Ameritron ALS-600 low power out

Discussion in 'Amateur Radio Amplifiers' started by W5UAA, Mar 19, 2016.

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  1. W5UAA

    W5UAA Ham Member QRZ Page

    Has anybody else in here had an Ameritron ALS-600 develop low power output over the years and successfully repaired it?

    Caveats: NOT bad finals, NOT a bad swamping resistor

    Mine is currently only putting out about 150 watts on 160 meters, and increasing up to about 400 watts out on 10 meters nearly linearly as I progress from 160 meters on up to 10 meters. Driving it with an ICOM 756 Pro II. About 60 watts input in all cases, about 20 amps input from the transformer power supply in all cases. Anything more than 60 watts input increases current input, but not power out.

    My transformer power supply is at a clean 55 volts DC unloaded (and pretty much stays there loaded).

    When I first looked at this amp, bias's on the MRF150s were ranging from 80 mA to 170 mA on each MRF150 and adjusting those to 250 mA had no effect. Replacing the MRF150s and adjusting to 250 mA had no effect. The datasheet test setup is set to 250 mA. Ameritron suggests setting them to 300 mA. I just can't believe bumping them up another 50 mA in bias is going to change anything--or has anyone actually seen going from 250 mA to 300 mA bias in each of the 4 MRF150s actually increase power out? And I have already replaced the swamping resistor (it was a little outside the 10% tolerance). Still no change. Cleaned relay contacts. No change.

    Tracing the RF path, I see there are several discrepancies between the schematic provided by Ameritron and the amp itself.

    Nothing obviously burned.

    Anybody else have any experience with this? I'm at a loss of what to do next. Can't really take measurements under full power. Can't test the capacitors in circuit with power off. Anybody?
     
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Is ALC working ?

    Good Luck.
     
  3. W5UAA

    W5UAA Ham Member QRZ Page

    Yes.
     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    What dummy load and metering are you using ?
     
  5. W5UAA

    W5UAA Ham Member QRZ Page

    MFJ-976 matched antenna. Multiple meters. The meter in the ALS-600 reads even lower.
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would test into a dummy load.

    Testing without a True 50 ohm load is not a very good test, Even if your tuner says the match is good.

    Plus if you use a antenna. RF can be getting back into your amp, and cause crazy problems.

    Good Luck.
     
  7. W5UAA

    W5UAA Ham Member QRZ Page

    Borrowed cantenna and a bird. Same thing measured. (adjusted ALS-600 meter to match the bird...) Anybody else have any ideas?
     
  8. REASTON

    REASTON QRZ Member

    I don't know how easy this would be but if you can measure the output of the finals by bypassing the low pass filter somehow, you may be able to isolate the problem. If you can remove the 10 meter filter and put a coax jumper across the input/output, that is probably the easiest way to test the amp board on all bands. If the output is still low from the amp pcb, you will know where to keep looking. Filter boards in Ameritron amps have always seemed to be a problem at least some of the time.
     
    Last edited: Mar 21, 2016
  9. K1MMI

    K1MMI Ham Member QRZ Page

    Join the Yahoo Group "Ameritron-Amps". The designer of the ALS-600 posts a lot of helpful suggestions on that group for people having trouble with different Ameritron Amplifiers. There is also a Yahoo Group "ALS-600" but the designer of the ALS-600 doesn't post on that group.

    For many years Ameritron adjusted the bias for each FET to 150 MA. The designer of the ALS-600 saw no reason to increase it to 300 MA and was going to check with Ameritron but if he did, he never posted the reason why they went to 300 MA on the Ameritron-Amps group.

    The schematic that came with my 1998 ALS-600 was for 1993 and doesn't match the amp. On the ALS-600 Yahoo Group there is a 2002 schematic that seems to match my amp. If you own a newer ALS-600 with remote control and auto band switching support the 2002 schematic will not help.
     
  10. W5UAA

    W5UAA Ham Member QRZ Page

    Thanks REASTON, I will consider that. I might actually be able to do that with minimal unsoldering/soldering. Us old-timers remember that being called the "split-half" method of troubleshooting. Makes perfect sense. I will probably attempt that.

    Thanks Edmond. Although I have a problem participating in anything having to do with "YAHOO," (I have a problem with their business practices and their privacy policy) if I become desperate enough, I may go ahead a join to see what's in there and see if the schematic you say is in there matches what I have. I bought this amp for my father back in 1992 and he just recently gave it back to me because he said he didn't use it, hardly ever used it.

    First thing I noticed was a noisy fan (bad bearings) in the power supply. Then, after finding a good replacement fan on Amazon and proceding to replace it, I noticed the original installation had the fan blowing "IN" the back, instead of sucking "OUT" the back. Not sure if that was a mistake made by an Ameritron technician back in the early 90's or if it was by design, but I went ahead and made an executive decision to reverse the direction so it was sucking air out the back. That makes more sense to me.

    Last time I chatted with an Ameritron tech, he said there're probably some capacitors on the filter board which have gone out of tolerance, but have they all gone out of tolerance together? At the same time?

    I wonder if Ameritron will just sell me a new filter board populated with new parts... I'll probably call and ask if the split-half method points to a bad filter board.

    Thanks once again to both of you. Anybody else have any suggestions?
     

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