[BC] Re: Smoking THE-1

[Xmitters@aol.com]

In a message dated 2/2/2007 11:27:57 PM Central Standard Time, 
broadcast-request at radiolists.net writes:


> Fixing one of these boxes for a friend. Whoever he got it from rode it hard 
> & put it away wet.
> 
> After I replaced most of the caps it finally stopped blowing fuses.  
> However, now it smokes the 10 ohm resistor in the modulator module.  This is between 
> the incoming 20 volt line and the -15 3 pin regulator.
> 
> Anyone else run into this? I've already changed the regulator.
> -- 
> 
> Hi,



I have never worked on one of these units directly but the 10 Ohm resistor is 
getting hot obviously because too much current is being drawn form somewhere. 
I would therefore look for shorted bypass cpacitors on the -15 volt power 
supply rail. I'm not sure how the various modules are connectorized, but I would 
disconnect modules in turn intil the -15 volt current draw decreases 
significantly. Hang your volt meter across this resistor so you can view any sudden 
drop of voltage, indicating a drop in load current. 

Tantalum capacitors are notorious for becoming leaky and it is also not 
uncommon for a disc type bypass ceremic to short to ground. You could also have a 
shorted active device someplace so feel of all the chips for one that is very 
hot. Some power supples in Harris stuff have a backward installed diode hanging 
across the power supply and ground. This is done to clamp any voltages that 
attempt to enter the supply output that are of the opposite polarity, Such 
voltages are destructive to three terminal regulators. You may luck out and find a 
protection diode such as this, shorted.


Good Luck,

Jeff Glass
Norhtern Illinois University