Repairing an RF module for a DX-50

I like these types of posts. Many people are intimidated by component-level repairs. I write this to show that it is possible, with minimal test equipment and easy-to-understand directions, to make repairs and get things back in working order.

Every year, we lose two or three RF amplifier modules from the DX-50s. Normally, this happens after a thunderstorm. Sometimes it is a spontaneous failure.

The project starts here; faulted RF module

These are fairly simple medium-wave RF amplifiers. There are no adjustments or tuning circuits on the amplifier board itself. They use eight IRFP-350 RF devices. There are two fuses to protect the transmitter power supply against device short circuits. If a fuse blows; Section C, RF amplifier Modules, of the DX-50 manual has a good troubleshooting guide. There is a very good chance that one or more of the RF devices is bad. Unfortunately, they have to be removed to be tested.

Gates Air recommends that if one device is failed, all four devices from that section are replaced at the same time even if they test good. The IRFP350s are inexpensive devices and it is easier to replace everything at once. The Mouser Part number is 844-IRFP350PBF and they are $3.81 each as of this writing. The PBF suffix means it is lead-free.

The other part that can be bad, but it is unlikely, is the TVSS diode across the gate and source (CR1-4). These are inexpensive as well. Mouser part number is 576-P6KE20CA and they are $0.38 each. It is good to have a few of these on hand.

Heatsink removed from the module

The most difficult part of this is dealing with the heat sinks. The devices get stuck to the heat sink pads after so many years, so it takes a little effort to get them separated. The manual recommends gently prying the device away from the heat sink with a small screwdriver. They can be reused if you are careful and do not rip the insulator pad. However, if the insulator pad rips, it needs to be replaced. Mouser part number 739-A15038108 ($0.86 each).

To test each IRFP350 after it has been removed, use either a DVM with greater than 3 VDC on the resistance setting or a Simpson 260 on the Rx10,0000 range. Connect the positive lead to the Drain, the negative lead to the Source, and then use a jumper connected to the Gate to turn the device on or off. Alternatively, you can use a 9-volt battery to turn the device on and off.

If the device does not turn on or off, it is defective.

The TVSS diode should measure open (>2M) in both directions. Anything other than that, the unit is defective and needs to be replaced.

DX-50 module, heatsinks removed

The first step is to remove the heat sink. I used a small screwdriver under the leads to gently pry the MOSFETS off of the heat pads. If you are careful, all of the heat pads will survive. Once the heat sink is off, I remove all four of the suspected MOSFETS. The leads are heavy gauge, so it takes a little bit of work with the solder pump and solder wick. I tested each MOSFET and found one shorted unit, the other three test okay. However, since these are inexpensive devices, I replaced all four.

DX-50 module, MOSFETS removed

Good device:

Device off
Device on

Bad device:

Device shorted

Assembly is in reverse order. Make sure that none of the insulating pads were torn during disassembly. I like to get everything attached to the heat sink before soldering the leads. It helps with lining everything up. Take care and make sure that the ferrite beads on the drain leads of Q3,10 and Q4,11 are re-installed with the new devices. These are necessary to prevent high-frequency oscillations.


Of course, the final test is in the transmitter. Generally speaking, I test the standby transmitter into the test load every two weeks. This is done in conjunction with the generator load test so as not to spin up the demand meter.

Excel spreadsheet formulas for Broadcast Engineers

There are many times when some mathematics is needed in this profession. For one-off situations, the calculator applications found on most smartphones will work just fine. However, sometimes the calculation is complex or is needed to be repeated many times. Excel Spreadsheets have many mathematical functions built in. Plugging a formula into an Excel spreadsheet is a handy tool.

I recently acquired this rather nice precision power meter:

Mini Circuits precision power meter

It has an input power range of -60 to +20 dBm with a stern warning not to exceed +23 dBm. Since we will be using this for a variety of applications, I thought it might be useful to know approximately how much power will be presented to the instrument in any given situation. For example, we are installing a 30 KW FM transmitter soon. The directional coupler that will be used has a coupling factor of -48.5 dBm. The TPO is 28,000 watts.

The formula to convert Watts to dBm is dBm=10 X Log10(Pw) + 30, where Pw is power in Watts. Thus dBm=10 X log10(28000) + 30 or 74.4715 dBm minus the 48.5 dBm coupling factor which is 25.9715 dBm. That is too much input for this power meter. A 20 dB attenuator will need to be used.

Since I will be using this meter in other places, rather than doing that calculation over and over again, why not build an Excel spreadsheet? That would make it easy to check.

A simple Watts to dBm calculator in Excel looks like this:


This is copied into cell C11. C6 is the cell in which the Transmitter output power in watts is entered. The other cells contain the coupling factor (C5) and external attenuation (C7) In application, it looks something like this:

Excel spreadsheet power meter calculations

You can arrange these any way you like, just change the cell numbers to suit your needs.

I like to make the data entry cells green. You can lock the formula cells so that the formulas don’t get changed accidentally. Below the Approximate port power cell, is the IF statement that will return either a “LOW”, “HIGH”, or “OK” depending on the result value in C11. That looks like this:


The spreadsheet itself is downloadable: Power meter port calculator

It would be very easy to make a system gain/loss calculator for using the licensed ERP to calculate the proper TPO.

Other examples of useful Excel spreadsheet formulas:

To convert from dBm to watts:


B22 is the cell in which the power in dBm is entered. These can be any place you want on the spreadsheet.

