Has anyone noticed that after some time running (hours) Tl494's can ignore the output control (pin 13) and cause the output from a single drive transistor to = the oscillator frequency?
Normally the drive transistor = 1/2 the osc freq with pin 13 held high, to pin 14, Vref.
What I see on the 'scope is that the pulse starts to get twitchy, cutting by half the period in a random manner, creating a lot of obvious audible noise rather than a fixed hum. Osc. Freq = 1Khz. Normal drive tranny pulse freq = 500hz, pulse widths around 25 µS normally
Then it can lock onto half the normal period with a consequent half pulse width , ultimately maintaining the same load control.
Powering down for a few minutes and then restarting stops the prob for a while, then it restarts.