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Author Topic: AL-1500 Low Power 15M  (Read 4091 times)
N4CR
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Posts: 1650




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« on: March 01, 2013, 11:17:29 PM »

I haven't had the amplifier apart since a couple of questions in another thread here, but here's what I do know.

1. The amplifier had Rich Measures parasitic kit in it. It was installed in a way that caused a lot of arcing, so I removed it. It doesn't work any differently now than it did before, other than it stopped arcing to ground. Whoever put it in was clearly not sufficiently schooled in handling HV. It ran 1/16" parallel to ground... I could have moved it around but it appeared that 4 of the 8 resistors had gotten hot so I just pulled it out and put it back to stock configuration.

2. I don't have a 75M antenna right now so I haven't tested it there. I'm not sure my cantenna is up to the task of this amp and I haven't tried it yet. I could probably get by with it for a few seconds. Maybe I can borrow the big one again.

3. I'm having no problem getting 1500W out on 40 and 20.

3. My antenna has a 1.2:1 SWR on 15 meters. Cushcraft R-8 on 275' of LMR-400.

4. The loading control always seems to be counterclockwise from the suggested manual settings.

5. The cap plate orientation to knob alignment looks good.

I'm getting no more than 600 out on 15 and 17 meters. If I apply more drive, the grid current starts to rise. There are two doorknob caps in series with the plate, then the plate tuning cap, then two more doorknob caps on the plate tuning circuit. I haven't checked the value of those doorknob caps, but I will next time I have it apart. When I'm tuned on 40 or 20 meters, the Tune knob is left of the band position.

The radio doesn't indicate high SWR playing into the 15 meter input section.

Haven't yet cut the green wire so I can't test 10/12.

Is lower output on higher frequencies a symptom of a bad 8877? I have no experience with that tube.
« Last Edit: March 01, 2013, 11:20:29 PM by N4CR » Logged

73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
KA5N
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Posts: 4380




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« Reply #1 on: March 02, 2013, 03:07:14 AM »

Since you found previous diddling with the Measures mod installation and you are seeing incorrect tuning results from band to band,  a dollar to a hole in a donut there is more didling left to be uncovered and repaired.   This amp needs to be gone over with a fine tooth comb
and find and correct whatever has been done to it.  Either the previous owner cobbled things
up or the the owner before him did and he couldn't fix it. 
I bet you gets loads of advice on this one.
Allen KA5N
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W1QJ
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Posts: 1439




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« Reply #2 on: March 02, 2013, 03:42:50 AM »

please send a few digitial photos of the interior of the amp on the rf side of the amp, not ps side to me at gudguyham@aol.com  I will look it over and see if any alterations other than stock have been made.  It seems that once someone starts with the Measures voodoo magic they go the gamit.  Lou
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KC4MOP
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« Reply #3 on: March 02, 2013, 09:18:23 AM »

All due respect to Richard Measures, may be his magic was needed in older design amplifiers and even transceivers from the 60's thru 80's. Stuff nowadays should be a lot better design.
The AL 1500 has circuits to protect from over-driving the sensitive 8877 final. About 10 years ago, many unhappy hams were around who damaged the tube from over-driving the grid.
Back to the OP:
All of these tests are done in a dummy?? The amp still can not give rated power in a dummy load?
No mention of test in dummy load.
There may be wild reactances that the final tank can not overcome in the antenna, preventing full power. A plane Jane MFJ 259 is not enough of a meter to show the X factor in the antenna system...Reactance...capacitive or inductive. You'll need the MFJ 269 to show X
Fred
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W1QJ
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Posts: 1439




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« Reply #4 on: March 02, 2013, 12:54:44 PM »

Fred, I was thinking the same thing.  I could never stress more the impotance of testing amps into a dummy load.  When testing an amp into an antenna and claiming there is a problem with the amp when it works on most other bands holds no water with me.  Unless I am told the amp performs like that into a dummy load I suspect something wierd with the antenna.  Especially a vertical and that kind of power.  Antennas and antenna tuners and feelines and connecting cables all presetn variables that can't be overlooked.  A radio, an amp and a dummy load reduces variables greatly and eliminates guess work.  If one is to own a high power amp (or any amp for that matter) a decenct dummy load is a necessity!! 
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N4OI
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Posts: 200




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« Reply #5 on: March 02, 2013, 02:02:26 PM »

[...] If one is to own a high power amp (or any amp for that matter) a decenct dummy load is a necessity!! 

A high-power dummy load should also be a necessity for even low-power shacks.  When troubleshooting, the last thing anyone wants to do (or will do) is let a low-power dummy load cool between tests...   But if that is not the norm, the cause of all that tuning up on the air is becoming more and more clear....

73 ES GOD BLESS U ES URS DE KEN N4OI  Grin
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W1QJ
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Posts: 1439




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« Reply #6 on: March 02, 2013, 02:19:01 PM »

[...] If one is to own a high power amp (or any amp for that matter) a decenct dummy load is a necessity!! 

A high-power dummy load should also be a necessity for even low-power shacks.  When troubleshooting, the last thing anyone wants to do (or will do) is let a low-power dummy load cool between tests...   But if that is not the norm, the cause of all that tuning up on the air is becoming more and more clear....

73 ES GOD BLESS U ES URS DE KEN N4OI  Grin

Much of the on air tuning disappeared after the onset of solid state radios.I'd assume any carrier you hear tuning up is an amplfier being tuned and not an exciter.  I wonder what the percentage of "load em up" amps are still being used.  I know there are some still but the percentage must beway down from the 60's 70's.
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G3RZP
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« Reply #7 on: March 02, 2013, 02:22:39 PM »

You must remember that solid state amps especially can go into oscillation if looking into a non 50+j0 load at a frequency removed from where they are operating. The Swan 100MX was especially bad for this on 15m.

