That's what happens when you are running in the AM mode with the VOX turned on and you forget to turn the radio off before you turn the "Hi-Fi" or Television on. Screaming kids and loud XYLs don't help.
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The failure rate on the ZD model was higher than on the E - EE versions. Back during that time period, if a ZD model came through the doors, the boss wouldn't touch them, always referred the owners back to factory service.