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Author Topic: Ameritron AL-80a questions.  (Read 1497 times)
N1NRK
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Posts: 13




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« on: May 07, 2008, 03:11:52 PM »

Hi all,  I recently purchased a used (and older) AL-80a amp.  The amp is currently connected to a regular 110v outlet, although I do have plans to upgrade that.  Anyway, when on, my plate voltage reads 2600 and not the 3100 indicated in the manual.  Then when keyed, the voltage drops to 2100.  Is this adequate?  The amp does seem to work, although I need a third meter to check for accurate output, as the meter on the amp indicates much less than the meter in my tuner indicates.  According to my amp, max seems to be about 650w at 200ma grid current, while my tuner's meter shows about 800w. I know meters can vary, so that's not really a question, just giving all of you that information to help here. Anyway, I'm inexperienced with amplifiers and hoping some of you who know about these things can lend some insight here.  Is the voltage/output low just because I need 220v at the outlet or does it sound like other issues?  Any insight would be helpful.

Thanks and 73s

de Keith, N1NRK
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K6AER
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« Reply #1 on: May 07, 2008, 10:05:28 PM »

When you configure the amplifier for 240 volts AC and run a dedicated 240 volt AC circuit for the amplifier you will see much more power and stable working voltages.

Your current 120 volt line is being overloaded.
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W1QJ
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« Reply #2 on: May 08, 2008, 06:18:29 AM »

The 2600 volts you are seeing on the plate voltage meter is pretty low.  I don't think that is because you are running on a 120v line.  Most likely the meter is reading low becasue the metering resistors have changed in value and you are getting a false reading.  I think there is (4) 1 megohm resistors that make up the metering string and they will need to be replaced with new ones.  As far as the "wattmeter" reading low, there is an adjustment for that on the meter board.  You can adjust the meter to read the same as your meter in the tuner.  That is very simple to do with a long screwdriver.  The problem is, it is hard to get the meter to read accurately on every band.  I suggest that you install a good 220v line into your shack.  Be sure to bring in a neutral along with the 220v in the event you should ever require a nuetral with an amplifier you may obtain down the road.  Leave the (white) nuetral wire buried in the box in case you need it.  You won't need it for the Al-80A, but with "some" amps you may need it someday.  You only need a 20A circuit, but I would pull a 10-3 wire that is good for 30A.  You can breaker it for 10A and increase it to 30A if need be in the future.  If you need the 1 megohm resistors, I have them.   Good Luck 73 Lou
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W3LK
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« Reply #3 on: May 08, 2008, 07:31:01 AM »

If you are going to run it on 120v, you need a 20A dedicated circuit - nothing else on it.  I had no problem getting 3100v on the plate and the drop was only 250v.

Now that I am in a new home, I have a 240v circuit for the amp.

73,

Lon - W3LK
Naugatuck, Connecticut
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N1NRK
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« Reply #4 on: May 08, 2008, 11:36:19 AM »

Thanks for all of your ideas.  I did connect the amp to a dedicated 20 amp circuit, still only 2600v.  Then I tried a new meter into a dummy load, only getting about 500w out with full 100w drive, and the ma reading is about 230.  So that's an issue.  I wonder even if I do connect it to 240v, I'm still going to have a problem because the output should be higher than it already is, even on a 110v outlet?  I know all of your replies are just guesses, but they are appreciated just the same.  Thanks again.

Keith/N1NRK  
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W3LK
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« Reply #5 on: May 08, 2008, 01:59:46 PM »

(1) What is the actual line voltage on that line and
(2) What tap are you using on the primary side?

There is are built-in buck-boost taps to match the primary side to the actual line voltage.

73,

Lon - W3LK
Naugatuck, Connecticut
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N1NRK
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« Reply #6 on: May 08, 2008, 03:20:29 PM »

I'll have to check, Lon.  To be honest, I have no idea, I just assume it's where it should be, around 120v.  I'll stick the meter to it tomorrow to make sure.  The outlet is on it's own 20 amp breaker in the panel, and NOTHING else was plugged into this outlet other than the amp.  Tomorrow I'm taking the cover off to re-check the taps.  Will post again after checking some stuff there.
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AB4D
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« Reply #7 on: May 08, 2008, 05:34:04 PM »

I've heard quite a few instances here and there about the AL-80a being suspectible to HV problems, which apparently are traced back to the transformer and/or HV components.  Specifically, what I have heard most often is when the HV drops to 2500-2800 volts at idle the transformer is going bad, and when it actually goes, it takes out quite a few of the HV components with it. The root cause seems to be that the transformer is marginally adequate for this amp, and over time it develops serious problems.  The cure seems to be to replace the old HV transformer with a new HV transformer for the AL-80b before the old transformer takes out the HV supply components.  However, it is not just a simple exchange, as the wiring color coding is supposedly quite different on the AL-80b transformer.

All of this is just based on what I've heard around the bands listen to different qso's.  Therefore, I would contact Ameritron about the problem and see what they have to say.  I assume that they must be aware of the transformer aging problem with the AL-80a, considering all of the AL-80b transformers they have sold as replacements for the AL-80a. I hope this information helps with your problem.

73

Jim
 
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W8JI
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« Reply #8 on: May 09, 2008, 05:01:22 AM »

I designed that amp and have access to service records and field failures.

There is **** NOTHING **** in a power transformer that gets weak and causes HV to drop before a transformer failure. Zero. Never happens. Never can happen. There isn't any mechanism of failure in any power transformer than can cause that.



It is unlikely to be power line voltage causing that error, although a long power lead on 120V will cause a no load to full load sag that is excessive.

