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Author Topic: Hammarlund SP-600 sporadic noise/performance  (Read 1892 times)

Posts: 139

« on: May 14, 2013, 10:14:55 AM »

After more than a moderate amount of time spent trying to find the cause of sporadic noise and degraded performance on my SP-600 JX17, I found the culprit. It was a not-so-obvious mechanism, and due to a construction detail that I would never have expected from Hammarlund.

Resistors R72 and R73 (voltage divider) provide the screen grid bias for 1st and 2nd IF 6BA6's. The resistors are 20 watt and end-mounted to the chassis side beneath the power transformer, using screws/nuts/mica washers/conical washers. The chassis ground side of R73 employs a lug sandwiched between the chassis and the conical washer that secures the resistor to the chassis. Mica washers are employed on each end of the resistor between the conical washers and the ends of the resistors.
Given the limited amount of compression that should be applied by the screw/nut to prevent damage to the ceramic resistors (and possible compression creep of the mica washers), the result is a "flaky" chassis connection of the sandwiched ground lug of the voltage divider.

This was the cause of the noisy, sporadic performance in my SP-600. My solution was to relocate the chassis ground-lead lug to it's own dedicated chassis mounting, NOT compromised by limitations of the resistor mounting and it's associated hardware. I relocated the R73 ground-lead lug to the chassis side with it's own 4-40 screw/nut (requiring an added hole in the chassis side). But, the ground-side lead from R73 could be replaced with one with a larger lug and attached to chassis using either of the two screws which secure the adjacent subassembly (containing C161 filter cap) to the hex standoff posts. I opted to use a dedicated ground to minimize the number of interfaces between R73 and chassis ground.

No more aggravating sporadic noise, or flaky performance from this great receiver!

If you have similar symptoms to those described, relocating the R73 ground lug could be the remedy.
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