Ameritron soft start: How do I stop the arcing?

[357]

I notice when I turn my al1200 on, theres a spark inside from the soft start relay.
I bought new replays since they were both pitted, 1 burnt up pretty good.
Is there a way to save on the contacts?

thanks

 

[357]

I put 1 relay in, found a match that was rebadged to NTE.
The other relay is a couple days out.
The original wasnt too bad but might as well replace it while I'm there.

 

[brandon7861]

This is from the manual

I wonder if you could replace that resistor with a higher resistance to further limit the current.

 

[nomadradio]

A bigger resistor lowers the current limit, but makes the buildup take longer. As a result, the resistor needs to have a higher watt rating. When we rebuilt HV power supplies for boxes with a lot of 3-500Zs in them I would make that resistor large enough to dissipate the entire 240 Volt line, since a short on the amplifier's HV would never allow the 'full' relay to ever kick in. Would call for a 240 Watt resistor, but there would never be a 'surprise' (BANG!) turning it on with a shorted tube. Just a lot of toe tapping waiting for the second "click" that never comes. And the scent of hot dust on the start resistor.

If you make the resistor twice the original 20 ohm resistance this suggests a need to double the Watt rating, at the very least. Making it much more than 20 ohms will call for a power resistor that needs its own mount brackets. And it won't serve as a fuse quite so well.

Worst case would be for the HV in the Ameritron to have a short on it. A 20 ohm resistor on the 240 Volt line will draw 12 Amps. Makes 2880 Watts. The 10-Watt wirewound resistor fails pretty fast. I remember one power supply that used a 500 ohm start resistor. Took it a little while to charge up, but a 200 Watt resistor was big enough to survive any overload.

As the old saying goes, your mileage may vary.

73

 

[brandon7861]

What about a NTC thermistor and a fuse where that 20Ω resistor is?

 

[357]

I was thinking a .02 across the contacts?

 

[linearone]

Most 120v amps fall into a range of needing 20ohms to 30 ohms on the hot lead soft start. It depends on a couple things but it's usually close. I go for chassis mount 25+50 watt resistors. The big aluminum ones. Sometimes bigger. Depends on the box.

 

[nomadradio]

A customer brought in his AL1500 today. It was smoking the 20-ohm 10 Watt start resistor every time you powered it up. Turned out his tube was bad. That resistor had done its job as a fuse, no damage to the rectifiers or filter caps. A good tube and a new 20 ohm resistor put him back on the air. Happened to notice a repair we had done a few years ago. The single-pole "run" relay that looks so damaged in the pic above had gone bad a few years back. We replaced it with a 3-pole version of the same relay, with all three poles wired in parallel. Hadn't thought about that fix in a while. I don't stock the single-pole part, and using the 3-pole version to fix Ameritron will be our standard replacement for now on.

This may be your only hope, a relay with a higher current rating than the original single-pole part.

73

 

[357]

A customer brought in his AL1500 today. It was smoking the 20-ohm 10 Watt start resistor every time you powered it up. Turned out his tube was bad. That resistor had done its job as a fuse, no damage to the rectifiers or filter caps. A good tube and a new 20 ohm resistor put him back on the air. Happened to notice a repair we had done a few years ago. The single-pole "run" relay that looks so damaged in the pic above had gone bad a few years back. We replaced it with a 3-pole version of the same relay, with all three poles wired in parallel. Hadn't thought about that fix in a while. I don't stock the single-pole part, and using the 3-pole version to fix Ameritron will be our standard replacement for now on.

This may be your only hope, a relay with a higher current rating than the original single-pole part.

73

thats a good Idea.

I did get one rated 25 amps at 300V 528-389FXAXC1-120A

The original was 10a at 240v and I couldn't really find much about it as relayservice co is long gone

 

[nomadradio]

Looks like an upgrade to me. We used the 3-pole part because it was all we had in stock that would fit. Just made sense at the time to wire all three poles in parallel. It appears this one has lasted longer than the single-pole relay that it replaced.

73

 

[357]

only thing is no back stud so you gotta drill a hole

 

[linearone]

only thing is no back stud so you gotta drill a hole

Pfffft, that's what double stick foam tape is for

 

[357]

 

[357]

Well I got it all put back together. Had to drill a couple new holes and it still shoots a green spark when it fires up.

 

[nomadradio]

The whole point of a 'soft start' feature is to put a limit on the inrush current demand. Lets you use a cheaper power switch. Empty filter caps require a burst of current to charge them up from zero. A 20-ohm resistor limits a 240-Volt circuit to 12 Amps, more or less. This is all the current that the "start" relay or a power switch should ever see.

But only for a short moment.

What should happen is for the HV filter caps to get charged up enough that they don't pull as much charge current going from half to full charge as they would pull totally empty.

But a big arc makes it sound as if there is still too much surge current when the 'run' relay closes and shorts across that resistor.

I don't have the schematic at hand right now. Seems to me if the 'run' relay closes too soon this is what will happen, a big arc. The 'run' relay has a 120-Volt AC coil. Can't remember how they reduce it from 240 Volts. Either wired across one of the HV transformer's 120 Volt primary windings, or with a resistor dropping 240 to 120 at the relay coil. A missing coil resistor would make the 'run' relay kick too soon. Just don't remember how this model takes care of that issue. Just seems like a 120-Volt relay with 240 on its coil would die fairly fast, so this can't be the picture.

Shouldn't be possible for it to close the run relay too soon. Sure sounds like that's what's happening.

Really ought to keep a copy of my shop library on a machine at home.

73