Otrona Attache power supply woes...

Josh Dersch derschjo at msu.edu
Thu Sep 18 17:13:17 CDT 2008



Tony Duell wrote:
>
> That's fine.
>
>   
>>> Now check for the 5V reference voltage from pin 14 of U4 (the TL494).
>>>   
>>>       
>> Hmm, I get 6.75V (measuring from Pin 7 (gnd) to 14.)
>>     
>
> Ouch!. According to the TI datasheet I've just looked at, the reference 
> should be between 4.75V and 5.25V. Either the TL494 is defective or 
> soemthign else is pulling it up (which may have damaged the TL494 
> anyway). Or I suppose youre meter could be way out of calibration, but 
> since the 8V line checks out I think that's less likely
>
>   
>>> Now, I think that TL494 should be oscillating, at least at startup. See if
>>> you get anything on pin 8. A TL494 datasheet would be a useful thing to have!
>>>   
>>>       
>> 7.91V on pin 8.  (I have a datasheet at 
>>     
>
> That sounds as thouhg it's not oscillating and that the output transsitor 
> is never turned on.
>
>   
>> http://yahozna.dyndns.org/scratch/tl494.pdf).  I scoped it out, and pin 
>> 5 is oscillating, a nice sawtooth.
>>     
>
> OK, the oscillator is running, but it's not driving the output stage.
>
>   
>>> What voltage do you see on pin 3 (Comp) of this chip? That's the output of the 
>>> error amplifier IIRC. It appears there's an overcurrent shutdown circuit 
>>> connected there, maybe that's shutting it down.
>>>   
>>>       
>> 0.06V on pin 3.
>>     
>
> Well. if it's as low as that the outputs should certainly be oscillating. 
>
> But shouldn't the protection circuit designed to shut the supply down if 
> the 8V rail drops too low have operated at this point? It compares the 8V 
> rail after 5 diode drops with the reference voltage, if the former is 
> lower, then pin 3 is pulled high, shutting the supply down. So check pin 
> 3 again...
>
> I am beginning to suspect that ther TL494 may have failed. It's not a 
> difficult-to-get chip.
>   
I replaced the TL494 this afternoon (thanks to a local electronics store 
that actually has useful stuff!) but it does not appear to have made any 
difference.  I get the same voltages on the TL494's pins as I did 
before.  Any other thoughts?

Thanks once again...
Josh
> -tony
>
>
>
>   



More information about the cctalk mailing list