My pre-WWII National HRO was very easy to repair because all the components
were easily accesible and you could easily get a meter probe to all the tube
socket pins. Components under the chassis were laid out in a single layer.
The 75A4 is more difficult because of the depth of the chassis and the
component density. In some cases it is nearly impossible to get at certain
components without unwiring other components, and Collins liked to wrap the
wire around and through the solder lug several times (and tie a square knot,
it almost seems) before applying solder. It is hard to unsolder a
connection without melting the insulation or heat damaging nearby components
and wiring.
I recall working on a 75S1 receiver. It was a nightmare. The same deep
chassis, with components literally packed several layers deep.
Don K4KYV
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