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Midnight special mf400 low wattage problems

Discussion in 'Amplifiers' started by That_guy_in_wv, Oct 25, 2018.

  1. That_guy_in_wv

    That_guy_in_wv New Member

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    I've got an amp in that had the original 4 erf7530 mosfet and we changed them out for the 4 erf9530 supposed drop in replacements. Before the swap the amp would output around 175 watts peak but would only dead key around 4 to 8 watts with a swing of 175. With the new erf9530 the dead key is about the same but now only swings around 70 to 100 watts. I changed a few caps out that were reading weird on my meter and 2 of the erf7530s were bad I checked everything else I could and still I hear myself through something in the amp and have a low dead key with not much swing. Would anyone know why this amp seems like it's output is low or why it's acting like something is holding it back. I'm running good voltage of 12.8v or better into the amp with a dead key of 3 watts swinging 20.


     

  2. Ranch55

    Ranch55 Well-Known Member

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    Increase your supply voltage to 13.8 to 14.5 volts D.C.
    With your dead key of 3 watts, and 20 watts PEP, about all you would expect the amp to produce would be 175 ~ 190 watts output with the 7530's, and really not much more than that with the 9530's ..... The original amp dead key of about 8 watts is a little on the low side, but not unexpected.
    If your output decreased with the install of the 9530's, you have an issue, or issues.
    Did you check/test the 9530's before they were installed?
    Can you post some photos of the internal's of the amplifier.
    I have changed the 7530's out with 9530's on several amplifiers and have had no issues at all. Sometimes a slight input tune capacitor was required, but not much.
     
    #2 Ranch55, Oct 26, 2018
    Last edited: Oct 26, 2018
  3. That_guy_in_wv

    That_guy_in_wv New Member

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    Ranch55 I'll post pictures this evening when i get home and thanks for the info. Yes I've checked the 9530's they all checked out ok but I'm not sure what else to do.
     
  4. Robb

    Robb Yup

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    If you have that much voltage drop; then either your power leads are too long or your power supply may be inadequate under load - or both. You didn't say how long they are. I would run no more than 13.8v under load as well. What is the rating of your PS?
     
  5. Ranch55

    Ranch55 Well-Known Member

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    This amplifier would be considered a HD (high drive) amplifier.
    It is exactly the same as a Palomar HD 450 FET .....
    Look on the board in the front left corner and see what version board it is.
    See if it is a " 4 FET PAL V3" board.
    It is meant to be used with a transceiver that will output up to 70 ~ 90 watts PEP.
    100 watts PEP drive is fine using the 9530's ..... But that is peak watts, not average ....
    It will work just fine with the radio output you posted above, but the typical amp output will also be right at the same you posted above with a low drive radio.
    The reason I suggested increasing the operating voltage to 13.8~14.5 VDC is that is the typical alternator voltage output of most mobil installations.
    All three of my trucks (2 GMC's and 1 Ford) all level out at 14.2~14.5 volts DC when driving.
    So that voltage will be just fine for your amplifier, but of course you do need the power supply to be able to provide the current the amp needs. Typically up to 45 amps for this amplifier.
    We can get this "fixed". It is not a complicated design. I do have one of these I recently converted from a LD (low drive) unit to a HD (hi drive) unit. And I also replaced the 7530's with the 9530's ...... It will output up to 475~500 watts using my Stryker 955 radio.
     
  6. That_guy_in_wv

    That_guy_in_wv New Member

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    I ran it on everything I run my boomer 600HD on so it should be enough. I'm using a 104 amp hour marine battery and a deka dozer battery not sure the amp hour on it though but its as big as 2 and a half batterys with 6 amp charger short leads big wire. I run solar at my house so it stays charged unless the sun isnt out i use the charger. I Get close to 500 on the boomer so it should be fine for the midnight special. Another thing I noticed on the midnight special is the standing wave is 0 nothing on dead key and swings up to close to a 2.0 or more when I audio into it on dummy load.
     
  7. That_guy_in_wv

    That_guy_in_wv New Member

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    Thanks Ranch55 I'll see if i can get my buddy's connex or stryker to see if the extra swing changes anything. I'm still not home but will be tonight with pics.
     
  8. Ranch55

    Ranch55 Well-Known Member

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    Well, you should still be getting 175 watts PEP or more using a low drive radio.
    What does the VSWR reading look like since the FET swap.
    Need to measure both input and output swr's ....... It could be just a capacitor tuning issue.
     
  9. That_guy_in_wv

    That_guy_in_wv New Member

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    It had a swing of about 175 before the swap now it's just under 100 watts with the 9530s. I think It is a cap tune issue. I've been in radios for a while now about 9 years but I'm not sure about how to check the VSWR on the input side. I'm used to working on tube amps but got tired of dealing with them so I went to transistor type now. Just trying to learn what I can.
     
    Robb likes this.
  10. Ranch55

    Ranch55 Well-Known Member

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    I run a VSWR/Power meter between my radio output and amplifier, and I run another VSWR/Power meter between the amplifier output and antenna .....
    And as far as tuning the input and output of the amplifier accurately, a quality dummy load is essential.
    Otherwise, you are tuning to the antenna, which will work, but the amplifier will not be as good for other antennas, unless your antenna is almost perfectly tuned itself.
     
    #10 Ranch55, Oct 27, 2018
    Last edited: Oct 27, 2018
    Shadetree Mechanic likes this.
  11. That_guy_in_wv

    That_guy_in_wv New Member

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    1540602650779-1208934121.jpg 1540602729436-750254559.jpg
     
  12. That_guy_in_wv

    That_guy_in_wv New Member

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    It's a little dirty and looks like someone replaced one of the transformer turns wire but that's about all I can see is different all diodes and caps seem to be ok.
     
  13. That_guy_in_wv

    That_guy_in_wv New Member

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    Sorry Ranch55 I wasn't understanding what you meant earlier lol yes I can check it between the radio and amp.
     
  14. nomadradio

    nomadradio Analog Retentive

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    Try putting a SWR meter in between the radio and the amplifier.

    Measure the SWR with the amplifier keyed. If the reading is high, the amplifier's input impedance is too far from the ideal 50 ohms. This will serve to reduce the output, if there is a mismatch between the radio and the amplifier.

    73
     
  15. loosecannon

    loosecannon Sr. Member

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    WV, the solder connections on the power wires look pretty bad from the pics.
    especially the negative lead.

    you might try re-flowing those using a properly rated soldering iron.
    LC
     

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