SX 71

 


Home Up HT-17 SX-24 S-38 S-40B SX 71 SX-62a TW-2000

 

 

 

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.  

 

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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.

 

IMG_1315.JPG (175968 bytes) IMG_1317.JPG (121927 bytes) IMG_1321.JPG (221854 bytes) IMG_1322.JPG (127482 bytes) This is the second converter stage that operates at 2.075 MHz.   IMG_1323.JPG (127768 bytes) With new resistors.  But we will be seeing this again.
IMG_1325.JPG (156294 bytes) Cap is good on the second converter transformer.  But we will be seeing this again. IMG_1337.JPG (887131 bytes) Sensitivity circuit under the headphone jack.  Something is strange.  It was put back to original.       
IMG_1342.JPG (123413 bytes)I told you so.  The alignment showed a strange reaction to adjusting T7 top slug. IMG_1343.JPG (201228 bytes) Oh crap!  A broken slug made from Unobtainium.  It seems the top of the slug is a plastic filler type of material.   IMG_1344.JPG (153269 bytes) 30 minute Epoxy to the rescue.   The glue works great!    
IMG_1347.JPG (105445 bytes)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. IMG_1351.JPG (187936 bytes) Photo for the record.  The transformer had to be removed to coax the broken slug out of the coil form tube.  IMG_1352.JPG (116142 bytes) Epoxy the upper plastic looking section.  IMG_1354.JPG (110977 bytes) 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.  

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

  2. 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.  

  3. 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. 

        

IMG_1661.JPG (112850 bytes)  Bad audio output transformer. IMG_1786.JPG (200452 bytes) 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 transformerBuilt by Hammond in response to requests from the "Collins Collectors Association" for a matching audio transformer for older equipment with 600 Ohm audio output.

119DA Photo 

 

 

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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