----- Original Message -----
From: "2" <2@vc.net>
To: "Michael J. Tubby B.Sc. (Hons) G8TIC" <mike.tubby@thorcom.co.uk>; "
AMPS" <amps@contesting.com>
Sent: Sunday, August 11, 2002 3:43 PM
Subject: Re: [Amps] Commander-II 144MHz amp PSU woes (and Tempo 2002A)
>
>
> >
> >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.
>
Yes, I know that hence why I said "suspected" - one thing it isn't
is short to ground - checked that... but it does blow a 10A fuse in
about 1-2 seconds :-(
> >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.
And they are...?
> >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.
Exactly what I thought, hence why I turned it off...
>
> >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.
Well, its an old amp. Its had a hard time, bashed about in contests, stored
in a garage and/or truck - probably got damp in it, specially with "british
weather" in contests - we have had events with significant "fog" inside the
operating tent before when sat 2200' up in the wilds...
Guess its taken it toll...
> >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
> well.
Cap stack off one of the 1990 ARRL 4CX1000A 144MHz amp I've
been building (slowy) for years... used "lashed up" on the Tempo the 12
caps will have a bit under 200V across each, hence the power dissipated
in each 22K 11W resistor will be 1.8W or thereabouts ... don't think it'll
get too hot :-)
> >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 ageing.
> >
> >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?
>
> ** Given their sordid history, changing out carbon-comps for MOFs would
> be my choice.
>
Agreed, I'm off to order some...
> cheerz, Mike
>
> - R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
> www.vcnet.com/measures.
> end
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