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Author Topic: Astron 35A - 13.95v good enough?  (Read 33356 times)
K5LXP
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« Reply #15 on: September 11, 2013, 03:24:18 AM »

They are infinite in the reverse direction, but the forward direction is a little high. I've ordered new diodes and a replacement for the transistor. We'll find out in a few days if that was it!

The diodes wouldn't cause the higher output voltage.  The drop (forward voltage) across a silicon diode depends on the current through it.  Look up the data sheet for the diodes you have and check the specs.  I've seen a few different failure modes for rectifier diodes, basically shorted, open or leaking, and they're not subtle like too high a voltage drop.  For the cost you can shotgun them easily enough but I'd be doing careful measurements and compare to the schematic to see where the real problem is.

Mark K5LXP
Albuquerque, NM
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K8AXW
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« Reply #16 on: September 11, 2013, 08:33:11 AM »

I  agree with Mark.  Replace the diodes but don't expect any change.  It would be time to go through the PCB and check voltages according to the chart Astron so conveniently provides.

You mentioned "lightning strike."  Most of the time.....but not always of course....but most of the time the items that take the hit are solid state devices and MOVs.  Then comes the caps....especially bypass caps.  This is where I would concentrate my efforts. 

Of course buying a new PCB is always an option.  Depending on your available time and patience.  Sometimes it's simply better to replace than repair.
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KA5IPF
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« Reply #17 on: September 11, 2013, 09:35:15 PM »

You're going to nickel and dime your way up to a new board. Not counting your time. The diodes are not the problem.

Clif
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K8AXW
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« Reply #18 on: September 14, 2013, 07:57:11 AM »

Clif:  You're absolutely correct!  No argument there.  But..... that can always been considered as a last resort, when a guy just gets tired of playing. 

In the meantime, an Astron power supply is great fun to troubleshoot because of their simplicity and readily available components.

There's nothing like the satisfaction of actually fixing something!  This is especially important for new hams.

Al - K8AXW
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KD8VMZ
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« Reply #19 on: October 14, 2013, 09:31:48 AM »

The new circuit board definitely fixed it. It's a little frustrating not knowing which item was the problem though!
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K8AXW
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« Reply #20 on: October 14, 2013, 10:04:22 AM »

VMZ:  Congratulations on getting the Astron back on line.  However....... now you'll never know which one of those little things on the board caused the problem, will you?   Grin Roll Eyes
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KD8VMZ
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« Reply #21 on: October 15, 2013, 08:09:51 AM »

I still have the old board, and I have some of the parts. I think it's down to one of the caps or the transistor on the board. I'll keep it all and if I get a second PS I'll make it a project to find out what actually happened.
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K8AXW
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« Reply #22 on: October 15, 2013, 09:18:58 PM »

VMZ:  To give you an idea how good those Astron boards are, I "Chinese copied" one and built a 50A 13.8V power supply.  Since 35A was the most I could load it with at the time, It would go from zero load to 35A and the analog meter needle wouldn't even flicker! 

So, perhaps if you scrounge up some parts you could build your own power supply.........
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W6EM
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« Reply #23 on: October 19, 2013, 04:43:24 PM »

Astron did one helluva job with their design in getting the bugs out of designs using the LM723.  Transients can and do kill the chips easily.  Been there, and seen that with competitor supplies.  Astron uses a 40V zener on the power supply line input to the chip.  Also, they liberally use reverse-biased diodes across transistor junctions to prevent reverse transient damage.  They also use MOVs or TVSSs on the AC side as well.

Another Astron "trick" is series connected DC supplies, to keep the dissipation of the main series regulator transistors down.  The second, very small DC source is in series with the main DC source to raise the LM723 and the first driver to well above the regulated output to compensate for the junction voltage drops. (0.6V for the paralleled main regulator, and 0.6V for the series driver)  So, just the chip and drive transistor see upwards of 30V, and the paralleled main regulator transistors only see just about 20V.  That cuts down on a lot of heat dissipation.

They also use base-spreading resistors on each paralleled main regulator transistor to help prevent thermal runaway. (hogging current by the ones with the highest gain)

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