This faulty but complete amp was bought for a very reasonable price by Chris G0EYO, who asked if I could have a look at it. Initial test showed the power supply had several faults, first and foremost the voltage selection jumper was incorrectly wired putting the 110 volt and 117 volt transformer taps across the 230 volt mains. This was corrected and the 850 volt secondary of the HT transformer was disconnected from the rectifier board. On switch on the mains fuse remained intact and a healthy 950 volts AC offload was present ( much to my relief as this would be a difficult item to obtain ) and both the 572's had glowing heaters, both cooling fans were also spinning round although one was a bit sluggish. Further investigation into the power supply revealed four of the rectifier diodes in one leg of the voltage doubler circuit ( a half wave doubler configuration is used here ) were short circuit and three of the equalizing resistors had gone high in value, the purpose of these resistors is to maintain an equal voltage across each diode when they are reverse biased every alternate half cycle. It was also noticed one leg of the smoothing can chain had unsuitable capacitors fitted ( two were only a 450 volt DC rating and were physically too big) also one of these was also leaking electrolyte. All rectifier diodes were replaced ( I used 1N4007's ) and all of the equalizing resistors. The old smoothing cans were replaced with PC power supply types, 150uF @ 500 VDC. A healthy 2400 volts DC was now being produced by the power supply. The original valves were Cetrons and both were faulty, one had no emission and the other was low emission. A new set of Chinese valves were installed however, after a merry dance these turned out to be duff. Another set of valves was installed and the amplifier was producing the manufactures spec output on all bands. Both cooling fans had the sleeve bearings re-greased and now spun like new. The two anode anti parasitic resistors were checked due to an earlier experience with a FL2100Z having one of these resistors open circuit causing instability and oscillation, both were ok. Also the two 33 ohm grid stopper resistors were checked and found to be fine ( I think these were replaced some time in the past ). As a point of interest, no neutralization is used in this amplifier unlike the FL2100Z ( who's effectiveness is somewhat questionable as the neutralization is taken from the output of the pie network thus giving a different phase shift on every band ). Lethal voltages are present in this amplifier so extreme caution should be exercised when fault finding . This is a old but very nice linear and should now give good service, the idea is to use it in conjunction with a Sommerkamp FT200 restored by Chris, at various vintage events that Wythall Radio Club attends throughout the year. The photographs show various stages of the rebuild.
Barry Zarucki M0DGQHOME