atwater kent 545 distortion problem

I got a B&K TV analyzer and restored it, just so I could sort a problem I was having with a TV chassis.  Even though the components test equipment was built with was the best there was, old test equipment can suffer from the same ills as old radios or old Tv sets or old stereos have.  I have found it to be almost as much fun getting the test equipment to run dependably as it is to use it on what you are trying to fix.  It turns out the problem I was having with the TV was found and fixed before I got the Analyzer going.  But I used the audio 1000hz signal from the Analyzer to set the balance on a stereo amp I was working on.  Good test equipment can be found for not much money if you look around and aren't picky about perfection.

wildcat445 wrote:

As to not wanting to buy test equipment, there is an old saying that works well here:  "If you are gonna dance, you gotta hire a fiddler".....

I will second that. In any hobby the quality of the work that you do is often related to the equipment that you are using. In recent years I returned to amateur photography. I have spent several thousand dollars on a DSLR and other photographic equipment, but it allows me to take the quality of photos that I desire (which blow away the images taken with any of my other cameras from the past 60 years).

Some work on antique radios may require equipment that you do not have. A fellow collector once brought me in to help to solve a distortion problem in his Zenith shutterdial console. I easily found it using my Tektronix 453 oscilloscope. He didn't own a 'scope, and most antique radio repairs don't require one. When you have a tough nut to crack, though, sometimes the only cure is a more sophisticated nutcracker.

As to not wanting to buy test equipment, there is an old saying that works well here:  "If you are gonna dance, you gotta hire a fiddler".....

Seriously, I don't know what output tube you are using, but you might be getting some grid emission in an audio tube as they get warm.  Put your voltmeter, set to DC, on the grid of the output tube.  See if the grid voltage slowly rises as the tube warms up.  If it does, you have grid emission from the output tube.  When a tube is built, sometimes some of the emitting substance finds its way to the grid.  As the tube warms up, the grid actually starts emitting electrons and becomes more positive in relation to the cathode.  If you don't find emission in the output tube, go to the first audio tube(s) similarly.  Good luck.

You can also short the grid of the output tube to ground thru a capacitor.  If the distortion clears up or goes away, you have confirmed that you are getting DC on the grid.  You might also check the coupling caps to make sure they are of the correct value.  Mistakes happen.

Have tried using different good tested tubes with no improvement.
Don't think I want to go out and buy more expensive test equiptment and then try to learn how to use it.
Will just try to moniter voltage when cold and again when warmed up and see if there is a difference.

Radionut wrote:

All tubes tested on my hikock tester

That just tells you that those tubes are not grossly bad - it does not tell you if they are good.

I will second Frank's suggestion of using an oscilloscope (my preference) or signal tracer to find the source of the distortion. Another approach would be to measure the voltages at the tube pins at startup and again after the problem develops. Either approach would probably be easiest with socket extenders or a chassis stand.

Hello,
All tubes tested on my hikock tester and all caps and resistors are new.
Definitely a warm up issue.

Often it is a tube causing the problem. Also could be a bad capacitor or cap.

More derails would help.

Did you use new parts or salvaged ones?
Were the parts tested before installing?
Are the replacement tubes used or NOS? Tested?

Need a bit more info. Could be a tube failing as it warms up. Have seen shorts and gassy tubes cause the problems.
What tests have you run?
 An oscilliscope or signal tracer would be handy to track the distortion too.

Hello, Have a model 545 AK that I recapped..
Has an issue with distortion after warm up.
Sounds great when first fired up, but then starts getting distortion issues after about 10 minutes.
Have replaced all caps and most resistors as well as a couple weak tubes.
Any suggestions?
Thanks,