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

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Subject: [Amps] Commander-II 144MHz amp PSU woes (and Tempo 2002A)
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Date: Sun, 11 Aug 2002 07:43:15 -0700
>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 :-(
**  Shorted turns do not show up on an ohm-meter.

>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
**  A FWB has advantages and disadvantages over a FWD.

>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...
**  not the case. However, a FWD requires a transformer with minimal 
Cu-losses and it needs twice the filter-C.  The pluses are low-Ripple and 
a transformer design that requires less paper insulation, which can 
thusly have more Cu. 

>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.
**  Hissing =s new filter caps.  

>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 :-(
**  Wow.  A new world record. 

>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...

**  22k-ohms makes lots of heat - which electrolytics do not tolerate 
>My question is, do the carbon composition resistors used in US
>made amps tend to go high resistance with age? 

**  yes.  However, typical change is less than 470k going to 1,8M. 

>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

**  Given their sordid history, changing out carbon-comps for MOFs  would 
be my choice. 
cheerz, Mike

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,  

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