SX 71

This is the grand Hallicrafters SX-71. A five band receiver covering 560 kHz to 56 MHz. Mode of reception include AM/NBFM/CW. Riders schematic 21-21 to 21-30. This unit is in for possible restoration.

This is a second SX-71. The challenge with this radio was finding and repairing two cracked IF slugs. The slugs were discovered during the alignment. It was a lucky break that the slugs were repairable. They are made of Unobtainium.

			This is the second converter stage that operates at 2.075 MHz.	With new resistors. But we will be seeing this again.
Cap is good on the second converter transformer. But we will be seeing this again.	Sensitivity circuit under the headphone jack. Something is strange. It was put back to original.
I told you so. The alignment showed a strange reaction to adjusting T7 top slug.	Oh crap! A broken slug made from Unobtainium. It seems the top of the slug is a plastic filler type of material.	30 minute Epoxy to the rescue. The glue works great!
Guess what? The T8 secondary (top slug) was cracked too. The green sealing glipt (breakable paint/glue) did not snap when I first tweaked the slug. It seems it has been tweaked before.	Photo for the record. The transformer had to be removed to coax the broken slug out of the coil form tube.	Epoxy the upper plastic looking section.	The squeezed out hardened glue has to be sanded smooth to slip through the coil form tube.

Once this radio was fully aligned its sensitivity climbed tremendously. It must have been a pretty deaf radio for its previous owner. Most of the alignment adjustments were curiously far from the proper alignment. A few possibilities occur to me.

Some one played with the alignment with out the proper procedure and/or equipment
The components drifted over time and a proper alignment followed that drift. I measured a few of the resistors to compare their measured value with their manufactured value. This could have contributed to the way out original alignment.
The broken slugs killed the receive capabilities of the radio. Usually I can receive some stations immediately after replacement of the components. Or get a signal through with the RF signal generator on maximum. This radio was totally deaf prior to finding the first broken slug.

Unfortunately, the audio output transformer primary opened up soon after this unit was returned to the owner. A direct replacement seems not to be available. However Peter Dahl Company A subsidiary of Harbach Electronics, LLC, can custom make a new transformer. The cost is in the area of $170 - $190. If you want original functionality (500 ohm speaker impedance) then this is the way to go.

For this unit the owner chose a universal Single Ended output transformer. The 500 ohms speaker impedance is the only feature compromised with this Hammond HX-125CSE Transformer. The cost of this transformer is about 33% of a custom wound transformer.

Bad audio output transformer.

Good audio output transformer. The 8 ohm output is wired though the head phone jack that interrupts the speaker. That is found on the 3.2 ohm speaker screw terminal. The 16 ohm secondary (use only one speaker) is wired directly to the 500 ohm speaker screw terminal.

To match a 500 (600) ohm speaker with a speaker matching transformer. Built by Hammond in response to requests from the "Collins Collectors Association" for a matching audio transformer for older equipment with 600 Ohm audio output.

In its day, owning a Hallicrafters SX-71 must have made many a Ham envious as for I find it both accurate and sensitive. Today it is a handsome radio that is a pleasure to "scan the bands" with.

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This is the grand Hallicrafters SX-71. A five band receiver covering 560 kHz to 56 MHz. Mode of reception include AM/NBFM/CW. Riders schematic 21-21 to 21-30. This unit is in for possible restoration.

This is a second SX-71. The challenge with this radio was finding and repairing two cracked IF slugs. The slugs were discovered during the alignment. It was a lucky break that the slugs were repairable. They are made of Unobtainium.

Once this radio was fully aligned its sensitivity climbed tremendously. It must have been a pretty deaf radio for its previous owner. Most of the alignment adjustments were curiously far from the proper alignment. A few possibilities occur to me.

Some one played with the alignment with out the proper procedure and/or equipment

The components drifted over time and a proper alignment followed that drift. I measured a few of the resistors to compare their measured value with their manufactured value. This could have contributed to the way out original alignment.

The broken slugs killed the receive capabilities of the radio. Usually I can receive some stations immediately after replacement of the components. Or get a signal through with the RF signal generator on maximum. This radio was totally deaf prior to finding the first broken slug.

For this unit the owner chose a universal Single Ended output transformer. The 500 ohms speaker impedance is the only feature compromised with this Hammond HX-125CSE Transformer. The cost of this transformer is about 33% of a custom wound transformer.

To match a 500 (600) ohm speaker with a speaker matching transformer. Built by Hammond in response to requests from the "Collins Collectors Association" for a matching audio transformer for older equipment with 600 Ohm audio output.

In its day, owning a Hallicrafters SX-71 must have made many a Ham envious as for I find it both accurate and sensitive. Today it is a handsome radio that is a pleasure to "scan the bands" with.