Why Does That Receiver Hum?

Because it doesn’t know the words? Guess again. John Hutson found out
Author:
Publish date:
Social count:
0

Continental Electronics’ John Hutson adds to our recent tube discussion.

He was trying to get hum out of the audio in an old “boat anchor” receiver that used a 12AX7 as the audio driver. He knew he had a mechanical issue because wiggling the tube would cause a static noise and the hum level would change. Different tubes acted the same way.

John sprayed Caig Labs’ DeoxIT cleaner on the tube pins, and he plugged and unplugged the tube a few times, but heard no improvement.

The next step was to wiggle the tube socket terminals under the chassis with a plastic alignment tool. All the terminals had the “problem” so John resoldered a few that didn’t look good. Still, he had no luck resolving the problem.

Next came the ohmmeter to check for bad resistors or leaky capacitors. Everything checked out fine.

John then turned the unit on and started to check voltages. While checking one pin, his probe slipped off the tube socket terminal and hit the metal mounting ring of the socket. The hum stopped momentarily.

Image placeholder title

Fig. 1: “How Not to Mount an Antenna 101.” Here’s what had happened: The tube socket had four ground lugs around its bracket that were used to make the ground connections for the resistors and a capacitor that went from various pin terminals to ground. The tube socket was attached to the chassis with rivets. The rivets had loosened over time, and there was rust between the tube socket and the chassis.

A simple wire added from one of the socket ground lugs to the ground lug of an adjacent terminal strip solved the problem.

The hat’s off to you, John, for your persistence in solving this problem, although I wouldn’t have expected anything less from a member of the Continental service team. Thanks for taking us through an organized and effective troubleshooting process.

****

Directors of engineering come across interesting situations in their travels. Saga DOE Tom Atkins sends along Fig. 1, a case in point.

While traveling in North Carolina recently, he glanced out his car window and saw this tower/tree combination. It was on the side of a mountain. Do you think the tree right in front of the bays may add some directional properties — especially when the tree ices up?

That’s another reason to inspect your tower/antenna sites regularly.

****

Recall our suggestion about lying on an ironing board to help gain access to certain hard-to-reach components under a transmitter’s power amplifier.

Radio Bob wrote to take us to task for this:

“Really hate that ironing board idea. Understand the necessity, but they’re not built to hold the weight of a person laying on them. Wonder what’s worse? Being stiff after sitting on the floor and being a contortionist to access what you need, or the injury when that damn board buckles in the middle and … you’re involuntarily and instantly placed in a contorted position (and your head is cracked against metal or protruding parts)? The ironing board idea is a bad idea waiting to happen.”

Image placeholder title

Fig. 2: Most ironing boards have adequate reinforcements to support a body. But it’s still smart to check. The problem with sitting on the floor is not so much about being stiff, but rather twisting yourself to obtain visual and manual access to components. A plank or, in this case, an ironing board, may make access easier.

Certainly, though, assess your ironing board first for your weight and size. I pulled out my ironing board and noted the metal sides are supported and rolled, to avoid buckling; see Fig. 2.

****

Friend Weller is chief engineer for Utah Public Radio. He sends a few more eagle-eye comments about the mystery radio station photo we published in the Feb. 1 edition of Workbench.

As we noted, “Saturday, October 7” tells us that the year was 1967 or 1972. Friend notes a blue/green MGM record label. If so, the date was no earlier than 1968.

The cheap plastic headphone lying on the console is another giveaway, indicating that this was perhaps the early ’70s, because 1960s “cans” were still kind of big and clunky.

The Tapecasters and the Ampex reel-to-reel appear to be of a more recent vintage, perhaps the early 1970s.

If the scan was of a higher quality there might be chance of picking something off the program log, a record label or the memo on the glass.

Also, Kitchen’s Field is the name of the football field at New Albany’s high school.

Friend writes if he were a gambling man, he’d identify the station as WNAU, New Albany, Miss. Perhaps this was their production room in 1972, or it may have been the control room.

WNAU signed on the air on June 1, 1955.

Contribute to Workbench. You’ll help your fellow engineers and qualify for SBE recertification credit. Send Workbench tips to johnpbisset@gmail.com. Fax to (603) 472-4944.

Author John Bisset has spent 45 years in the broadcasting industry and is still learning. He handles West Coast sales for the Telos Alliance. He is SBE certified and a past recipient of the SBE’s Educator of the Year Award.

Related

Image placeholder title

We Love That 'Mr. Clean Attitude'

Aren't solid-state transmitters great? You just plug them in, and forget about them!...Too often that's what happens, unfortunately; and this "out-of-sight, out-of-mind" attitude eventually will cost you. Dirty air is no friend of the broadcast engineer and it can have a dramatic impact on the operation of solid-state transmitters.

Image placeholder title

I Ain’t Climbin’ That Thing!

A tower owner calls in a rigger to climb his tower. Before he even sets foot on the structure, the rigger makes a visual inspection...Sorry, there will be no climbing today...Figure 1 shows why: the hollow legs of the tower have split.

Image placeholder title

Little Things That Mean a Lot

Consider those solid-oak studio doors that you specified for that added soundproofing touch to the studio project. They’re great, except they lack a window.

Image placeholder title

Rebuilt Tubes That Work

I've only worked with one engineer, now many years retired, who made soldered RCA plugs a work of art. As widely used as these plugs are, they are without a doubt the most cantankerous.