FRR/MRR/SRR Crank Arm Replacements

by Joe Bartolick

The die cast switch arms used in these receivers are a chronic source of trouble.   This problem that showed up while the receivers were still in active service.   Later units had steel switch arms in place of earlier die cast alloy ones.   For the majority of units in this family, however, replacement of cracked or completely broken switch arms is an ongoing problem.   While a parts receiver may seem to be good source, there is still demand for these receivers and a limited supply.   Why trash one just for the switch arms?

Original and replacement crank arms

Making serviceable replacements is not as big a deal as it first appears.   Unless you are seeking to keep the receiver in a museum quality state, the original design need not be followed precisely.   The two drawings here show serviceable and easily made substitutes for the original switch arms.

The first type of arm is used on the antenna, RF, mixer and oscillator boxes, all located in the bottom of the receiver chassis.   This lower arm is simply a straight steel bar 1/4'' square, made from what was sold as key stock at my local hardware store.   The pin is similar material from the same source.   The pin is silver soldered into the bar, but could be epoxied in.

The lower crank arm diagram

The other type of arm is used on the 2nd IF, AF and BFO modules, all located on the top of the receiver chassis.   The upper arm has a different shape than the lower arm.   Because of the thinner section of the upper arm the pin needs to be more securely fastened than epoxy will allow, so silver solder or something equivalent is needed.

The upper crank arm diagram

The tools used in making these were a hacksaw, file, #4-40 tap and drill press.   A hand drill would work as well, with care.   A nut could be used in place of the tapped hole but would need to be held from rotating during adjustment.   Other materials could be used, particularly brass, but the steel stock was available so I used it.   The pins, however, should be steel in any case.

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