electrical noise affects amp - solution?

[gmgraves]

There are scopes that can measure up to 600V directly with the isolation built into the scope. Amec, Tektonix, Agilent and Fluke produce these models.

 

Most "TV service quality" scopes have the isolation you speak of and are perfectly safe - at least in the USA. As I said earlier, every scope I've ever owned, from a $39.95 Eico kit built by my dad in the mid-fifties, through a $150 Dick Smith 'scope purchased in the '80's, to my current Tektronix 465, has had that isolation built-in, which is probably why I've never given the danger aspect any thought. To me it's no more dangerous than measuring ones mains voltage with a volt/ohm meter. Sure, you have to be careful not to get shocked, but that goes without saying - I would think.

[gmgraves]

Which CRO do you think I was using when I stupidly blew up rectifier diodes in that old Sharp transformerless TV?

Some DSE CROs may have had this feature, but mine didn't .

 

 

When I was in college, I worked for a TV repair shop. I used to hate working on so-called AC/DC TVs (which is what transformerless TVs were called at the time). Here in the US, in those days, plugs were two-pronged, and non-polarized. All you had to do was plug one in wrong (and you had a 50/50 chance of doing that - which meant that you'd do it wrong 90% of the time!) and you had a hot chassis. The worst were the ones with painted metal cabinets. They became "hot" too. Add to that the fact that all of the tube filaments in these sets were in series (you had to test all of the tubes to find which tube had 'blown out". You couldn't look at what the set was doing wrong, and think to yourself - I have to test the tubes in the horizontal hold circuit, because that's where the problem is. With all the tubes dark, the set did NOTHING), and working on these cheap clunkers was a royal pain. Muntz TVs were the worst in this regard. My boss at the time used to call 'em "gutless wonders".

[4est]

I thought those were the cars from the seventies when emission control started...

My boss at the time used to call 'em "gutless wonders".

[gmgraves]

I thought those were the cars from the seventies when emission control started...

 

 

I suppose, that too. But the connotation is different. "Gutless" WRT early automotive emissions controls refers to the cars having no power, in Muntz (and several other cheap TV brands), it referred to the fact that compared to an RCA or a Zenith, or a Magnavox or Motorola, they had only the barest minimum of components that allowed them to function as televisions,, and thus had few "guts". For instance, aside from having no power transformer, many had no RF amplification in front of the tuner and only one or two IF stages after it. This meant that these TVs were useless in anything but urban environments, close to the transmitters. Get more than 15-20 miles from the transmitter tower, and you couldn't get an acceptably noise-free picture.

[One and a half]

Just to nudge the post along....to the OP, could you update the thread on your findings, thanks.

[firedog]

Determined the problem was DC on the line. Have a DC blocker on order from Van Alstine Hi Fi. Will update after it arrives - probably in 2-3 weeks.

[firedog]

Hi-

Just installed this: the Avahifi Humdinger , which is a DC Blocker.

 

Costs $125 Plus shipping. It's small and not heavy, so shipping is reasonable.

 

Worked like a charm. No more noise.

[One and a half]

So there is DC on the mains. OW.

 

At least no more transformer buzzing.

[firedog]

Yeah. We didn't do an actual measurement, b/c we don't have the proper equipment available. But by process of elimination of other possible sources and the actual symptoms involved it became pretty obvious that DC was the problem.

 

I originally thought that a nearby industrial site (about 500m away) must be the problem, but turns out it is on a different transformer on the local grid, so that isn't the problem. Our guess at the moment is that the DC is probably coming from neighbor(s) satellite TV dish/cable box combos. Apparently they aren't setup well by the provider and it's thought they could be polluting the local grid. We are in semi-detached houses and there is a shared dish setup on each building (individual households each have their own boxes which connect to the shared dish.)

 

 

So there is DC on the mains. OW.

 

At least no more transformer buzzing.

[Speedskater]

While satellite TV dish/cable box combos can cause all sorts of AC power problems, DC offset isn't one of them. It takes appliances or machines using significant power to cause DC problems. Also you wrote the the problem was intermittent, so that would indicate that the problem child is turned on/off not something that runs 24/7 (like a TV dish/cable box).

 

Have you checked your power line voltage both during good & bad times?

[4est]

I thought the same thing.

While satellite TV dish/cable box combos can cause all sorts of AC power problems, DC offset isn't one of them. It takes appliances or machines using significant power to cause DC problems. Also you wrote the the problem was intermittent, so that would indicate that the problem child is turned on/off not something that runs 24/7 (like a TV dish/cable box).

 

Have you checked your power line voltage both during good & bad times?

[Ellsworth]

After reading the material, it seems that the problem is DC offset. I came across this (avahifi - AVA Humdinger) and it seems like this is just what I need.

I had the exact same problem as you describe and bought the AVA Humdinger last week. It works perfectly and takes care of the problem without hurting sou d quality. It's nice when that happens.

[firedog]

While satellite TV dish/cable box combos can cause all sorts of AC power problems, DC offset isn't one of them. It takes appliances or machines using significant power to cause DC problems. Also you wrote the the problem was intermittent, so that would indicate that the problem child is turned on/off not something that runs 24/7 (like a TV dish/cable box).

 

Have you checked your power line voltage both during good & bad times?

 

I'm not a knowledgeable person on these subjects, just know what I've read. Apparently devices like printers, photo copiers and espresso-machines are also known for causing enough DC offset to be a problem.

[Speedskater]

Yes equipment and appliances that use lots of watts (an espresso-machine is a good example) could cause a DC offset. But an intermittent problem like these should be easy to track down.

[firedog]

Yes equipment and appliances that use lots of watts (an espresso-machine is a good example) could cause a DC offset. But an intermittent problem like these should be easy to track down.

 

Yes, but not if it isn't in my home, but at one of the neighbors.

[Speedskater]

How true.

Ham radio operators have lots of interference problems with grow lights in neighbors homes. It's not safe to talk to the neighbors about their grow lights.