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Repair / Re: Tek FG502 fails at high frequency end of dial - help needed!
« Last post by gcwills on Today at 08:53:48 am »Success
After a lot of extra time spent on this challenging function generator I stumbled upon a fix for the oscillator dropout. I focused my attention on the area around Q292 and Q290 as it appeared that there were bias issues associated there.
Partly out of desperation I added some resistors from the collector of Q290 to ground and the oscillator was able to maintain oscillation a little further up the dial. I then tried the same approach to the collector of Q292 to no effect so I wondered if increasing the value of R155 (the Q292 load) would have a positive result. As it turned out it did and adding a 100 ohm resistor in series with R155 (820 ohm) allowed the generator to operate fully to the high end of the frequency dial. I then removed R155 and the 100 ohm and substituted a 1K 1/2W resistor. I was able to confirm that the generator worked fine across all of the ranges and waveform settings. The generator was then calibrated according to the manual.
So was this a bodge or a repair? I think it is a repair as the generator didn't have any faulty component and worked albeit in a limited way. It just failed to oscillate on certain settings. My guess is that there was sufficient drift in components around the oscillator causing this issue. Increasing the R155 value a small amount increased the loop gain enough to resolve the issue. Of course I am open to better brains than mine with better explanations?
After a lot of extra time spent on this challenging function generator I stumbled upon a fix for the oscillator dropout. I focused my attention on the area around Q292 and Q290 as it appeared that there were bias issues associated there.
Partly out of desperation I added some resistors from the collector of Q290 to ground and the oscillator was able to maintain oscillation a little further up the dial. I then tried the same approach to the collector of Q292 to no effect so I wondered if increasing the value of R155 (the Q292 load) would have a positive result. As it turned out it did and adding a 100 ohm resistor in series with R155 (820 ohm) allowed the generator to operate fully to the high end of the frequency dial. I then removed R155 and the 100 ohm and substituted a 1K 1/2W resistor. I was able to confirm that the generator worked fine across all of the ranges and waveform settings. The generator was then calibrated according to the manual.
So was this a bodge or a repair? I think it is a repair as the generator didn't have any faulty component and worked albeit in a limited way. It just failed to oscillate on certain settings. My guess is that there was sufficient drift in components around the oscillator causing this issue. Increasing the R155 value a small amount increased the loop gain enough to resolve the issue. Of course I am open to better brains than mine with better explanations?