[sdiy] Prophet 5 -15v rail is shorting... Need advice

[ASSI]

On Donnerstag 08 November 2007, Gil W. wrote:
> Could it be that the transfomer used it not 100%
> suitable for the P5 applications, therefore I both get
> fried tantalums on the -15v rail, and lack of voltage
> stability on the +15v rail ?

If the specs for the transformer you've given earlier are correct, the 
transformer should be adequate.  You can however check if the voltage 
directly at the secondary windings and after rectification is 
reasonably stable (under constant load and with load variations).  
Depending on the type of regulator you're using, you need a minimum 
voltage of at least 18V into the regulators.  Since the positive side 
feeds both the +15 and +5V regulators, it has more load to cope with.

As someone else has already mentioned, the ground layout for the power 
supply requires some consideration.  You do need a proper star layout 
to ensure that the reference path for one regulator is undisturbed by 
load variations in the other.  I would add that the bypass caps in 
front of the regulator need to be as close as possible to the same.  
This rule should also be followed for the filter caps after the 
regulator.  I also hope you have not missed that D506 diode in front of 
the +15V regulator bypass cap: it is absolutely necessary to have it 
when you have a second bypass cap for the 5.6V rail! (I would rather 
put it in front of the 5.6V regulator or use a second rectifier bridge 
to decouple the two bypass caps, but that's another story).  This part 
of the schematics is a bit funny, it appears that the +15V rail was 
added after the rest of the circuit was already designed and then later 
the 12V and -5V rails were removed (if your Prophet has 2708 EPROM it 
still needs those voltages, if it has 2732 ones it doesn't).



Achim.
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