heath: T-4 (T4); Signal Tracer

The Heathkit T-4 Signal Tracer is very similar to their model IT-12 (see ID 72.708). The circuit diagram is almost identical. Only the two rotary buttons are of an older model in the T-4. This confirms the dates of production: 1965 for the T-4 and 1968 for the IT-12. 

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