[Amps] Commander-II 144MHz amp PSU woes (and Tempo 2002A)

[Michael J. Tubby B.Sc. (Hons) G8TIC]

I have a Command Technologies Commander-II 3CX800 amp
on 2m, well that is until last night operating WSJT in Perseids!

Was running the amp at only about 350W o/p - derated for the
50% duty cycle (30sec TX, 30sec RX), and in the small hours
of the morning it went bang - got up today and investigated and
it looks terminal for the HT transformer - has a DC resistance of
about 4 ohms but I suspect has a shorted turn - blows 10A fuse
even with the rectifiers disconnected :-(

So, what to do to replace it:

a) get an identical transformer from Command Technologies and
leave the PSU configuration the same (voltage doubler)

b) get a replacement transformer made in the UK and leave the
PSU configuration the same

c) get a replacement transformer make in the UK but change the
PSU configuration to more conventional fullwave bridge

I have never been a fan of voltage doubler power supply designs -
okay they save a few diodes, but they charge one half of the caps
on each half-cycle of the mains, and if memory serves rely on a
low impedance transformer design, and the output voltage under
load is more susceptible to quality of the mains supply...

Comments/suggestions welcome...


I switched over to my friend's Henry Tempo 2002A (which hadn't
been used in over 2 years) and it would only produce about 500W
out max. I was concerned by a slight ticking and hissing noise
coming from the 8 x 180uF 450V caps in the EHT supply, so I
shut it off and had a measure around.

The 8 caps each have a 470K 2W carbon resistor across them
for voltage equalisation - found that they varied in resistance from
about 900K to 1.8M and one was open :-(

So I've lashed up and external cap stack off a bigger amp project
(12 x 470uF 450VDC SMPSU caps with 22K 11W ceramic R in
parallel) and am back on the air...

My question is, do the carbon composition resistors used in US
made amps tend to go high resistance with age? I had a similar
thing on a friends 2006, and the HT meter multiplier resistors
(chain of 1M 2W carbons) in the Commander-II also went high R.

In homebrew amps I use much larger (physical and power) and
lower values for voltage equalisation (22K 11W or 33K 7W, etc.)
either ceramic or vitreous enamel types. Ok, so I dissipate 40-50W
of the EHT supply as heat in the resistors but the voltages are
kept close on the caps and the whole stack discharges pretty
quickly when turned off...

For meter multipliers I tend to use strings of the Philips high
voltage resistors (VR37) which have given no trouble with aging.

Do we need to be checking our amps regularly if they have the
carbon composition resistors that Henry and Command Technologies
tend to use - or should we just change them out for something
better?



Mike G8TIC/M0VHF




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