When spring came and I prepared to get my RV all un-winterized and ready to take out, I discovered that my trailer battery was nearly dead. Since it was new from just the year before, I thought I wasn’t getting very good mileage on batteries but I thought I would do some other checking before indicting the battery as defective. First, doing some various voltage checks, I found that I had good 12 volts DC to my interior lights and to the voltage monitor on the control panel. Hmmmmm….. that’s odd. That voltage was coming from the power converter 12 volt supply (although not from the converter's battery charging section).

Checking the cigarette lighter socket, it only showed 5 volts and I determined that this was coming directly from the battery which was, by then, in a near discharged condition. Although the trailer had been plugged in all winter, the battery had not received any charge since the battery charge feature in the power converter had failed in my Magnatek Power Converter, and I didn’t know when that had happened. The battery was OK, the Magnatek higher current 12 volt section was OK but the battery charger section of the converter was not OK.

Checking at the Magnatek panel in the bathroom, I found no voltage between point C (which was the positive side from the battery) and point D (which was the negative side from the battery).  Now I knew where I had to concentrate my attention.  NOTE:   Points C and D are actually labeled as such on the unit.

At this point I spent several hours combing the Internet in search of a schematic for the Magnatek converter….. totally without success.  I read jillions of messages from RVers, several asking for the same thing I sought.  I wanted any kind of information, particularly a schematic, for this unit.  I even contacted some of these people to ask if they had been successful in the search.  Nope…..nada…..nothing....... no such luck.

I found a telephone number for the factory which built the unit and even tried to call for info.  Human help was available for warranty work but my unit was several years out of warranty.  I was out of luck.

I needed that schematic and since it was unavailable, it looked like I would have to generate one myself.  If you’ve ever tried to do this it is akin to somewhere between trying to write down a cake recipe from only having the completed cake in front of you and unscrambling an egg.  Not much fun, especially if the parts have non-standard markings or no markings at all........  like my converter.

I was ultimately successful and the schematic I have included is close enough, if not an exact rendering, of what is found in the Magnatek power converter.  I hope it helps others as it helped me. Now, with schematic in hand, I returned to the project of repairing my converter.

 PIC #1: The circuit schematic is actually in two parts.  The first part, shown above, is the main power supply part of the converter.  Thanks to a reader named Baldy in California, you'll be able to see the pictures better than they originally were displayed.  Baldy took my original pictures, which were upside down and sideways, and corrected them for me.  Thanks Baldy..

PIC #2:  The schematic above is the battery charger part of the power converter.    It will charge the RV battery but then automatically taper down to a trickle charge so as not to destroy your expensive battery. 

PIC #3:  I included this picture but I'm not absolutely sure of its total accuracy.  I drew this out several times and I think this was the final version.   Don't hold me to it, however, and if there is a discrepancy between the schematic and the underside of the PC board, you should believe the schematic first.

UPDATE NOTE:  Since publishing this original article someone sent me the actual schematics on a Magnatek RV power supply.   Click here to go to copies of those two new pages.

CAUTION: When you decide to work on your power converter, be sure you unplug your trailer’s power cable from the 117 volt AC mains.  Once that is done you can remove the screws holding the panel/cover onto the converter.  This will expose the transformer, diodes, solenoid, fan and battery charge board.  This solenoid is energized whenever the trailer is plugged into external AC power and it allows the converter to send charging voltage to the trailer battery.  Magnatek chose to use a solenoid to activate a switch to perform this function rather than use a relay.  Why?  I don’t know.

The following are steps required to remove all connections so you can pull the whole power converter out to work on it:

1.  Remove the 4 hex-head screws which hold the front board.

2.  Remove the 2 hex-head screws holding the other board.

3.  Remove the 12 volt wiring to the 12 v. distribution board – the white and red wires have screws, the blue (12v) wire is attached with a screw and nut.

4.  Open the cover to the 117v AC circuit breakers.

            (You did disconnect the 117v AC didn't you?)

5.  Remove the white wire which goes to the left side vertical screw strip.

6.  Remove the black wire coming from the far right-hand circuit breaker.

7.  Carefully feed the 2 AC and 3 DC wires from the top box so they are inside the bottom power supply box. Now you can remove the power converter pieces.

8.   With the power converter box on the workbench, remove 2 hex-head screws holding the nylon posts which are holding the battery charger circuit board.

9.   NOTE: With the power converter box on the workbench, the top cover can be removed with 4 hex-head screws. This allows full access to the inside components.

The next step is to remove the board containing several electronic parts including resistors, silicon controlled rectifier, zener diode and a capacitor.  This board, on my unit, stood vertically and was attached to the right-hand wall of the supply.   NOTE: Do NOT attempt to pop the board off the nylon posts where it is mounted. It will not come off and attempting to pop it loose will result in a cracked board, which is made of rather fragile phenolic material.  Breaking that board can separate traces on the printed circuit board and you will have additional problems.   The board can be removed but only after the whole box, holding the supply, is removed. (A word to the wise….. I hope.............. ask me how I know this...... No......... never mind.)

It is now that you must use your electronic trouble-shooting skills to determine which component or components might be causing your problems.  That is rather difficult for me to tell you what might be causing your converter to fail but, hopefully, this information and the schematic provided will allow you to find it.

