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Author Topic: Murphy strikes!  (Read 2599 times)
KC9JCH
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« on: October 29, 2011, 08:36:41 AM »

Working CQ contest this morning, station is an FT-2000 driving an AL-1200 through a palstar AT4K tuner to a hex beam.  I only use the tuner on 10m as the band is much broader than my antenna.  Was running about 800 watts out on 10m just to try and get through pileups and my amp cut out.  HV reads fine but when I try to key it up, I get high SWR on all bands from the exciter.  Was running very close to 1:1 SWR when this happened, had the headphones on, so don't know if it made any funny noises, but no smoke, no smell!  I checked the two fuses on the back panel, are OK.  I doesn't make that satisfying clunk of the relay closing when I key it up now.  I suspect the keying relay is the culprit.  The manual from Ameritron stinks and tech support not available on Saturday.  Any suggestions as to how to troubleshoot?
Tim
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K8AXW
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« Reply #1 on: October 29, 2011, 08:45:00 AM »

If you have high SWR showing between the exciter and the amplifier, this tells me you have a bad cable between the two units.  The exciter not keying the amplifer at the same time sounds strange.

Suggest you try the exciter into a dummy load by itself.  If all looks OK key the amplifier manually and see if the relay in the amp pulls in.

This is for starters.
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KC9JCH
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« Reply #2 on: October 29, 2011, 09:30:18 AM »

Exciter alone loads into the antenna system/tuner with the amp off or with the standby engaged, only get high SWR when trying to key the amp.  I don't know how to key the amp manually, ground the keying line from the rig?
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KC9JCH
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« Reply #3 on: October 29, 2011, 11:01:30 AM »

update:
I have a Heath SB-200 that I used before I got the AL-1200.  I hooked it up and it keys fine with the same jumpers/cables etc.  I have one of the ARB-704 keying interface in line from when the heath was the only amp and just left it in line when I set up the ameritron.  So now I know it isn't the exciter, coax or jumpers.  My original theory that it is the keying relay in the amp still seems a good bet.  Anybody know if this is a user serviceable part?  Can it be opened up/inspected/cleaned etc? And how should I go about determining if this is the failure?
thnx
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K8AXW
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« Reply #4 on: October 29, 2011, 01:16:09 PM »

If the amplifier rely wasn't working then your exciter would be bypassing he amp to the antenna.  Your SWR should be good.  What happens when you run the amp into a dummy load?  If your SWR goes to zero then your probably right about the keying relay.

If your keying relay is pulling in and not making good contact, you would show an infinitely high SWR.

I can't tell you how to manually key your amp because I don't know rather it is a "pull to ground" or if it requires something like 12V to key it.  You're schematic will show you this.
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W8JI
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« Reply #5 on: October 29, 2011, 02:03:38 PM »

The T/R relay in the AL1200, if you have a later relay board with two relays, has a 7 mS delay.

Some radios, and the FT2000 might be one of them, apply full RF without enough delay and eventually eat up the input relay.

This may NOT be the case, but it might be.

I think the FT2000 has a delay setting in the menu. Look in the manual. If TX delay is not set at 5-7 mS or more, you will hot switch almost any amplifier.

This may not be it, but it is something that could be it.
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KC9JCH
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« Reply #6 on: October 29, 2011, 02:34:00 PM »

K8AXW,
High SWR into a dummy as well.
W8JI,
Will check the menu on the FT-2000, but if this is the case, the relay is toast, right?  Are these something I could potentially repair?  Clean?  I did send an email to Ameritron earlier and will call them Moday.
Tim
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W8JI
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« Reply #7 on: October 29, 2011, 03:43:50 PM »

See what the relay delay is set at.

It could be something else, but that is a starting place.
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K8AXW
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« Reply #8 on: October 29, 2011, 09:09:25 PM »

Tom: What's getting by me is he doesn't hear the relay pull in.  If that's the case then the exciter should still be bypassing the amp and the SWR should be OK.  Unless, the relay is hanging OPEN.
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W8JI
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« Reply #9 on: October 30, 2011, 08:35:10 AM »

Tom: What's getting by me is he doesn't hear the relay pull in.  If that's the case then the exciter should still be bypassing the amp and the SWR should be OK.  Unless, the relay is hanging OPEN.

Some radios can be set to hot switch the relay, and this can eventually make the relay burn up the contact from arcing.

I think the FT2000 has a menue that allows setting the TX delay from zero to quite a long time. With most non-vacuum relays, it should be set to 10 mS delay.

 
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KC9JCH
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« Reply #10 on: October 30, 2011, 09:38:33 AM »

Tom,
Seems there is more than one setting for CW, none for SSB.  CW shape gives a rise time of 1/2/4/6 msec and defaults to 4 msec.  QSK time (which I think is the one at play here, the manual defines as "time delay between when the PTT is keyed and carrier is transmitted")  Has available 15/20/25/30 msec with default of 15 msec.  There is no specific menu item that defines keying delay on SSB, And I cannot find anywhere in the manual where the delay is defined.  Sorry this seems disjointed... big halloween party last night, wicked hangover this morning.   Need coffee
Thanks,
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KC9JCH
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« Reply #11 on: October 30, 2011, 11:06:58 AM »

Tom,
The schematic for the AL-1200 shows three relays.  One for input, one for output and one to the meter board.  Once I talk to the Ameritron folks, I'll open it up and see if anything obvious jumps out at me.  Just out of curiosity (and because the 3CX1200A7 tube is made from unobtainium), what would be the symptom of a tube failure?  This amp is old, I bought second (or third or fourth) hand, and although I tune carefully and never drive hard (grid current of 200 or less, always!), I don't know what the previous owners were like.  When I bought this thing, I paid considerably less than what a new tube costs from RF Parts today, and in part, bought this model because the 3CX1200A7 tube could handle the abuse an "inexperienced" ham could give it.  Seems like it has to be a relay as it just doesn't key, doesn't click/clunk when I attempt to key and shows normal HV.  I will also attempt to contact Yaesu on Monday and see what the default keying delay is, but on SSB would the rf level not be at or near zero until a modulated signal was present?  Just trying to understand the problem a little better.
Tim
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W8JI
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« Reply #12 on: October 30, 2011, 01:27:11 PM »

You might be OK on the radio.

It sounds like a relay problem, because you said you cannot transmit through it on standby.

Make sure the center pins of the SO239 are not spread out. Sometimes someone will screw an oversize male pin, usually from excessive solder on the outside of the pin, into the female and spread the center pin contact in the female open too far.

You can just ohm between the input pin and the output pins to check bypass status, and from the pins to ground.

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