The most common problem by far is a meter multiplier resistor. Specifically a single 1.5M ohm resistor when used (it should be replaced with a pair of metal film 750k resistors in series) has gone up in value. The only common trouble part was a 1.5M resistor used only in early production runs.  It is on the front panel side of the electrolytic board.

Very much less likely is a bad electrolytic or electrolytics. This is VERY rare. If you have a bad electrolytic string you will have bad hum on a carrier or a gurgle on SSB as well as unstable HV from no load to full load. The NO LOAD voltage will be about normal however. You also will get lower than normal output. I’ve seen one case of this in a 22 year old amplifier!!

There is actually nothing that can cause what you describe while having full output and no smoke except the bad multiplier resistor. Be especially cautious if it is a single 1.5 meg resistor in series with two brown 1 megs. The updated string was two 1 meg HV resistors in series with two 750K metal films. The 1.5 meg was a problem.


73 Tom
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W8JI
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« Reply #9 on: May 09, 2008, 05:15:06 AM »

Jim,

Where ever you got that information, they are full of beans.

That transformer is actually overkill for the AL80A.

The TRUTH is during a short period in the mid 1980's we switched for 100 or 150 pieces to a Better Coil transformer instead of the normal Schumacher transformer. The Better Coil had inadequate primary insulation and that lead to some instances of primary to secondary shorts.

The Schumaker had a virtually zero failure rate, the Better Coil had significant failures in the first year or two so I imagine there are failures in the field today.

Fortunately we went back to Schumaker.

When the transformer fails from primary to secondary, if the fuses are the CORRECT 250V rated 10A fast blow fuses, nothing happens. Just the transformer is bad. If someone uses a 32V AG fuse or a slow blow, or if they upsize the fuses, sometimes it may take out other components.

The transformer in the AL80 was a more conservative design than the Heath SB220 transformer. It was so much overkill that Amp Supply bought them and used them in their pair of 3-500Z amps for a long time.

Whoever gave you that other info really misled you.

73 Tom








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N1NRK
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Posts: 13




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« Reply #10 on: May 09, 2008, 08:18:46 AM »

Hi Tom, thanks for your replies.  I was hoping you'd take a look at this topic.

There has obviously been work done on this amp.  It appears that I have a 1 meg resistor, then a 620k soldered with a 470k resistor in place of the second 1 meg (rather than replacing it with the right one).  Then it appears that I do have the 2 750k resistors in series after that(to the right when looking from the front).  All resistors do measure resistance across them but the 1 megs seem to be on the low side for their rating (770k and 850k from left to right, respectively).  Also, to the left of those resistors, there's a 47k that I'm not getting anything out of when I measure across it, so maybe that is part of the problem too?  Ugh!  I'll keep digging.  Continued thanks to all.

Keith/N1NRK
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W8JI
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« Reply #11 on: May 10, 2008, 01:50:58 AM »

You would have to open that string to measure the resistance.

Those resistors are shunted by the bleeders and electrolytics.

You can measure them all accurately by lifting the leg nearest the center panel of the 1 meg ohm resistor. Place the meter in Ip position to read the resistances and you can read them directly.

You clearly have the wrong resistor values in your unit from a bad repair. I would order all four resistors from Ameritron. That would be  two 1-meg ohm high voltage resistors and two 750k metal film resistors. They will know the correct part if you simply say they are for the HV metering in the AL80 series of amplifiers.

The 47K is NOT important. It can be from 150k down to 20 K and everything would work the same. It does not need to be special in any way and it virtually never fails.

It sounds to me like what happened is one or BOTH of the 1 meg HV resistors went off value and someone doing a home repair fudged the values of the 2x750K resistors with the wrong type and value of resistor to make up for one or two of the 1 megs going off value.

Instead of changing the part going bad, they readjusted a good part to the wrong value to make it work.....for a while.

I'd look around to make sure they didn't do other similar repairs.

73 Tom


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N1NRK
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« Reply #12 on: May 12, 2008, 08:00:16 AM »

Again, thanks for all.  Called Ameritron today.  The tech there said that if the metering resistors were bad, it should still put out higher power on transmit.  So I changed to another tube I have here (Eimac 3-500Z) and the output is still only about 500w.  So maybe there's two problems?  I'm awaiting another answer from Ameritron but may give them a call in a bit.

Thanks,

Keith/N1NRK
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W8JI
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« Reply #13 on: May 14, 2008, 04:29:38 PM »

The later 80A amps had a buck-boost winding. If your amp has buck-boost it is possible it has been set for higher line voltage.

That's worth a few hundred volts.

 If I were you I'd change the resistors first. Then you will have a better idea what is going on.

You also have a big disagreement in your power meters. What type of meters are you using?



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N1NRK
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« Reply #14 on: May 15, 2008, 02:56:36 PM »

I think this is an early amp.  Serial number is 0268.  Ameritron told me to disconnect the ALC.  Did that, and changed to Eimac tube.  I think I'm getting more power now.  The external meter (3rd meter) was a swan 1500w pwr meter.  Tuner is mfj-989c with cross-needle meters.  And of course, the amplifier's own meter.  Right now, the MFJ and the amp meter are pretty close to each other, whereas before they were not.  Don't know why.  Maybe new tube.  Maybe own circuit.  Who knows, but on whistle I can get almost a kw, maybe 900.  On a steady cw tone, for some reason, it's down to like 600.  I'm going to replace the resistors and take it from there.  Ameritron couldn't think of any reason that it's not putting out power except bad caps.  But it IS putting out some.  Would just like to know how much for sure.

Keith/N1NRK
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