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Author Topic: Ameritron AL-80A Low Output  (Read 1538 times)
N7YV
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Posts: 28




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« on: October 27, 2005, 05:06:20 PM »

Dear Fellow Hams,

I bought an AL-80A at the Denver Hamfest and just now hooked it up to test it.  My transceiver is an Icom IC-756Pro.  The photocopy of the manual indicates the HV meter scale is 0-3500V and lp, Plate Current is 0-700V.  The meter in mine is HV 0-4000V and lp 0-1000V.  

According to the manual HV should be 3100 volts no load.  My meter reads 3500 volts no load.  

All tests have been with CW.  When I key the exciter with no drive the plate current should be between 50-100 mA.  Mine reads 225 mA.  When I increase exciter drive the amp will put out only up to 95 watts as measured by my Bird 43 into a dummy load.  

When I checked the power output of my 756 when the amp is in the OPR mode I see only 7 watts as indicated on the internal meter of the 756.  It seemed there might have been high SWR from my 756 to the amp thereby causing the 756 to reduce power.

So I switched the amp to standby, keyed the 756 and I can see 100 watts output on the 756 and Bird 43.  Can I assume the cables are not the problem?  Does anyone have an idea if high SWR may be the problem and if so where?  

In 1989 I built a Heathkit SB-1000 which I think is basically the same as the AL-80A.  I remember making some adjustments to match the exciter to the amp.  Could this have anything to do with the problem?  

Thanking you in advance for your help.

Best Regards,
Roger, N7YV
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WB2WIK
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Posts: 20542




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« Reply #1 on: October 28, 2005, 08:47:08 AM »

>Ameritron AL-80A Low Output  Reply  
by N7YV on October 27, 2005  Mail this to a friend!  
I bought an AL-80A at the Denver Hamfest and just now hooked it up to test it. My transceiver is an Icom IC-756Pro. The photocopy of the manual indicates the HV meter scale is 0-3500V and lp, Plate Current is 0-700V. The meter in mine is HV 0-4000V and lp 0-1000V.<

::I think you mean 700mA and 1000mA, not "V."  The current product is as your manual shows.  The AL-80A hasn't been manufactured in several years (my AL-80B is 5 years old, and wasn't the latest thing out then, either), so sounds like they changed the meter scale over time.

>According to the manual HV should be 3100 volts no load. My meter reads 3500 volts no load.<

::You should probably actually measure this.  The panel meter could be wrong, and HV might be something different than indicated.  If you verify the HV really is 3500V, that could be dangerous because you're right at the limit of the HV filter capacitor ratings.  The tube itself can take 3500V just fine, but the caps could fail.  Also, if it's really 3500V, it's possible the filament voltage is too high, since both HV and filament secondaries share a common transformer primary. If it's really 3500V, you can likely reduce it by using a different transformer tap as described in the manual.  

>All tests have been with CW. When I key the exciter with no drive the plate current should be between 50-100 mA. Mine reads 225 mA.<

::Again you might want to verify this by actual measurement before trusting the panel meter.  225mA idling current is way high, wastes a lot of power for nothing, and may indicate a problem in addition to very high B+ voltage.

>When I increase exciter drive the amp will put out only up to 95 watts as measured by my Bird 43 into a dummy load.<

::What's the plate current read when it does this?

>When I checked the power output of my 756 when the amp is in the OPR mode I see only 7 watts as indicated on the internal meter of the 756. It seemed there might have been high SWR from my 756 to the amp thereby causing the 756 to reduce power.<

::Probably.

>So I switched the amp to standby, keyed the 756 and I can see 100 watts output on the 756 and Bird 43. Can I assume the cables are not the problem? Does anyone have an idea if high SWR may be the problem and if so where?<

::What band(s) did you try?  Do you get the same result on all bands?  The AL-80A has tuned input networks for 160-80-40-20-17/15-12/10 meters (six networks, bandswitched), and they're all adjustable from the rear panel.  Possibly somebody messed with them, but it would be surprising for them "all" to be off (all bands).

>In 1989 I built a Heathkit SB-1000 which I think is basically the same as the AL-80A. I remember making some adjustments to match the exciter to the amp. Could this have anything to do with the problem?<

::Sure it could, see above.

-WB2WIK/6
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N7YV
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Posts: 28




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« Reply #2 on: October 28, 2005, 03:09:00 PM »

Dear Steve

Thank you for your help.  Sorry I made a typo.  Yes, I meant 700mA and 1000mA not V.

I have just tested the HV again with both a Simpson 215 and a Heathkit HV probe.  It appears the HV is about 2650 volts at idle, not 3500V as show on the amplifier's meter.

