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

[2]

>
>----- 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...?
>
FWB:  More efficient transformer possible, less ripple due to ripple 
cancellation feature. Higher V easier to obtain.  Switchable to FWB for 
RTTY/FM/AŘ with spst switch [half-V].
>
>> >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...

Weather seems an unlikely reason.
>
>> >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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-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
end