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   Home   Help Search  
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Author Topic: 811 Tuned Input for 20/30 M  (Read 3608 times)
N8DAD
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Posts: 2




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« on: February 12, 2018, 05:33:11 PM »

i have a problem i have bought a AL-811 from a guy and the input swr is fine except on 20meters its like a 2.5vswr. now 30m is shared with the 20m input tuning. but could it be
one of the caps on the trace of the input coil. and is there more then one cap or not. im new to this but , trying to figure out what to do. 30 meter input is fine but i can adjust the coil for 20 and it just dont adjust down. can anyone tell me what the value is of the "CAP" i cant see what it says... or what can the problem be.
anyone that can help or help me understand this would be great.

thanks,
barry-n8dad(at)charter(dot)net

73, n8dad
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K6AER
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Posts: 5743




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« Reply #1 on: February 13, 2018, 11:14:01 AM »

I know this seems silly but what did Ameritron say about your problem?
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AF6LJ
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Posts: 1




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« Reply #2 on: February 13, 2018, 11:23:25 AM »

It is my understanding the input circuit can be tuned to 20 meters or 30 meters but not both.
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KD8GEH
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Posts: 620




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« Reply #3 on: February 13, 2018, 01:22:02 PM »

Some opt to adjust half way between 20 and 30. It depends on your fav band.  I am big on 20 so I live with the reduced output on 30 when I use it.

73,
Dave KD8GEH
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N1RND
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Posts: 62




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« Reply #4 on: February 14, 2018, 03:38:57 PM »

Some opt to adjust half way between 20 and 30. It depends on your fav band.  I am big on 20 so I live with the reduced output on 30 when I use it.

73,
Dave KD8GEH
Same here. You can not have it optimized on both bands.  I tuned mine for 20.
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W8JX
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Posts: 13268




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« Reply #5 on: February 14, 2018, 05:03:23 PM »

Given that there there is a limit of 200 watts on 30m (if you operate legally) what do you really hope to gain with a amp on 30m CW??
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Ham since 1969....  Old School 20wpm REAL Extra Class..
K9MOV
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Posts: 106




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« Reply #6 on: February 14, 2018, 09:24:10 PM »

Great Question--W8JX, Why do a lot of people run their AMPS. on 30 meters when there is a 200 watt power limit?
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MM0IMC
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Posts: 255




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« Reply #7 on: February 15, 2018, 01:08:35 AM »

Great Question--W8JX, Why do a lot of people run their AMPS. on 30 meters when there is a 200 watt power limit?


Whilst I appreciate that the OP has an American callsign, it's a valid point for us outside the US where different power limits apply.  Here is the UK, the legal limit on 30m is 400W PEP.  That's the thing with the Ameritron amps, you have to choose whether you want the low input VSWR on 20m or 30m, 15m or 17m and to some degree 10m or 12m. Roll Eyes
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W8JX
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« Reply #8 on: February 15, 2018, 04:57:35 AM »

Great Question--W8JX, Why do a lot of people run their AMPS. on 30 meters when there is a 200 watt power limit?

I really do think that many Hams do not understand that the field strength difference between 100 and 200 watts is only 3 db or about 1/2 a S-unit. Minimal increase at best.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W1BR
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Posts: 4196




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« Reply #9 on: February 15, 2018, 05:28:15 AM »

You can use an external tuner to help get a better match.  But, the Q of the input matching network has to be kept high enough to do its intended job--the trade off being bandwidth.

A Pi Net resembles a low pass filter.  Tuning lower may adversely affect 20 meters more than it improves 30 meters, but you can try to split the difference.

Pete
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N8DAD
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Posts: 2




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« Reply #10 on: July 01, 2018, 11:08:42 AM »

im not concerned about 30m , only 20 i have seen where someone else has the same issue with a 2.5:1 swr on 20m on the input slug, its as good as it will get
so im not the only one that has this issue. i run a amp on 20 when i need to..  N8dad
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W1QJ
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Posts: 2984




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« Reply #11 on: July 02, 2018, 03:14:37 AM »

Sticking to the question at hand,  those  amps I believe may have ( I think) a trace that has a jumper across them.  If the trace is cut the coil maximizes the adjustment for 30 meters.  Not cut it best for 20 meters.  If I’m correct you’ll have to go back and jumper the trace.  This will give you a better swr on 20. I know these traces are on the AL80b amps and may be on that one also.  If I had the 811 I worked on a few days ago I’d look for you but it’s been returned.  You will have to  look for yourself.  I do know one thing for certain, all bands (not warc) should be able to adjust to a flat swr.  Also someone may have altered the input circuit.  These issues need to be investigated in your case to see just what was done.  All mica cap values are found on a chart in the manual.
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K4RVN
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Posts: 261




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« Reply #12 on: July 02, 2018, 07:26:02 AM »

Hello Barry,
As I understand it when you try to tune 20 for the lowest SWR on 20 meters for the tuned input turning the slug does
nothing to reduce the SWR or make any changes. Mike K6AER suggested  words to the effect for you to check with Ameritron. I would agree and here is a link to get help from them that has worked for me if you so desire. I did peak all my favorite bands on my AL811 which sure helped.

http://www.mfjenterprises.com/HelpDesk/index.php?a=add&company=2

You would normally get an answer by email in a few days in my experience.
Howdy Mike, I talk to your not too distant neighbor Jimmy Ka5DOB quite often and he mentioned you helping him.
Good luck Barry, I used a dummy load to adjust mine and low drive although that might not be the right way. It sure worked for me. If you don't have a manual they are available on the Ameritron site free to download and print if you need one. On the schematic page of the manual is the tuned input chart listing the caps and slug coils for each band.
For 20 meters C3 is 330 PF C4 is 180PF. L1 is 9.75T. The part numbers are shown and you can order them from Ameritron a sister co. of MFJ. Technical assistance phone number is also shown.

Frank

« Last Edit: July 02, 2018, 07:46:13 AM by K4RVN » Logged
W8JX
Member

Posts: 13268




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« Reply #13 on: July 02, 2018, 11:01:47 AM »

Given that the "legal" power limit is 200 watts for this band which is 3 db at best, what do you hope to gain unless you plan to run more than 200 watts...
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W1BR
Member

Posts: 4196




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« Reply #14 on: July 02, 2018, 11:36:27 AM »

He'd gain 3dB.  Same as doubling the boom length on a beam.

Regardless, if that is a pi-net input, the bandpass may not be symmetrical, acting more like a low pass filter.  Peaking the network for 10 MHz may cause more problems at 14 MHz than it is worth. 

Pete
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