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Radio Broadcast Technical Consulting and Sales
 
10032 Island Drive, Brainerd, MN  56401

 


Technical Tips from Mark W. Persons
BE RF Amplifier Repair/Upgrade


 
 
Friday, August 23, 2013: Many will recognize this as a 250 watt FM IPA amplifier from a Broadcast Electronics B series 1.5, 3.5, 5, 10, 20, 25, 30, and 35 KW FM transmitter.   Later production runs have a beige color.  It is used to drive a tube power amplifier.   When this one arrived, the 31 lb. power transformer was flopping around after two of the four securing bolts had pulled-through the aluminum chassis.  Part of it was due to being dropped during shipping and the other was the fact that tapered holes, for the flat-head 1/4-20 bolts, were bored too far into the aluminum.  They were deeper than just plain flush.  That left them unable to hold the transformer during rough handling.  I bored new holes nearby that were not as deep and actually left the bolt heads sticking slightly through the aluminum bottom of the assembly.    
 
 
Here is the RF amplifier assembly where the top of the uppermost RF power transistor is missing.  It blew off when the transistor failed.  These are SD1460 or MS1281A NPN planar VHF transistors that are difficult to get. 

June 2016: Broadcast Electronics says it will not sell transistors to the end user, but will instead repair modules in their facility for about $850, depending on how much work needs to be done.   A complete amplifier chassis swap can be purchased for $1592. 
 

This RF amplifier that was damaged by lightning.  Serious trouble....not repairable by just replacing transistors.
 
 
Here is a Broadcast Concepts SD2942/FM350 Pallet Amplifier installed in place of the original amplifier. 
RF input to the new pallet has nine resistors, which form an RF pad.  It takes about 9 watts of RF drive to get the amplifier to full power.  
 
Repairs and modifications, with the original power supply, run $900 plus shipping at the M. W. Persons facility.  That gives up to 150 watts of RF output for BE 1.5, 3, and 5 KW B series transmitters.  There will be an additional charge if the power supply regulator card is not working properly when received here.

It takes 48 volts to get a full 350 watts from this amplifier design.  To make the modification work, I replaced the original power supply with a 48 VDC/600 watt switching power supply and substituted the 120 VAC cooling fan with one that requires 240 VAC.  The unit now  runs only on 240 VAC, unless other modifications are made.

The amplifier's DC controller card needs to be included in the final design.  That card turns DC on only after the transmitter's tube has high voltage, it regulates DC voltage to the amplifier, and it cuts the IPA power back in case of high VSWR.  In the end, the pallet amplifier only gets 42 volts under normal operating conditions, but easily makes 250 watts.  

The update with new pallet amplifier, switching power supply, and cabinet fan for 250 watts output  is $1400 plus shipping at M. W. Persons.  There will be an additional charge if the power supply regulator card is not working properly when received here.


Warning!!  I do not recommend doing this modification if the IPA amplifier is to be combined with another existing Broadcast Electronics RF amplifier as there is likely a phase difference, which will not work well when the two are brought together in a combiner.  Amplifiers need to be very similar in characteristics in order to add power together in a hybrid combiner to feed the tube.  


August 26, 2013 email:  Just read your latest pallet amplifier install in the BE IPA. Very nice job as usual.  I'd say you've got this kind of repair/rebuild down to a science.  Bob in Hamden Connecticut.


The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   Ham WMH      

Questions?  Email Mark Persons:  teki@mwpersons.com        

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page last edited 07/02/2016