ad: elecraft

mfj 4225 SWITCHING power supply demons

Discussion in 'Radio Circuits, Repair & Performance' started by WB8VGE, Nov 20, 2021.

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: Left-2
ad: L-Geochron
ad: L-MFJ
ad: abrind-2
  1. W9GB

    W9GB Ham Member QRZ Page

    Apple dealer I worked for (summers during grad college) automatically sold a switched surge power strip with every home Apple II system. Over 50% of his sales were education market (elementary/middle schools), so those systems were rarely turned off. :)
     
  2. G4COE

    G4COE Ham Member QRZ Page

    Once repaired a high end tape recorder that had a squeak that couldn't be pin pointed, in the day's when I clodded ale down my throat I hit on an idea..... I asked the land lord to give me a snip of the plastic tubing used on the pumps...... 'Dr COE will see you now Mr Recorder'..... OK, OK, I had no receptionist, let mi' have mi moment of glory won't you?

    I got the tubing and poked around like as if a stethoscope, usually the clutch that squeaks but not this time, t'was a cam wheel buried beneath the rest of the mechanism..... what a pain!

    Make sure its not wet or greased up or you might have some voltage down ya' arm n' ears!

    Dave
     
  3. WB8VGE

    WB8VGE XML Subscriber QRZ Page

    Everyone!

    I thought I'd poke on this supply one last time before it became a speaker.

    After I cooked the switching transistors, my fault btw... I ordered in new and replaced the shorted ones.

    It powered up and life was once again 22 V.

    So, there is a 3.2kKΩ resistor that monitors the output. The main voltage control pot (the one on the front panel with the center detent) appears to be wired in series with the 3.2KΩ.
    I took the 3.2KΩ out of the circuit and in it's place I put a 25K multi turn trimmer. Should have used a 5k, but didn't have one in multi turn.
    Set the front panel control to center, and fired it up. I adjusted the trimmer until I got 14.0V on the terminals.
    I loaded the supply to 18A and held it there for ten minutes. The voltage drop .3V (thin wire to the load)

    So, I'm going to call this one fixed.

    Measure the final value of the trimmer and solder in a close fixed resistor? Thought about it. Just might hot melt glue the trimmer in place and call it MFJ done.

    I'll tidy up the mess and dress the wiring.

    I'm mike, wb8vge
     

    Attached Files:

    WA9UAA likes this.
  4. G4COE

    G4COE Ham Member QRZ Page

    Good work on getting it back working, one thing though 3.2K is not a standard value, unless it's a high tolerance type.

    All I can think of is some component drifted with age, I wouldn't be surprised if you got slight variation in voltage at the detent point when comparing same power supplies.

    Dave
     
  5. WB8VGE

    WB8VGE XML Subscriber QRZ Page

    I thought the same thing about the resistor value. However that would explain how the front panel voltage adjustment didn't have an 'adjustment' to set 13.8V at the detent.

    However that wouldn't explain why sometimes I could fix the problem by cycling the power on and off several times. Unless, and I'm guessing, there was a micro crack in the resistor. Hell, I don't know. I'll be damn sure to reconnect the overvoltage shut down circuit before I put it together.

    mike, wb8vge
     
  6. G4COE

    G4COE Ham Member QRZ Page

    Whoooa, avva look right here at this thread. Well worth following links in the post.

    I wonder if its related to that sizzlonmg noise you mentioned? I'd be checking the snubber, resistor capacitor combination across the high voltage primary winding by substitution, rememberingit'll be a high voltage capacitor, changing the two electrolytic in series with the two bases for high termp. 105 deg. C low ESR types, increasing their working voltage is a bonus.

    Theres what looks a like D12 diode from the secondary going to a 33 Ohm with two 47uF capacitors either side.... without looking the chip up I guess this would be the referance voltage, if so I'd be looking around here.

    I've had diodes check OK out of circuit but under stress, they can play funny tricks.... that sizzly is foxing, it could be something or nothing I'm afraid.

    Dave
     
  7. WB8VGE

    WB8VGE XML Subscriber QRZ Page

    You know the silly thing? After I put in the trimmer in place of the fixed resistor, the frying bacon sizzle had gone away. Was it the replacement trimmer or perhaps my fingers pushed and nudged something and that stopped the sizzle/cracking

    Mike WB8VGE
     
  8. G4COE

    G4COE Ham Member QRZ Page

    One may never know, I've had resistors doing strange things in Stereo units and TV's. Remember one Sony Stereo amplifier the sound would just quit..... t'was the sound muting circuit up to it's funny tricks..... I can't remember if it was a 1M or 10M resistor, it was quite a high value it checked perfect on the meter but certainly wouldn't work in circuit.... I tried three times., a new one cured the fault everytime.

    I've had diodes and capacitors playing funnies in CRT TV's. LOL, had a telly that kept going off, it could run all day, it could run half an hour or less. I was doing something else, I looked daggers at the set and thought of the lump hammer the telly KNEW....it just went off - The darn mains fuse was intermittent, the only thing I never suspected - it caught me out!!!!!

    I connected the fuse to a 100W 240V mains lamp, bang the fuse do what ya' want but lamp would only go off when the fuse told it.... that devil didn't go to the fuse yard in the sky.

    Dave
     
  9. W9GB

    W9GB Ham Member QRZ Page

    Mike -
    In early 1980s, my Elmers found two (2) intermittent resistor issues.

    1.) When this specific 1/2-watt resistor was manufactured someone in production spot welded the wire (giant spools) and that weld ended up inside the carbon composition.
    SO, as the resistor heated its characteristics changed.
    An amateur jeweler, he painstakingly removed the molding to find the spot weld (poor job BTW) inside. He kept it for awhile — to show mfg (Allen Bradley, I think).

    2.) ETO Alpha 76A amplifier control board, resistors on 24 Volt line.
    Intermittent shutdown, drove K9EWJ (sk) crazy trying to find cause.
    Sent back to Alpha in Colorado (warranty), but they could not find any issue.
    He never fully used the amp.

    My Elmer acquired that Alpha 76A, a few years later, and found the culprit.
    Hairline fracture (likely due to heating) of resistor on control board (bias),
    since wattage value selected was right at edge of current level (no margin).

    ETO Alpha raised wattage of 3 resistors used in 24 V part of control circuit (Service Note)
     
    Last edited: Dec 4, 2021

Share This Page

ad: w5yi