[TenTec] best power supply for Orion II and other items

[Dr. Gerald N. Johnson]

The failing in vintage Astron supplies is that the emitter resistors 
have too little voltage drop to force decent sharing so the transistor 
with the highest current gain gets most of the load. When it fries, 
usually burning off the emitter connection internally after the junction 
shorts and the crow bar protects the radio, that leaves one less 
transistor to shoulder the load. That progesses through the entire 
collection of pass transistors. It would be preventable if the emitter 
resistors had a larger value so to drop as much as a volt when each 
transistor is carrying its fair share of the load. E.g. 35 amp supply 6 
pass transistors, means each one carries almost 5 amps, and that emitter 
resistor needs to be 0.2 ohm to drop one volt. Last I noticed Astron 
resistors were more like 1/4 that value.

73, Jerry, K0CQ

On 11/22/2010 6:56 PM, Stuart Rohre wrote:
><SNIP>
>
> As Jerry says, the paralleling of transistors means if one goes, the
> others can fail under the next load.
>
> -Stuart Rohre
> K5KVH
>
>
>