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Frank Hertel Revives a 1 kW FM Transmitter

Here’s how he brought a BW Broadcast box back to life

Engineering consultant Frank Hertel writes in to describe a fix for a power supply failure in the popular BW Broadcast 1000W transmitter.

If you’re not ready for a construction project, you can get service support for the BW lineup from Progressive Concepts, which introduced the transmitters to the U.S. in 1997. The company has expertise servicing BW equipment and has once again been appointed as BW’s sole sales and service center for the United States. For information, contact Eric Hoppe at 630-736-9822.

Frank took a DIY approach. The original power supply is a semi-propriety design modified for special use, making his efforts to find a replacement harder, but Frank set out to find a workaround to revive this otherwise good FM transmitter. 

In the end, it was a fairly simple fix. What was lost was software control of the transmitter’s power output; his repair causes the transmitter power to be adjusted using the output voltage of the 48 VDC supply. You will also need to add a 12VDC power supply, rated at 3 to 4 Amps, since the original power supply provided both 48 and 12VDC.

Frank described a series of steps to replace the defective power supply. First, for the 12VDC supply, Frank selected a “brick” power supply from Amazon. They are inexpensive (under $10) and widely available. The 48VDC supply is manufactured by Jingmaida and can be found on Amazon for under $300. Figs. 2 and 3 show where to connect the wires from the new 12VDC and 48VDC power supplies, inside the transmitter chassis. For the 48VDC, Frank used #8 silicone superflex wire. 

Click on the photo to toggle between Figs. 1–7.

Frank adds the following notes:

  1. No holes were drilled in repair. The external power supplies can be replaced easily, should they fail.
  2. Velcro brand hook-and-loop fastener was used to fasten the new power supplies to the top of the transmitter chassis.
  3. Frank chose to remove the defective internal 48VDC power supply. The void provides easier routing of the new power supply wiring.
  4. If desired, the on-board voltage adjust potentiometer can be wired to an external larger potentiometer, to make power adjustment easier, as seen in Figs. 4 and 5.
  5. The 48 VDC power supply also has a terminal strip where you can add a switch to MUTE the 48 VDC power supply. Frank used this feature for easy shutdown of the transmitter. These MUTE contacts can also be wired to a set of dry relay contacts on your remote control, to remotely shut down the transmitter. 
  6. When the new power supply is received, the MUTE terminal strip has a jumper on the bottom of the circuit board.  You will need to remove that jumper to use the MUTE function.
  7. The value of the Voltage Adjust Pot is 5 K Ohms. Frank and his team added two small trim pots (10K each) to the lower and upper terminals of the larger external 5K pot (Figs. 4 and 5). One of the small 10K trim pots will let you set the Minimum Voltage Platform (it should be approximately 27 VDC) and the other small 10K Trim Pot will let you set the Maximum Voltage Platform (which should be approximately 42 VDC). These settings help protect the internal regulators.

Other models of BW Broadcast transmitters have similar configurations but deserve a close review to make sure the procedure is suitable. 

The new adjustable 48 VDC power supply also has a current limit adjustment. It is factory set at approximately 33A. You should not need to reset this adjustment. Note that the replacement power supply also has over-current shut down. If something in the transmitter should fail and draw excessive current, the power supply will automatically shut down.

Fig. 6 shows the completed project, and the restored RF output power is seen in Fig. 7. 

If you complete this modification, snap some pictures and let us know how the upgrade went. Email them to me at johnpbisset@gmail.com.

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