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Ameritron AL-1500 ALC Problem

R

Ranch55

Sr. Member
Jan 18, 2016
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Fredericksburg, Texas
Ameritron AL-1500 ..... I purchased this amplifier used, very little use, 4 years old.....Excellent condition and power output.... The inside looks brand new, no dust and no work done inside. It arrived in three different boxes (original Ameritron boxes inside of additional packed boxes). I had to install the U.S. made 3CX1500/8877 tube, and the transformer .......
My problem is that the ALC control will not function with my Icom 7300. Actually, there is not any ALC voltage coming out the ALC jack. Well there is a little..... -270mvdc ~ -290mvdc (-.290vdc).... This is not enough to activate the ALC control.
I removed the ALC board and disconnected one leg of each component for checking. All diodes check good with a diode specific test meter. All capacitors check good with their stated value. And all the resistors check good also.
However, what I did find was the ALC out control adjustment pot (R2) is a 1K ohm pot. According to the schematics and parts list for the AL-1500 and the AL-1200, this pot should be a 100K ohm potentiometer.
Could this be the issue ? Not enough resistance to get enough voltage drop ?
Thanks for any help or assistance ?
 

In the manufacturing environment it is not uncommon at all to end up with the wrong part in a circuit board. Especially if it is a "Hand Pop" part. That is a part that is added after wave solder for through hole components or oven heat for surface mount boards.
More times than you can imagine an assembler will run out of parts and then somebody comes along and just dumps "Pots" in to the bin. It could be the right part or, maybe not.
 
Does the ALC control on the rear panel look as if it has been replaced?

A look at the diagram makes the 100k value look right. But this would not cut the ALC voltage that much. There is a 47k resistor R1 in series with the rear-panel ALC control. Even with the wrong control you should get more ALC voltage than that. The 1k control won't work right, but the only likely cause of readings this low would be blown ALC diodes, D1 and D2 on the ALC/Power board.

If this amplifier has been used for 11-meter AM, that could have overloaded those two diodes.

Pretty sure the 1k control is bogus, and that the two diodes are blown, both.

73
 
nomadradio said:
Does the ALC control on the rear panel look as if it has been replaced?

A look at the diagram makes the 100k value look right. But this would not cut the ALC voltage that much. There is a 47k resistor R1 in series with the rear-panel ALC control. Even with the wrong control you should get more ALC voltage than that. The 1k control won't work right, but the only likely cause of readings this low would be blown ALC diodes, D1 and D2 on the ALC/Power board.

If this amplifier has been used for 11-meter AM, that could have overloaded those two diodes.

Pretty sure the 1k control is bogus, and that the two diodes are blown, both.

73
Click to expand...

I appreciate your input .......
The ALC control pot (R2) on the back panel does not appear to have been changed. I cannot confirm that. It looks original, inside and out. But I do not find where a 1K ohm pot is used on the ALC on this amp, just a 100K ohm pot. The 47K ohm resistor also checks good.
The D1 and D2 diodes check to be good with my diode specific tester. And all four diodes check to be identical. There may be a diode issue, but all check good.
 
Tallman said:
I have schematics for you.
Click to expand...

Thank you Tallman. That is the same manual and schematics that was packed with the AL-1500. My "used" purchase did include all the original documentation, including the packing slips .....
It is because of this schematic that I am questioning the 1K ohm potentiometer on the ALC output control of my amplifier ......
 
The 7300 only takes -4-0v, the Ameritron can supply up to -20v so you have to be able to dial the voltage properly. The correct pot should be installed. You could easily run the amp without ALC connected just set your power output on the radio no more than the power you want the amp to produce at any given time. Then use the radio's ALC control.
 
BlowinSmoke said:
The 7300 only takes -4-0v, the Ameritron can supply up to -20v so you have to be able to dial the voltage properly. The correct pot should be installed. You could easily run the amp without ALC connected just set your power output on the radio no more than the power you want the amp to produce at any given time. Then use the radio's ALC control.
Click to expand...

Yes, that is the current way I operate the IC7300 with the AL-1500.
But I would like for the ALC control from the amplifier to operate also.
Even with my 7300 set at very low drive 25% ~ 30% (8 ~ 10 watts), it will still swing the amp up to 1600 watts plus output at times. I try to watch my modulation peaks to keep it down.
 
The one method that comes to mind requires a 'scope.

With the cover off, use a 12-Volt bench supply to power the antenna relay. The standby switch is the easiest place to clip the positive wire from the power supply. Key the relay. A RCA jumper to the amplifier's relay jack with a gator lead on the center pin works fine for this. Hook a coax jumper from a CB to the input socket. A CB turned down to a couple of Watts won't be damaged by the high input SWR it will be feeding into. Setting the amplifier to the 10 meter band will minimize this. Transmit an AM carrier of a Watt or so. Powering only the relay prevents the safety issues of poking deep into a large amplifier.

A 'scope probe on the center pin of the input socket will show the radio's carrier drive level.

Follow the signal through the relay to the input capacitor C1 of the ALC/Power board.

The RF level on the output side where C1 meets C2 will be roughly one-tenth the RF voltage from the radio. It should show both the attenuated RF plus a negative DC voltage so that the waveform you see is nearly all negative. With the scope input DC-coupled, the RF-plus-DC you see should be nearly all below the zero-volt baseline.

The anode end of D2 should show a DC voltage roughly equal to the RF+DC you saw where C1 meets C2. Turning the ALC control should reduce the DC voltage here.

But yeah, if a signal gets lost, following it downstream step by step with a 'scope is the only method I know to try next.

73
 
Okay, I do now have the proper negative voltage coming from the ALC control output on the back of the amplifier.
I changed the R2 potentiometer from the 1K ohm that was installed to a 100K ohm as per the schematic and Ameritron tech support. This change produced dc voltage up to -15vdc as it should be. And it is now adjustable with the ALC pot as it should be.
 
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