By Jim Liles, K9AXN

SR-2000 HUM problems:


K9AXN SERVICE NOTE 056

Hum problems with the SR-2000 date back to the first production trials. Hum was reduced but not quite enough to be acceptable. The origin of the problem is truly very simple. The 115vac hot line is wired to the power on switch. It is also tightly bound in the buss bundle along with every other high impedance low level signal in the radio. The Hallicrafters guys figured if they twisted it before installing it in the bundle it would be OK. Well in most of those wonderful radios the person on the line in charge of twisting was twisted and didn't do it properly. Twisted or not, that pair does not belong in that bundle without a shield. That pair can be anywhere in the bundle creating a number of different problems. Here's a quick and simple fix without tearing the bundle to pieces.

Reverse the incoming leads in the PS-2000 power supply routing the neutral lead to the power on switch. Now both lines in the twisted pair will be neutral after power on keeping that evil AC out of most of the radio. You will now have fixed a myriad of hum problems in transmit, receive, VOX etc.

For the SR-400, SR-400A, and SR-150 simply reverse the plug unless you have installed a grounding power plug. Then you will have to rewire the power supply to route the neutral to the power switch.

I've heard a number of complaints about other radios stating that after installing a grounding plug hum followed. Check for the same malady. Most of those vintage wonders have very high impedance lines running around that don't like AC lines in proximity.

Kindest regards Jim K9AXN


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