Radio Frequency to Wavelength in Meters:


Where B10 is the cell in which the frequency in Hz is entered. 299792458 is the speed of light (Meters per second) in a vacuum. If you wanted the input frequency to be in kHz, simply move the decimal point for the speed of light three places to the left, e.g. 299792.458. For MHz move the decimal four places to the left, GHz five places, etc.

Convert electrical degrees to Meters:


Where B10 is the frequency in kHz and B11 is the number of electrical degrees in question.

An example of that in an Excel Spreadsheet can be downloaded: Frequency to Wavelength converter

Audio Frequency to Wavelength in Meters:


Where B11 is the air temperature in degrees Celsius and B12 is the frequency in Hz. Room temperature is normally about 21 degrees Celsius (about 70 degrees Fahrenheit). Humidity and altitude can also affect the sound wave velocity, which will affect the wavelength.

Base (or common point) current calculator using base impedance and licensed power:


Where B12 is the License power in watts and B11 is the measured base impedance of the tower (or common point impedance of the phasor).

Convert meters to feet:


Where B11 is the length in meters

Convert feet to meters:


Where B12 is the length in feet.

Convert degrees F to degrees C:


Where B11 is the degrees Fahrenheit

Convert degrees C to degrees F:


Where B12 is the degrees Celsius. In this case, the order of operations will work without the prentices but I kept them in place for uniformity.

Convert BTU to KW:


Where B11 is the BTU/hr

Example of an Air Conditioner load estimation:


Where B11 is the TPO, B12 is the transmitter AC to RF efficiency. The output is in BTU.

There is an entire list of Excel functions here: Excel Functions (alphabetic order)

You get the idea. Yes, there are smartphone applications as well as online calculators for most of these functions. However, I have found smartphone apps are becoming more painful to deal with as time goes on, mostly due to the ads. App developers need to make money, and you can buy apps for things that are often used. However, it is nice to have these types of calculators available offline. Besides, it is fun to play around with Excel formulas.

Fixing the switching power supply

This particular power supply is used in Broadcast Electronics AM1A, AM2.5E, AM5E, AM6A, AM10A, FM1C, FM10T, FM20T, FM30T and FM35T transmitters. It is a Computer Products NFN 40-7610, 40 Watt, +5 VDC, +/- 15 VDC BE part number 540-0006.

BE AM1A ECU power supply, C-15 marked with pen for replacement

Generally, one component fails over time on this unit, C-15 which is a 680 uF 35 V electrolytic capacitor. When that capacitor dries out, the power supply will fail to start, do odd things like start and fail after a second or two, or cycle on and off. This will happen after the transmitter has been off for a few minutes. Replacing C-15 with a 1000 uF 50 V capacitor will fix the problem. There is enough room for the larger capacitor if the leads are left a little bit long.

BE AM1A repaired ECU power supply re-installed

We have several of these repaired units on various shelves at various transmitter sites.

As always, when replacing electrolytic capacitors, pay attention to the polarity otherwise this will happen:

Blown Electrolytic Capacitor installed backward

I suppose somebody was in a hurry to get home that day. After I installed this repaired unit, it ran for about 15 seconds and then there was a pop. I opened the door on the ECU and white smoke was wafting out from under the power supply cover. Since the Pope is still The Pope, I knew it was the electrolytic capacitor.

Back in business

Our beloved BE AM1A is back in service. This transmitter is 22 years old and we can keep it going for as long as parts are available.

Changing the Capacitors in a DX-50

This lovely transmitter is 33 years old! I thought we’d celebrate by changing the electrolytic capacitors! It appears that some of the capacitors were changed in 2009, but many are original to the transmitter. Gates Air recommends replacing the electrolytic capacitors during the course of regular maintenance cycles. Dried-out capacitors can lead to a variety of mysterious fault conditions including low drive levels, poor audio quality, difficulty starting the transmitter, and so on. See Harris service bulletin AM-579-JK.

DX-50 transmitter

Some of the Low Voltage Power Supply capacitors were at least partly dried out. They felt a little bit light and one of them rattled when I shook it.

Replacing large PA capacitors

Gates Air has a full replacement kit, part number 973-2100-391. When I looked into it, the price of this kit is very reasonable and is closely aligned with the prices from Mouser Electronics.

Old capacitors for disposal

Here is a full list of capacitors needed, I put the Mouser part numbers in just in case:

NomenclatureAmountGates Air PNMouser PN
Cap, 5,100 uf 350 V17524-0341-000598-DCM512T350DG2A
Cap, 5,500 uf 200 V4524-0219-000
Cap, 15,000 uf 100 v2524-0322-000
Cap, 76,000 uf 40 v4524-0342-000
Cap, 120,000 uf 40 V*2524-0380-00075-36DX124G040DJ2A
Large Capacitors for DX-50 transmitter

*See Harris service bulletin AM-474-TLH for information on the -8 volt power supply.

Board is green, Returned to service

Most of the capacitors in the PA section were original to the transmitter, as well as those in the 30-volt and 60-volt driver supply.

One important detail; pay attention when installing these things. You do not want to reverse the polarity on an electrolytic capacitor, it will likely explode. These units are pretty big and the explosion would be loud and messy.

With that done, I also went through the transmitter and reset all of the voltage levels according to the factory test data sheet. When it was returned to the air, it sounded great!