Mike asked:
"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."
Yes. Carbon resistors will often go high - even sitting unused. They also
can change when operated near the rated wattage. Years ago I took a 2 watt
resistor and dissipated two watts in it in open air at room temp. I
wondered if I could "condition" it so that the resistance would not change.
Amazingly, the resistance kept changing after each heat cycle. I drew a
graph which looked like a spiral. One of the problems may be that they
absorb moisture.
Best solution is to change to some other type resistor that is stable.
Make sure that the substitute has a proper voltage rating.
73, Colin K7FM
> [Original Message]
> From: Michael J. Tubby B.Sc. \(Hons\) G8TIC <mike.tubby@thorcom.co.uk>
> To: <amps@contesting.com>
To: <amps@contesting.com>
> Date: 8/11/02 5:41:37 AM
> Subject: [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 :-(
>
> 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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--- COLIN LAMB
--- k7fm@teleport.com
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