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Astron VS-70M AP Bleeder Resistor Mod

NightThumper

South of Pittsburg
Nov 8, 2017
655
1,550
153
Well I was just a tad bit bored today so decided to do a bleeder resistor modification on my Astron VS-70M AP. Astron meters lights whether led or older incandescent have a bad habit of burning out even with a resistor to drop voltage/current. They start burning out at about 5-6 months sometime less. They sell replacement light boards but I find it much easier to just replace the entire meter although much more costly than just the light board. Unfortunately the meter housings are enclosed so a small string of leds or single leds can’t be used as in an Ameritron amplifier meters which are clear on top.

The Astron voltage across the capacitors when energized is 22.60 volts DC. Total capacitance is 128000 uF. Decided to run with what I had on hand which was 1 watt 2.2k ohm resistors. Two were run in parallel giving a total resistance of 1.1k ohms and wattage rating of 2 watts. Installed a 5mm white led without the dropping resistor. This gives a nice visual when the lid is off or through the vents on the cover. Total voltage going to the led was measured at 2.98 volts. Didn’t measure current as the mod had already been performed and I didn’t want to un-solder the work already performed.

Running the power supply for a couple of hours resulted in the bleeders only being slightly warm to the touch.

After 15 minutes voltage has dropped to 3.39 DC and led voltage is 2.6 DC
After 30 minutes voltage has dropped to 2.68 DC and led voltage is 2.5 DC

The capacitors can now be safely shorted without any sparks/arc to bring voltage down to zero.

The reason for this modification is that many many years ago I did a meter replacement on an old Astron VS-50M. One of the wires when un-soldered touch something else and sparked resulting in the voltage regulator needing replacement. Through subsequent testing I realized that even though the meter read zero volts and there was no voltage on the output terminals that the capacitors retained their charge for a very long time…months. (128000 uF is a LOT) So even though one might assume having been off for so long it was safe to work around the inside this definitely isn’t the case. Also taught me never trust any capacitor to be discharged despite what the meters on the equipment say. Can you say “chicken stick” is your very good friend!

Yes Astron’s wiring definitely could use some serious improvement.

That’s all folks

Brad
KE0XS
South of Pittsburgh

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I have a Trip-Lite PR-40. The original 64,000uF capacitor went bad so I replaced the capacitor bank with a single 100,000uF I happened to have It is wild how mucg energy they will store. I wired the LED power ON indicator to the output. I also have an LDG FT-Meter with incandescent meter light tied to the output thru a DC distribution buss. That buss is in a homebrew box with voltage and current meters. All that draws the voltage down to zero as, indicated by the meter in the DC buss box, in less than 30 seconds.
 
Heck yeah, good wite up there Brad. I've never owned an astron but this is good to look back and read if one ever comes into play.

When I was a lad curious about electronics (because it seemed to start at a young age) I once decide to open and inspect one of those living room console TV sets (you remember those). Yeah well very quickly I learned that those transformers and capacitors hold one heck of a jolt even when they've been off and disconnected for a while.

I learned and remember that lesson!
 
Oh....as for the never trust a capacitor to be discharged. A lifetime ago I worked in broadcasting and was out servicing one of our tube type AM transmitters. When working on those TXers I would ALWAYS shut it down and use the Jesus stick ( so-called because if you did not use it Jesus would be the next person you see. One night I was working on the DC power supply section. It was a call-out and 90 miles from home so I was dead tired. In all the times I used that stick I cannot recall having a BANG or SNAP from a charged cap. The bleeders were that good. I reached in and touched a couple caps in the 600 volt section, then BANG! It woke me up a bit. I started checking things out and found the bleeder, which was a ceramic wire-wound resistor, had actually split in two and open. Seeing that woke me up a LOT.
 
Well I was just a tad bit bored today so decided to do a bleeder resistor modification on my Astron VS-70M AP. Astron meters lights whether led or older incandescent have a bad habit of burning out even with a resistor to drop voltage/current. They start burning out at about 5-6 months sometime less. They sell replacement light boards but I find it much easier to just replace the entire meter although much more costly than just the light board. Unfortunately the meter housings are enclosed so a small string of leds or single leds can’t be used as in an Ameritron amplifier meters which are clear on top.

The Astron voltage across the capacitors when energized is 22.60 volts DC. Total capacitance is 128000 uF. Decided to run with what I had on hand which was 1 watt 2.2k ohm resistors. Two were run in parallel giving a total resistance of 1.1k ohms and wattage rating of 2 watts. Installed a 5mm white led without the dropping resistor. This gives a nice visual when the lid is off or through the vents on the cover. Total voltage going to the led was measured at 2.98 volts. Didn’t measure current as the mod had already been performed and I didn’t want to un-solder the work already performed.

Running the power supply for a couple of hours resulted in the bleeders only being slightly warm to the touch.

After 15 minutes voltage has dropped to 3.39 DC and led voltage is 2.6 DC
After 30 minutes voltage has dropped to 2.68 DC and led voltage is 2.5 DC

The capacitors can now be safely shorted without any sparks/arc to bring voltage down to zero.

The reason for this modification is that many many years ago I did a meter replacement on an old Astron VS-50M. One of the wires when un-soldered touch something else and sparked resulting in the voltage regulator needing replacement. Through subsequent testing I realized that even though the meter read zero volts and there was no voltage on the output terminals that the capacitors retained their charge for a very long time…months. (128000 uF is a LOT) So even though one might assume having been off for so long it was safe to work around the inside this definitely isn’t the case. Also taught me never trust any capacitor to be discharged despite what the meters on the equipment say. Can you say “chicken stick” is your very good friend!

Yes Astron’s wiring definitely could use some serious improvement.

That’s all folks

Brad
KE0XS
South of Pittsburgh

View attachment 49592

I have a Trip-Lite PR-40. The original 64,000uF capacitor went bad so I replaced the capacitor bank with a single 100,000uF I happened to have It is wild how mucg energy they will store. I wired the LED power ON indicator to the output. I also have an LDG FT-Meter with incandescent meter light tied to the output thru a DC distribution buss. That buss is in a homebrew box with voltage and current meters. All that draws the voltage down to zero as, indicated by the meter in the DC buss box, in less than 30 seconds.


When I said I connected the LED to the output I meant the output of the bridge rectifier not the output of the PSU. That puts it right across the filter and not isolated by virtue of the pass transistors, thru with current must flow to discharge the caps. Basically leakage current.
 

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