My problem turned out to be the large rectangular resistor mounted with a pop rivet to the back panel of the box. The value was obscured on mine but an ohmmeter check showed the value to be more than a megohm which was much, much too high.  I would guess that its true value should have been less than an ohm but at a high power rating, perhaps 50 – 100 watts.  It was here that I had to do some guessing.  Since this resistor is located in the line providing charging current to the trailer battery, my guess is that its function is to drop the load a bit when first connected to a battery which is totally discharged.  A fully discharged battery would place a tremendous temporary current load on the power  supply components and this resistor helps to protect things during that initial surge.

After the repair was complete, I did some checking on the converter to see just how much current was provided to a battery for charging and to see whether the higher current stayed up at the high level or if it tapered off to a trickle charge as this type of circuit should do.  With a 0-3 amp meter in series with the battery charging line, and using no extra resistance in the line (the big white rectangular resistor was shorted to make zero ohms or there-abouts), I placed the charging circuit across a pair of 6 volt/7.7 AHr lead-acid batteries which I had on hand. The ammeter started at just above 2 amps charging, then tapered down to around 1.25 amps.

Then I tried it across a 12v/7 AHr lead-acid battery with was already charged.   The current started a just below an amp and within a minute, dropped to about 200 milliampres or .2 amps. I say "about" because the ammeter constantly wiggled the equivalence of .1 amp.  I suspected that this might be caused by the noticeable AC ripple in the DC line.  After all, there is no filtering on the rectified DC coming off the full-wave rectifier.

I located a 2 ohm, 50 watt resistor in my junk-box to replace the defective one which had originally caused my problem.  Placing the charger wires across another charged 12v/7 AHr battery it started the charge current at less than an amp.  It also tapered down, within a couple of minutes, to about .2.amps…… a shaky .2 amps.  The resistor did not even run warm but it had very little current through it.

I finally found a .25 ohm/5 watt resistor and placed it in the place of the original defective white rectangular unit pop riveted to the back wall of the converter.  I feared that the power rating on this resistor might be too low but I have used this one for several months and it is holding up well.

I reassembled my converter by going in reverse with the steps taken earlier to disassemble the unit.  Everything went back together as easily as it had come apart and I was a happy camper (so to speak).  I hope, if you are having problems as I was, that you can use some of this information and have as much success as I did.

One additional piece of information – the fan which you hear while the trailer is plugged into an external 117 v AC source, is actually running on 117 v AC rather than 12v DC and is thermostatically controlled. A temperature sensor is attached to the aluminum heat sink which holds the power diodes.  When they are doing their job and supplying 12 volt power to your trailer (not particularly to the battery) those diodes will run hot and make the heat sink also quite hot.  The sensor turns on the fan which blows across the heat sink as well as our now familiar power resistor. If you don’t hear the fan then the power converter is not having to do much work and the fan gets to rest.

One other benefit on my converter is, I was able to clean up all the dust and "grunge" around the fan and it has become much quieter.  I still hear it but nothing like it was previously.

Just as a final encouragement for you to try to repair your own converter, when I looked up the replacement converter in a trailer accessories catalog, the replacement unit was between $200 and $300.  That was enough inducement for me to attempt my own repairs.

Jim Pickett – K5LAD

Written June 29, 2002 ---- Updated 09/20/07

After learning it was the same resistor as in the old Dodges, the same ones that we ALWAYS had a spare of in the glove compartment,  I checked that first.  I jumpered it with my ammeter when I had the battery out and all the lights in my camper would work.  They have not done that since I bought it used!  No wonder my battery would not charge while plugged in!  I then removed it and ohmed it...  open circuit.  The new one cost me $8.99 at the Canadian Tire store  part # 18-4506 ( for us Canadians )

Thanks Tom

I replaced my fan with a 117v AC muffin fan which was about 4" square, as I recall. These fans should be available in electronics supply stores, hamfests, or even computer stores.  The one I used was from the surplus market somewhere and was one I had kicking around.  If you go this route, be sure you get a 117v AC unit as a lot of the fans you'll find surplus are 12 volt DC.  The Magnatek fan only runs when you're plugged into city electrical mains.

As far as other parts for the unit --- "official Magnatek parts" I once had a catalog I got from an RV store.  I tried to find it today but, alas, no luck. I dug back through some older messages and will copy what I sent to another RVer about this book:

Sounds like you found a pretty good deal and I think that might be wise.   I might recommend that you find a book (actually a catalog) called "2004 RV Parts & Accessories."  It's an 8 1/2 x 11 inch book that's just over 3/4" thick.  I got mine from one of the RV dealers in Tulsa and it was a freebie.  I don't know if you've got some of those around you but it  might be worth a call to a dealer or two.  It's one of those books which is published for "the world" but has this dealer's name, logo, address, and phone numbers on the cover.  They say that many of the items in the catalog are stocked but if you find something which they don't have, they can order it for you......... kinda like the old Sears and Roebuck catalogs.  In this catalog I found, not only brand new items (including power converters) but also some (note SOME) parts to repair existing equipment. For instance, on page 116 I'm seeing replacement parts for Magnetek Converters like: door latches, limit resistors, the PC board, fan motor, and relay.  The prices are surprisingly economical.

Thank you in advance,

Again, I hope this helps.

Last Updated - 09/20/2007

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