At about 7 watts input the amp puts out about 100 watts, the plate current reads 375mA and the grid reads 50mA.

I have tried the amp on 20, 40 and meters only with all results being near the same.

I have also noticed when I set the 756 meter to read SWR and key it momentarily the SWR reaches 3:1 before it falls back to 1:1 as the power is automatically reduced.

Thanks again,
Roger, N7YV

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N7YV
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Posts: 28




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« Reply #3 on: October 28, 2005, 03:24:12 PM »

Hi Steve,

I just checked plate current with my Simpson 215 and I get 90V, more like it should be.  Looks like I may have more than one problem but the meter circuit for sure.

Thanks again, Roger.
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N7YV
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Posts: 28




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« Reply #4 on: October 28, 2005, 03:46:48 PM »

Steve,

I just had something strange happen.  I was testing  output of the amp.  With 7 watts in I was getting 100 watts out as read on the Bird 43.  I turned the multimeter switch on the amp to ALC and the output goes up to 140 Watts.

Huh??
73, Roger N7YV
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W8JI
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« Reply #5 on: October 30, 2005, 12:09:21 PM »

Roger,

I designed the AL80A (and B and so on), and the Heath amp (as well as some other amps) also.

I suspect your EXTERNAL meter is off a bit, and the INTERNAL meter is off also. The nominal HV should be about 3000 volts. There isn't very much that will ever change that without having very obvious signs of trouble like bad hum or smoke!!

HV multiplier resistors are notoriously troublesome devices. In the AL80, the most likely weak link is a 1.5 meg ohm resistor in series with the other two 1 meg ohm resistors. Some later production runs used a pair of 750k 1% resistors to make up the 1.5M resistor. I'd look there.

I'm fairly sure your external meter, a very dangerous thing to use, is also a good bit off. You are probably using a 35kV probe to read 3,000 volts, so the error can be large in the direction of low reading if the meter is loading that probe. The probe could even be bad.

The plate current is normally a stable function. Some AL80's have a diode that clamps the negative rail of the HV to ground in the event of a HV to chassis arc or fault. If that diode is present and blown, the plate current will read low and grid current read high. This is because the meters will be electrically placed in parallel when the diode shorts. You can check this by watching grid current with no drive applied but the amp keyed. If the grid meter deflects forward, you have a blown protection diode.

Lastly, do you have the ALC connected? If so, remove it. The amp will actually work just fine without external ALC connections. If changing the meter positions to ALC causes power to be restored the ALC pot has a bad ground connection. IMO you should fix it, but I wouldn't bother using the ALC in that amp.

73 Tom



 

 

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N7YV
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Posts: 28




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« Reply #6 on: October 30, 2005, 04:14:09 PM »

Dear Tom

Thank you for your help.  I too thought the external meters as well as the internal one was off.  The Simpson is a very old unit and the Heathkit is for testing CRT's.  I had to interpolate the reading and only used it to try to determine that the amp's reading was high.  

Yesterday I disconnected the ALC and the exciter would produce higher output.  I began to tune the amp on 20 meters up to about 300 watts until I heard arcing and the output would drop to zero.  I changed to 40 meters and tried to tune it again.  I was able to get around 400 watts.  I looked into the plate capacitor and saw the arcing.  After a few more times the arcing stopped.  On the Bird 43 it then could be loaded up to 640 watts with 50 watts input.  I did not try to go higher because at that point the grid current was 215 mA. The amp's power meter would now only read up to 80 watts even when I adjusted the calibration pot.  Could you please tell me where the diode might be that might be blown?  Would it be on the meter board?

Is it Halloween or is there a ghost in my amp?  I located the HV multiplier resistors and tested them.  The 1.5 meg read 1.4 and the two others read 750K each.  I put the cover back on and tuned the amp.  I checked for grid current with no drive and I had 100 mA.  (This was not the case yesterday.)  So it turned it of, went to the store and came back about an hour ago.  Now all three resistors read open!  

And lastly, what does "IMO you should fix it" mean?

Thanks again,
Roger N7YV
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N3OQD
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Posts: 104




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« Reply #7 on: October 07, 2007, 01:29:34 PM »

Just curious.  Of all the Ameritron amps, which is the best one or ones.  Many other hams say that Ameritron amps in general have issues.  Today I had to replace the meter glitch D16 diode.  The parts list calls for a 1N4007 of which I replaced it with.  The one inside was a 1N4004. I wonder why they had the 1N4004 instead of the listed 1N4007?  Thanks.
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