Yaesu FT1000mp

input bandpass filter module input bandpass filter module

Callum brought his FT1000 Mk5 ( the 200W version ) to me for repair. Symptoms were, when the radio was switched on, the outboard 38V switched mode PSU would go into over - current shut down. Prior to the set croaking, it was noticed that the swr / power indicator on the set showed very low output power and high swr. After removing the millions of screws, the P.A. unit was accessible, only to find that both output FET's were short circuit ( thus causing the PSU to trip ). The devices used are BLF147's, very expensive FET's, from the U.S. they worked out about 55 each, still much cheaper than UK suppliers. A pair was ordered from the U.S. and promptly fitted. Lead free solder is used on this board ( higher melting point than lead / tin soler ) , so a very hot iron is needed, I used a100W solder gun. On switch on , all seemed ok, no PSU tripping.The bias pots for the FET's were set up according to the manual. However, when the mic was keyed in CW ( or any other mode ), about 280W was produced into a dummy load, even though the power out was set for 10W in the menu! Also, the power output bar graph on the set was only showing about 20W and the swr bar graph was showing very high swr, approx. 4:1. A separate swr indicator was put in line from the output of the P.A. and checked on key down, the same was then done from the output of the low pass filter board, all was correct. This led me to the swr bridge on the low pass filter board. After some investigation, it was found that one of the diodes, ( two are used in a voltage doubler configuration ) D6001 in the forward leg was leaky ( it read 300 Ohms both ways ). This was replaced. Forward and reflected power was now being indicated, but not correctly. A further delve into the swr bridge uncovered two more duff components, the surface mount coupling caps ( C60062 and C60063 ) from the current transformer, L6015, output to the two legs of the bridge were also leaky! One read 15K and the other read 3K. These were replaced with conventional caps tagged on the back of the board as I had no SMD caps. The bridge was balanced on key down by adjusting TC6001 into a dummy load. Finally, all was well, the set performed as it should ( now producing 200W on full power and swr / power bar graphs functioning correctly ) and I only had a few screws left over. So, for the sake of a few components costing pence, two very expensive components were destroyed as a result. Don't you just love semiconductors?

input bandpass filter module input bandpass filter module




Barry Zarucki M0DGQ

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