating from the time base or stimulator. Setbacks of this 

 kind are to be anticipated and should not be a cause for despondency. 

 Use the 'freezing' technique described in the last chapter to establish how 

 the interference enters the amplifier, whether via the power supplies or by 

 the input circuit, or by some other route. If the former, the output impedance 

 of the power pack must be reduced, or two separate packs used, or if 

 necessary the amplifier may have to be supplied from batteries. If by the 

 input circuit, the methods enumerated in Chapter 43 are available to 

 combat the trouble. 



676 



