pete,
the .68 ohm resistor does NOT set the current for the supply. That's done by
the four .15 resistors mounted on the pass transistor PCB.
the resistor monitors base current to the 2n5301 transistors. If it's cooked,
egads!
The circuit Clayton mentioned is a classic 78XX 'wrapped 'round' a pair of
transistors. Any of the handbooks should have a copy of the schematic.
The problem is, there is NO over current protection if you use that design
instead of the ten tec one. And with no over current protection you can really
wipe out a set of finals. At the least, get the magnetic circuit breaker from
airpax to protect the final output transistors if you use any other power
supply.
The current pot is what sets the 'trip current' for the supply. The output
current is monitor by the voltage drop across the four .15 resistors. WHen
excessive current is pulled, a transistor is turned on that supplies the scr
with gate voltage. that fires the scr and it removes base drive from the mje520
transistor, shutting down the base current to the pass transistors. To reset
the scr, you power the supply down and then back on.
there is another scr used for overvoltage shut down and that's located on the
pass transistor pcb.
this could be ugly.
the 723 is more than likely shot.
the MJE520 is toast, too
Ditto on the pass transistors 2N5301s
if you need any help, fire away.
mike, wb8vge
On Jun 2, 2012, at 3:19 PM, Clayton Brantley wrote:
> Sounds like the "normal" way for a '723 supply to die. I would keep the
> power transformer
> IF it's ok, then build a supply using the 7815 regulator controlling the
> base of a darlington
> pair. K4EAJ (SK) used this supply for all his repeaters for may years
> without any failures.
> The saving grace is when a '78xx regulator faults, it shuts down vice going
> to the max +v
> rail as the '723's do. Another neat thing is the parts count is less. I
> have been using three os
> these supplies for over 15 years and never a hicup. I might have the circuit
> that I could copy
>
> and email to you if you would like.
>
> Clayton N4EV
>
>
>
> ________________________________
> From: Peter Bertini <radioconnection@gmail.com>
> To: tentec@contesting.com
> Sent: Saturday, June 2, 2012 1:07 PM
> Subject: [TenTec] Rsc value for LM723 in 280/255 power supplies?
>
> Hello to the group
>
> I'm in the processing of rescuing a much abused TenTec 280 power supply.
> There is no amp meter on the front, but it has the speaker, so
> I assume it may be a model 255 despite what the label shows.
>
> The supply had major problems. Most the transistors were shorted, as well
> as the
> LM-723 regulator IC. The .62 ohm resistor (Rsc) was also vaporized. I'm
> curious
> what this part was used for? I assume that 13.8/0.62 means that the
> this resistor
> set the maximum available current at 22 amps? If so, is the adjustable
> current pot
> a second over current setting that works independently for setting a lower
> max.
> current output for this supply?
>
> regards
>
> Pete
> _______________________________________________
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