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 Post subject: Repair success using Heathkit AT-12 noise locator
PostPosted: Mar Sun 04, 2018 12:47 am 
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Joined: Dec Sun 02, 2007 11:59 pm
Posts: 140
Location: Alabaster, Ala.
I have had a Zenith A723 with the 7A02 chassis for many years. (a garden variety Zenith AM/FM radio) It had a constant scratchy, buzzy sound that made the radio a pain to listen to. I gave up on it long ago but decided to give it another try.
* The noise was present in both AM and FM modes.
* Audio signal injected to the grid of the 19T8 first audio was clean.
* 455 kHz modulated RF injected into 1st IF amplifier tube grid came out clean.
* 455 kHz modulated RF injected into the 12AT7 mixer grid came out with buzzy and crackly audio.
* substituting another 12AT7 tube - no change.

I used the Heathkit AT-12 signal tracer's noise locator function and put it on all components associated with the 12AT7. It produced the same sound when it applied voltage to pin 7, the oscillator grid. There are two components attached to this tube pin, a 10K resistor and a 50 mmF capacitor. I unsoldered both from the tube socket and tested both with the noise locator. The resistor made the sound, the capacitor only a click when voltage was applied.

I replaced the resistor and everything is great now. In fact this radio performs as well as any radio I have on weak FM stations.

Conclusions: 1.) The noise locator function of the signal tracer was a lifesaver. It found the bad component easily. 2.) All the Zenith AM/FM radios have basically the same circuit and even the low-end models are good performers.


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