Tranz4mr’s ZW Page


ZW 275 Hum Fix

Does your ZW make a loud humming noise?  Here is the method that I discovered to fix it. This only works on the last versions of ZW 275’s with a riveted core like the Type R or ZW 275’s made just before the Type R. It does not work on ZW 250's or older ZW 275’s since their core construction is completely different. If the bottom plate is stamped “Model ZW (R)” as shown in Figure 1 then your good to go.

Figure #4 - ZW 250 and older ZW 275 Core  


Before starting compare the photos below to your ZW core to verify it is of the riveted construction. Bottom plates can be changed. Remember, the core is the difference so open it up and check what type of core is inside.


  1. Unplug the power first.

  2. Remove the 4 bolts from the bottom cover.

  3. Remove the Red and Green lens covers from the top.

  4. Remove the 2 light bulbs.

  5. Remove the 4 screws that hold the ZW top cover on.

  6. Remove the ZW top cover.

  7. Look for the rivets of the Type R core. Figure 2 & Photo 3.

Photo #1 - ZW Type R bottom Plate

Figure #2 - ZW Type R Core  


The ZW 275 Type R transformer core is made up of layers that are riveted together to form two separate parts. See Figure 2 & Photo 3.

Part 1 is C-shaped and the primary and secondary coils are slipped onto the 2 legs.

Part 2 is T-shaped and is glued and then pressed onto the end of Part 1, closing the C.

What happens is one side of the C where Part 2 bonds to Part 1 de-bonds and vibrates at 60 Hz. 

The fix is to pry apart Part 1 and 2, clean the surface, and then re-bond it with epoxy. You can do this with 2 people without unsoldering anything if you are careful.

The older ZW 275 and ZW 250 have a layered type core. There are no rivets.  It is made up of many alternating layered plates manually inserted into the primary and secondary coils.

There is no glue and no rivets so these core plates are free to vibrate. Also because of the alternating layers there are small openings or voids at each end of the core. This makes the core less dense and probably less efficient.  See Figure 4.

Photo #3 - ZW 275 Riveted Core
Photo #5 - ZW 250 & older ZW 275 Core. Voids at ends between layers.
  1. Tie back the Roller Arms on each side to the lamp sockets to keep them away from the Core. I use heavy bent paper clips. Photo 8 & 9.

Photo #16 - Clean the mating surfaces with a Dremel Wire Wheel.
Photo #17 - Clean the mating surfaces with a Dremel Wire Wheel.
  1. Clean off the mating surfaces with a Dremel Wire Wheel tool.
    Photos 16 & 17.

  1. Clean the mating surfaces with Alcohol or other solvent.

Photo #18 - Mix some 2 part epoxy.


Photo #19 - Apply epoxy to the mating surfaces.

  1. Mix up some 2 part Clear Epoxy.
    Photo 18.

  2. Apply a thin coat of Epoxy to all of the mating surfaces. Photo 19.

Photo #20 - Align the 2 Core parts. Use a 6” C-Clamp.

Photo #21 - Tighten the
C-Clamp squeezing the Core Back together
  1. Align the 2 pieces of the Core with the rivet heads on the same side.

  2. Use a 6” C-Clamp to hold it together. Photo 20.

  1. Tighten the C-Clamp until the Core is completely clamped back together. Photo 21.

  2. A large Vise can be used in place of the C-Clamp. Photo 22

  3. Clean off any Epoxy that squeezed out.

  4. Leave the Core clamped for the minimum time that the Epoxy needs to set. More time is safer!

  5. Reassemble carefully. Check for:
        - pinched wires,
        - broken solder joints,
        - cracked Power Cords,
        - cracked or worn Rollers,
        - loose bits of metal.

  6. Let the Epoxy set for at least 24 hours before applying power.

  7. When you turn it on it should be dead quiet.

  8. Use an AC Voltmeter to check that the output on each handle is approximately 18 volts.

Photo #22 - A large Vise can be used instead of the C-Clamp.


  1. Now is a good time to pull out the popsicle sticks, lollipop sticks,  match book covers, toothpicks etc. Photo 6.

  2. Remove the 2 brass Coil Support Straps. Photo 7.

Photo #7 - Remove Coil Support Straps.


Photo #6 - The Lollipop fix.

Photo #9 - Tie back the roller arms on each side.
Photo #10 - Unscrew the Circuit Breaker.
Photo #12 - Remove the 2 Coil Support Brackets.
Photo #8 - Tie back Paper Clips.
  1. Unscrew the Circuit Breaker from the aluminum Coil Support Bracket. Photo 10

  1. Carefully lift the Core up. Photo 11.

  2. Remove the 2 Coil Support Brackets. Photo 12.

  3. Get a tool that is sharp and can be forced in between the 2 parts of the Coil as shown in Photo 3 above. I use a thick 1” wide putty knife with a chisel point end. Photo 13.

Photo #11 - Lift the Core up.
Photo #13 - Get a sturdy sharp 1” wide putty knife.
  1. Lift the Core up with the rest of the case and wires hanging below.

  2. Set the core on something sturdy like a work bench or flat anvil part of a vise. Set it down so that the joint crack between the 2 Core pieces is on the flat surface. Photo 14

  3. Make sure no wires are caught under the Core and the flat surface.
    Photo 14.

  4. Using a hammer carefully drive the putty knife into the joint separating the 2 pieces of the Core. Photo 14.

  5. Insert a screwdriver in the joint if needed and pry the Core apart farther.

  6. If needed, flip the Core over and drive the knife into the opposite side.

  1. The 2 pieces should separate.
    Photo 15.

  2. Notice the small amount of glue especially on the left side. Photo 15.

Photo #14 - ZW 275 Type R Core with the joint resting on a  solid surface.  Putty knife in the joint crack.

Photo #15 - ZW 275 Type R Core with the 2 parts separated..

The Fix