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Author Topic: AL-1500 Low Power 15M  (Read 4301 times)
N4CR
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Posts: 1666




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« Reply #15 on: March 14, 2013, 03:53:14 PM »

George, thanks for your input. I haven't had the amp apart yet to clip the 10 meter wire. The modifications you made to your coil spread is interesting. There's more than one coil there, which one is the one you spread the turns on?

Got a picture?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W1QJ
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Posts: 1446




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« Reply #16 on: March 14, 2013, 09:24:26 PM »

Spreading the coils on a tank circuit (making a larger space between turns) lowers the total inductance of the coil, so if you indeed spread the turns, the inductance was reduced not increased.  Doing this would simply shift the tuning to a higher section of the band.  Indeed it will make a difference in both the tune and load settings at the same frequency before and after.  To determine what you actually want to do to best model your amp to your operating habits you must use a steady input signal to the amp, load it up at the very bottom band edge of any particular band then go to the top end of the band and do the same thing.  Note the tune and load controls an both and most of all record as accurately as possible the output power.  There should be a slight difference in power output between the upper part of the band and lower part.  Which ever is the higher output will tell you what to do to the coil if you want to adjust it.  The best way to track the highest output level on a band is to have a good dummy load and a radio that can transmit outside the band about 100kc or so.  You want to adjust your coil so that the "power band" falls into the portions of the band you want to operate and surely not having a higher power output outside the band.  Another note, using a resistor on the input to the cathode or on the plate to tune a tank circuit is only a "ball park" adjustment.  This method should only be used when experimenting with a totally unknown tank circuit or input circuit.  Final adjustment must always be made according to full power measurements.  In the case of an Al-1500 you would never use this method to tweak up the tank circuit.  It is already better than a "ball park" setting.  Although the Al-1500 usually has a dead nuts adjustments on the input circuits, if one was to need to tweak them up, they are located in such a place so that one could easily slide the cabinet back about an inch and safely make adjustments under full load conditions.  Once a tank circuit is in the ball park the only way to fine tune it is to make slight adjustments  to the tank coil and try it each time until the power band falls into the desired area.  The higher you go in frequency the more a slight adjustment makes a difference.  Depending on what band you want to adjust you have to look at how the bandswitch taps the coils.  You only want to adjust the coils that will most likely affect the band of concern.  Because all the coils are in series there will be an effect on the lower bands but most likely not even noticeable.  Ameritron has a habit of laying some goop across the larger coil to keep the coils from moving.  You may have to break that seal if you find trhe coils need to be spread more.  Another variable of course is your antenna.  Dummy loads are the only way to test amplfiers.
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N4CR
Member

Posts: 1666




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« Reply #17 on: March 16, 2013, 04:26:34 PM »

Well, the problem finally came to a head a few days ago.

The SWR on my Cushcraft R-8 went more than crazy on 17/20/30/40. When I layed the tripod/antenna assembly over, I found where the top of the 20 meter trap had caught on fire. There was a carbon track across the rain boot at the top of the trap. I cut it off, cleaned the trap parts and reassembled it with some butyl rubber strips to provide watershed.

Now, I'm getting full power out of the amp on 15 meters.

All I can assume was that this carbon track was adding something to the antenna that made it present a strange load on 15 meters. I'm having no problems getting full power now.

Like was already mentioned, I need a big dummy load.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KC4MOP
Member

Posts: 734




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« Reply #18 on: March 17, 2013, 03:33:25 AM »

Well, the problem finally came to a head a few days ago.

The SWR on my Cushcraft R-8 went more than crazy on 17/20/30/40. When I layed the tripod/antenna assembly over, I found where the top of the 20 meter trap had caught on fire. There was a carbon track across the rain boot at the top of the trap. I cut it off, cleaned the trap parts and reassembled it with some butyl rubber strips to provide watershed.

Now, I'm getting full power out of the amp on 15 meters.

All I can assume was that this carbon track was adding something to the antenna that made it present a strange load on 15 meters. I'm having no problems getting full power now.

Like was already mentioned, I need a big dummy load.
Your CANTENNA would have been adequate for about a minute of QRO power.
Good thing the antenna had a major crap out before you modified the amp.
Fred
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N4CR
Member

Posts: 1666




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« Reply #19 on: March 17, 2013, 03:59:32 AM »

Your CANTENNA would have been adequate for about a minute of QRO power.
Good thing the antenna had a major crap out before you modified the amp.

I was afraid that I'd blow the resistor considering the amp is capable of 2.5 times the Cantenna rating.

I was NOT in a hurry to pull this beast off the shelf again. It's not a marshmallow.

(although I still need to get in there and cut the green wire...)
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
N4CR
Member

Posts: 1666




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« Reply #20 on: March 17, 2013, 11:35:35 PM »

I pulled the amp off the shelf yesterday and cut the green wire. When I bought the amp, I also ordered in the bias board (EBS1) because this amp is old enough that it didn't come with one. I didn't want to install that until I got the amp working normally. I drilled 4 holes, mounted it and wired it into the amp.

When I powered it all back up, I had no problem getting legal limit on 10 meters as well.

Looks like I can park this amp on the shelf for a while. What a beast these things are to move around. Not to mention that they kept a screw factory in business to attach the covers. 31 screws to take the cover off!

The EBS1 is working. I have 0 plate current between speech. Reports are that the amplifier sounds normal and doesn't introduce any chop or unpleasant artifacts while switching between class B and class AB. I believe this board comes stock on all newer Ameritron AL-82, AL-1200 and AL-1500 amplifiers so I expected it to work as advertised.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
N4CR
Member

Posts: 1666




Ignore
« Reply #21 on: March 24, 2013, 06:36:00 PM »

Here's a couple of pictures of the rain cap from the Cushcraft R-8 that melted and caught fire. The amp has been working fine since I fixed the part that caught fire on the antenna. I suspect that the rain cap was contaminated with bird droppings.

Strangely, the bands that would not tune, 20/30/40 all had crazy SWR (shorted trap). The 15 meter band which would not tune up to full power didn't have high swr, was at 1.2:1.

http://linuxceptional.com/img/ScortchedRainCap1.jpg

http://linuxceptional.com/img/ScortchedRainCap2.jpg
« Last Edit: March 24, 2013, 06:57:40 PM by N4CR » Logged

73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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