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Author Topic: Initial ALC "jump" when switching amplif  (Read 3027 times)
K7JQ
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« on: July 07, 2004, 12:15:15 PM »

Using an IC-746PRO/ALS-600 combo. Just acquired a Heil mic with the foot switch. The foot switch keys the amp first, and then the xcvr. However at times, on SSB, when I hit the foot switch, the ALC meter on the xcvr "jumps" past its limits, and gradually pulls back. During this jump, speaking into the mic produces no output indication on the amps' meter. When the ALC is back within its limits ( a second or two) all is OK with full output. Doesn't happen all the time. I adjusted ALC on the amp according to instructions. Thinking its a switching "timing" problem, I tried disconnecting the foot switch lead to the amp relay and went back to switching the amp thru the xcvr send relay, as I did before when using an Icom SM-20 mic. Thought that solved the problem, but now the same thing is happening, again not all the time. With the amp offline, the problem doesn't exist. Any ideas would be most helpful.
73, Bob, K7JQ    
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W7DJM
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« Reply #1 on: July 07, 2004, 03:13:37 PM »

I don't know about your particular case, but generally this has been caused by a spike in output from the tranceiver itself.

I would think that a peak reading wattmeter would show this, but if not, a scope certainly would, with or without the amplifier.
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K7JQ
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« Reply #2 on: July 08, 2004, 08:49:08 PM »

W7DJM: Thanks for the reply. What's weird is that an RF spike should show a response on a peak reading wattmeter (on my ALS-600 amp, but there is no RF output reading on that or on my xcvr output meter...just a spike in ALC reading on the XCVR with no corresponding spike in RF output. RF output won't happen until the ALC spike retreats back  into range. Only happens intermittantly..didn't happen at all today with about 2 hours of operating. Again, thanks for the reply.  73, Bob
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W8JI
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« Reply #3 on: July 10, 2004, 11:50:01 PM »

Feel free to forward this whoever you like, including the designer of that system.

The relays in MOST amplifiers require 12-15 milliseconds to close and to move past contact bounce. The expections are vacuum relays at about 7mS, and PIN diodes at about 1-5mS.

Proper radio sequencing delays are in the 20mS area for almost amps sold today.

There isn't any possible way to do correct sequencing with a simple mechanical stagger-contact foot switch, unless it has a rate of travel speed limiter. You have to be sure the velocity is such that the distance between activation points and the speed of travel guarantees you always have 20mS closure, and longer on release. That would require some fancy and complex systems in the switch!  

Using that system without a guarantee of 20mS sequencing on make and break would almost certainly be worse than using the radio alone!! The reason is most radios hang after release, and the start delay isn't corrected at all.

IMO, it is an ill-advised system compared to an electrical delay.

Many or most radios output AFTER PTT is released!! It is REQUIRED, not optional, that the radio's amplifier keying line parallel the amplifier keying switch ibn the footswitch. This way any hang in the radio release is passed along to the amplifer, preventing the relay from dropping too early. It also greatly reduces odds bad connections or switch problems in the FS might cause an amplifier hot switch.

73 Tom
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W8JI
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« Reply #4 on: July 10, 2004, 12:05:56 AM »

Bob,

I don't understand the sequencing you use. How do you get a foot switch to key the amplifier first, and the radio second?

Do you have some extra electronics that does the timing??
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K7JQ
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« Reply #5 on: July 10, 2004, 10:18:28 PM »

W8JI, Tom: The Heil FS-2 is a dual channel footswitch. It has 2 switches and consequently 2 leads out of it. Upon depression on a hinge, the first switch keys the amp directly and as you press further, the second switch keys the xcvr. This all happens in a fraction of a second and is intended to negate the need for an amp switching external interface between the amp and xcvr, if the send relay in the xcvr can't handle the voltage/current.
73, Bob, K7JQ
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K7JQ
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« Reply #6 on: July 11, 2004, 11:45:05 AM »

Tom,

Thanks for your advice. I can see that the sequencing/timing in the 2-switch footswitch would vary depending on foot speed/pressure exerted on it each time. Based on your advice, I think I'll stay with the amp switching from the '746PRO to control the amp. Would you recommend a keying interface (Ameritron ARB704) anyway, or should I stay with the xcvr send relay directly to control the amp? Seems to work fine the way it is. Thanks again for your input.

73, Bob
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W8JI
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« Reply #7 on: July 14, 2004, 10:00:48 PM »

You could parallel the keying systems with a diode block. Wire it so the radio OR the footswitch closes the amp relay.

I'm not sure how sensitive your radio's relay control port is. The ALS-600 only has 13V at 100mA or so on the relay line, and it has back-pulse cancelling.

You may not need a ARB system. I'm not sure. The radio manual might say, or the manufacturer should know.
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