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Author Topic: Astron 35A - 13.95v good enough?  (Read 29704 times)
KD8VMZ
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Posts: 19




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« on: September 08, 2013, 07:34:26 AM »

I have an older Astron 35A that I bought at a swap meet a few weeks ago, that promptly took a lightning strike. I replaced the 723 regulator, the 4 pass transistors, the rectifier diodes, and the TIP29 driver transistor. I think the primary things that were bad was one of the rectifier diodes and the regulator chip.

After replacing all that, it was putting out around 17v. I dialed back the voltage adjustment pot all the way, and can get it down to 13.95v. Is that acceptable? Or do I likely still have something not quite right?
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KA5IPF
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« Reply #1 on: September 08, 2013, 08:16:04 AM »

Something is still not right. It should adjust from abt 11-12v up to abt 16 using the internal adjustment. At least that's what I've found. The easiest way to repair those is to replace the AVR board. It is less than $30 (last price I had) and the savings in time and frustration are worth it.

Clif
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KA4POL
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« Reply #2 on: September 08, 2013, 08:35:19 AM »

Did you check the voltages according to the schematic? You should be able to adjust between 11 and 15 V. Being close may be an indication you require one or both of the parallel resistors marked Rx in the schematic. Are there any on your board? They change the output adjustment.
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W9GB
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« Reply #3 on: September 08, 2013, 08:52:20 AM »

EVERYTHING you need to know about ASTRON
Repeater Builders TIP
http://www.repeater-builder.com/astron/astron-index.html

CHECK R6 (2.2K) and R7 (3.0K)
R7X (? ohms) is placed IN CIRCUIT  .... as part of Factory Calibration.
http://www.repeater-builder.com/astron/pdf/astron-rs35m-2009-k2qde.pdf
MEASURE VOLTAGES at KEY Test points
http://www.repeater-builder.com/astron/pdf/k2qde-rs35m-eval.pdf
« Last Edit: September 08, 2013, 09:11:34 AM by W9GB » Logged
KD8VMZ
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Posts: 19




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« Reply #4 on: September 08, 2013, 08:55:35 AM »

The repeater builders site is what I used as a basis to buy the parts. Unfortunately, I'm not nearly experienced enough to understand half of it :-) Has the 723 regulator changed that substantially that I would have to change the resistance? Shouldn't it be a drop in replacement? I tried replacing everything like for like.
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KA4POL
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« Reply #5 on: September 08, 2013, 09:01:21 AM »

If you look at the spec sheet there are tolerances to all values. If by whatever happened inside the circuit a resistor value has changed there you are. Now keep in mind Astron has already foreseen these additional resistors. So the 723 surely will be fine. However....
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AC5UP
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« Reply #6 on: September 08, 2013, 09:24:27 AM »

You actually have two things that need your attention... If you're able to crank the output up to 17 vdc that tells me the crowbar (overvolt) circuit isn't working. It should trip at a shade above 15 vdc. Check the Zener diode CR4 and transistor Q1 (2N3906) as well as the SCR. Considering the PS has seen a lightning strike you absolutely want to be sure the overvolt circuit is working lest you smoke something painfully expensive. This is an easy check; if you can't go much above 15.5 vdc - and - the adjustment pot isn't pegged at the high end the SCR is doing what it should do.

As for R5, the voltage adjustment pot being way off center, top & bottom on the pot you'll find R7, R8 and R8x which set the adjustment range. R8x is a hand selected value and the pot should come in around midrange at 13.8 vdc when selected properly. More likely CR3 (1N4148) or CR5 (1N4002) or the 1N4002 between pins 3 and 12 on the uA723 have seen better days. Check them with your diode tester or just replace as cheap insurance.

Final Note: There are no exotic parts in the supply and you're far better off to replace a few parts that didn't need to go than not replace one that did. As Clif noted previously, if a fully stuffed regulator board from Astron is in the $30.00 price range that's way cheaper than hiring a pro or smoking a radio. Think big picture and don't risk anything more expensive than an used automotive headlight until the supply load tests as normal.
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KD8VMZ
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« Reply #7 on: September 08, 2013, 10:59:44 AM »

So, per the repeater builder site, I replaced the rectifier diodes, the pass transistors, the driver transistor, the regulator IC, and the scr. I think we're due for the board, and then there isn't much left to replace.
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AD4U
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« Reply #8 on: September 10, 2013, 08:48:12 AM »

In any and all linear Astron power supplies..........

If you ever have to replace the 723 regulator, do yourself a BIG favor and spend $0.10 to buy an IC socket.  Solder in the IC socket then insert the 723 into the socket.  You will not be disappointed.

Astron could do this but it would add $0.10 to the cost of the PS.   Grin

Dick  AD4U
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KD8VMZ
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« Reply #9 on: September 10, 2013, 03:25:13 PM »

Fortunately, mine has a socket. I'll be working this evening on pulling the diodes on the board and testing them to see if I can fix it without replacing the board.
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N4NYY
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« Reply #10 on: September 10, 2013, 04:08:28 PM »

Some of those components AC5UP mentions are nickel/dime items. You might as well shotgun it and replace them anyway. It should work fine at 13.95, but as Clif said, that is way above the minimum adjustment. No telling that another weakened component can go at any time, and then you have 16V out. That would be a disaster.
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KD8VMZ
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« Reply #11 on: September 10, 2013, 04:28:47 PM »

I tested the diodes and they all show forward resistance between 800 and 1000 with the diode check on my meter, is the normal?
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N4NYY
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« Reply #12 on: September 10, 2013, 05:50:14 PM »

I tested the diodes and they all show forward resistance between 800 and 1000 with the diode check on my meter, is the normal?

When I do diode checks, I look are the drop at the junction. .6V for silicon, and .3 for germanium.
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K8AXW
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« Reply #13 on: September 10, 2013, 06:02:58 PM »

VMZ:  I usually see around 600-650 ohms in the forward direction and infinite resistance in the reverse direction.  You did check the reverse, didn't you?

I've noticed that the Astrons built in recent years now have an IC socket.  Took 'em long enough!

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KD8VMZ
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« Reply #14 on: September 10, 2013, 06:10:41 PM »

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!
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