So before you can start chasing faults, you need a dummy load. W1QJ, N4OI, KC4MOP have it right. You need a good dummy load. Get the highest power one you can.

Having one is at least as important as a having, as a minimum, a cheap Harbor Freight VOM.

In general, it seems many hams avoid getting even basic test gear. In the long run, it's more expensive and you don't learn anything if you have to pay someone else every time something goes wrong...except how to write cheques!

I have more investment in test gear than rigs.....
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K8AXW
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« Reply #8 on: March 02, 2013, 04:49:37 PM »

4CR:  I you found the Measures mods on the PA plate circuit then check the grid circuit for additional Measures mods. 

His modifications consisted of grid protection resistors, HV glitch protection resistor and  parasitic suppressor(s).
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N4CR
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Posts: 1650




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« Reply #9 on: March 02, 2013, 07:55:04 PM »

4CR:  I you found the Measures mods on the PA plate circuit then check the grid circuit for additional Measures mods. 

His modifications consisted of grid protection resistors, HV glitch protection resistor and  parasitic suppressor(s).

It's an older amp with a glitch resistor, so that may be an RM mod. It looks nothing like the big resistor in the newer amps. Where were the grid protection resistors put?

I have a Heathkit Cantenna, but it's seriously underrated for this amplifier. Didn't want to bake it. I had borrowed a larger bird load when I first got it but it was arcing and I never got much out of it before it zapped and blew fuses.

Also, this has the pink resistors on the capacitor bank. I should probably change those out post haste.
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73 de N4CR, Phil

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

Posts: 1439




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« Reply #10 on: March 02, 2013, 09:30:59 PM »

"IF" someone ran the gambit of Measures mods then there will be changes down under the chasiss of the tubes.  But I doubt only one band would suffer.  Although you never know.  Might be worth a check under the inspection plate of lifting the chassis to look underneath.
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N4CR
Member

Posts: 1650




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« Reply #11 on: March 02, 2013, 11:44:45 PM »

"IF" someone ran the gambit of Measures mods then there will be changes down under the chasiss of the tubes.  But I doubt only one band would suffer.  Although you never know.  Might be worth a check under the inspection plate of lifting the chassis to look underneath.

Darn, I had that apart during the sound abatement measures but didn't know to look.

It's not that hard to get to. I'll inspect.
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73 de N4CR, Phil

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

Posts: 1439




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« Reply #12 on: March 03, 2013, 04:18:22 AM »

I'm not sure if Measures would tell someone who wanted his advice on an AL82 to buy his "one size fits all" voodoo kit for the 82.  Since the 82 has directly grounded grids one would have to lift the grids and insert his grid grounding caps and resistors.  Since many amps I have in for repairs have caught that plague, they needed to be quarantined, operated on and rehabbed.  Changing the grid grounding scheme may require readjustment of input coils especially on 15 and 10 meters.  Rarely do I see an 82 with good tubes needing any kind of input tuning.  In your case someone may have revamped the input coil for 15 and made it a dedicated 17m coil. Check to see if the coil slug is still in the coil on 15.  From what I have seen over 20+ years of doing amp repairs I won't put passed anything.  When I work alone in my repair shop downstairs of my house, my wife thinks I am losing it because sometimes I break out into hysterical laughter when I open up the covers of some amps in for fixing.  Then I start crying because I have one hell of a time removing and restoring the amp to original design.  Keeps the chump change flowing.
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N4CR
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Posts: 1650




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« Reply #13 on: March 06, 2013, 02:29:37 PM »

I haven't had this beast off the shelf since I started this thread. If it were light like my Collins 30L-1, I'd have it apart in a few minutes but this one is a whole 'nuther story.

Anyway, as I operated the contest over the weekend, I did find frequencies where the amp would provide 1500 out. So perhaps this is all revolving around the complex impedances the antenna is presenting. Which is strange since 15 meters has always been the best band on that antenna for broadly low SWR across the band and it's always worked like a hose on that band. The amp hasn't arced a single time since I removed the alien components. Seems very reliable.

My LP-100A does display complex impedances, I just don't have it set that way, so perhaps that's something I can look at.

Gotta get that jumper cut.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
K5TR
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« Reply #14 on: March 12, 2013, 10:56:40 AM »

I a few things:

Your input power?

Plate current for 600 watts out?

Where is the load control?
You say as you drive it harder the grid current goes up - is there no more adjustment left on
the load cap? 

It would be interesting to know what your results on 10 meters are - since you could have a tube that is going soft.

I have used about 5 different AL-1500 amps over the years.  I have two at my station now that I use in my SO2R setup.  Several of them - including the one I have owned the longest always took more drive on 15 meters. 
When I got my second AL-150 a few years ago I noticed that it was much better on 15 meters. One day I tuned them both up and looked at where the caps were set.  On the one that was harder to drive I noticed that the load control was at or near the limit (I do not recall if it was fully messed or the other way around) but there was almost no more adjustment left. I think indicating the inductance needed to be adjusted.  I opened both amps up and noticed right away that the coil on one had turns much closer together than the other - on the one that was taking more drive - so I spread them out.  The next thing I noticed was one end of the coil was bolted to the cap in a different location - like one was on the outside of the cap and the other was on the inside.  The difference was not much but it was different. So I made them the same.  These small adjustments made all the difference.  The net effect was that I increased the inductance on the one that was taking more drive to get full output. 

If your loading capacitor is not in the mid range I would look at adjusting the inductance.

But first I would make sure you have a good tube. 

There are ways of tuning the tank circuit of an amp without high voltage on it using a resistor and an antenna analyzer - you can tune your tank circuit.

As outlined here:

http://tinyurl.com/dyxacbo   (pdf)



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George
K5TR
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