MFJ-4035MV RV2 Max pot adjustment?

Discussion in 'Radio Circuits, Repair & Performance' started by KD9MXA, Apr 2, 2020.

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  1. KD9MXA

    KD9MXA Ham Member QRZ Page

    I have my MFJ-4035mv power supply opened up to make some modifications to. It has 4 pots, 2 for meter adjustment, one to adjust the 13.8v detent, and a 4th labeled RV2 Max. My assumption was that this was to adjust the max voltage to 15v, however adjusting it has no effect on the voltage or the onboard meter.

    Potentially a bad pot, but before going further I thought I'd ask here if it's indeed an adjustment for something else. Schematic attached.



    Attached Files:

  2. G4COE

    G4COE Ham Member QRZ Page

    This circuit is used in many other 'branded', power supplies one is that of the Daiwa PS304 Mk2. Another source would be to have a look on Repeater Builder, they have a section on PSU repair.

    RV2 is current limiter set. See Schematic attachment.


    Attached Files:

    Last edited: Apr 4, 2020
  3. KD9MXA

    KD9MXA Ham Member QRZ Page

    Thanks for the helpful resources Dave!

  4. G4COE

    G4COE Ham Member QRZ Page

    Glad they was of use. I have the Daiwa PS-30 Mk2, same circuit as discussed. I've put meter LEDs in using super bright white, using two in each meter, these give a white clean look, warm white LED's will give that incandesecent warm glow.

    I keep saying I'll put a crowbar in using a MC3423 crowbar chip, all my homebrew psu's has one of these chips, you can set the trip very close to where you want it and it stay's 'put' unlike a zener. I've also built a replica circuit incorporating the crowbar, pcb slightly different mind, for some reason I prefer these op-amp circuits over the LM723 regulator chip.

    Good luck, Dave
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    What mods does it need ?
  6. KD9MXA

    KD9MXA Ham Member QRZ Page

    I added a pot to limit the voltage to the fan to quiet it down. Also added a 3-prong socket for the main line connection (added ground). Added some caps to the main 12v terminals to remove any RF on the lines...
  7. G4COE

    G4COE Ham Member QRZ Page

    I'd be inclined change that 100nF across the secondary to about 0.47nF using a proper AC rated components.

    I built a 'fixed tuned' Radio 4 LW and FM band radio for the shack (sometimes Radio 4 LW carries different programs), when powered from the mains the noise was horrendous on long wave band so much so I had a job hearing the station..... a 100n across the 12V secondary did nothing so I tried a 0.47uF AC rated cap, think it was a class X cap. wow what a difference it made the reception as as clean as the FM transmission.... except when thunders about. Just a simple TDA7000 FM chip and a NE602 front end into a two transistor IF amp.

    It cleared all the trash up that the national grid picked up, switching spikes, flash-overs, squeals and other noises, seemed to make no difference if it was on the primary or secondary side, a 100nF did nothing in my case , keep in mind our voltage is 240V. I lobbed at least 0.47uF minimum in all my linear power supply's.

    Be aware some power supplies may look very similar and may have a component value difference here and there, most important is yours as a 'booster winding' of a few volts and some don't, they just have a single winding for the regulator like my Daiwa PS30XM-2.

    Last edited: Apr 15, 2020
  8. KD9MXA

    KD9MXA Ham Member QRZ Page


    That's an interesting anecdote and I'll have to give it some thought.

    I'm still trying figure out an exact process I need to follow to set the max current pot without needing expensive test equipment. Based on some of what I read through on Repeater Builder it seems that I'd have to hookup a known resistive load and then watch for a voltage drop based on Ohm's Law to see that current is being limited. I'm also curious if there is any harm in setting the max current pot to maximum and assuming that I won't place the PS under a load to exceed the max current spec of 30 amps.

    Last edited: Apr 16, 2020
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    Why not just use a fuse ? That adjustment protects the power supply.

    A bigger fan would be a better idea, Instead of slowing down a fan that is to small too cool properly and is noisy.
  10. KD9MXA

    KD9MXA Ham Member QRZ Page

    I've always been a student of Socrates. Beyond practicality, for me this is an exercise in seeing a problem through to a solution to build wisdom.

    Under a good load and the fan operating at 13.8v it sounds like a jet engine, and the exhaust is cool to the touch. Retarding the fan to 9, 7, or even 5v has no noticeable change in exhaust temperature or overall case temperature. Modern case fans with better bearings operating at 13.8v would certainly be even quieter than the original fan at 7v, but I was able to make the mod with parts on hand which is a great benefit right now.

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