[ICOM] IC 756 Pro III fuse over-heating?
Martin Sole
msole at loxinfo.co.th
Tue Dec 16 10:54:13 EST 2008
I will admit to not having read back very far on this thread but think I
can offer a comment here.
The 20.5 or 23 foot length prescribed by Collins back in the day was an
attempt to ensure that any reflected power from the amplifier,
irrespective of the band in use, would be essentially in phase with the
internal feedback operation of the very carefully designed Collins
transmitter power amplifier and not add any objectionable distortion to
the legendary Collins audio. Tales tell that one of two things happened,
either a careful analysis at a later date revealed that there was not a
problem and that any reflected energy from the amplifier back to the
assumed Collins transmitter could not and did not cause a problem or
alternatively that minor changes and service bulletins resolved the need
for this cable. In either case it is now generally accepted that
thespecific length requirement can be ignored in all cases. Furthermore
it only ever applied when interconnecting a Collins amplifier to either
an S line transmitter or KWM2 series transceiver, 20.5 feet, or a KWM-1
transceiver, 23 feet. Use of a transmitter other than those would render
the length irrelevant since it would be unlikely to find the same design
of power amplifier employed as used by Collins.
The theory says that it should in effect be irrelevant what the cable
length is. The transmitter output is assumed to be 50 ohms and
essentially resistive. The amplifier input is likewise assumed to be 50
ohms and essentially resistive, I'm discounting some earlier cheap tube
amps with untuned inputs, if the cable used is of decent quality and
nominally 50 ohms then any mismatch at the radio resulting from a less
than apparently resistive load at the amplifier should be minimal. There
will be occasions when a grounded grid tube amplifier reflects back a
significant amount of energy but it should be severely off resonance to
do so. Once tuned it should not. It is possible that on 10 metres with a
certain length of cable and a particular amplifier mismatch that you
might see odd things at the radio but they should be consistent and not
time variable. If they are I would look elsewhere.
In the case of the Alpha 91B mentioned I'm not sure but I would consider
several areas. Check the cable connectors carefully for hot spots or
weak connections. If PL259/SO239 connections look carefully at the
sockets. Do the plug centre pins appear to make firm contact with the
socket? Replace the amp with a dummy load and see if you can see a
similar thing happening. If the cable is good you will not. Beware of
hot switching in the amplifier. This is when the amp change over
contacts happen fractionally after power is applied causing the radio to
see a brief high SWR during which time the rig may do much more than
instantaneously reduce power. If the ALC control loop is not
sufficiently fast or properly damped you might see power actually
increase then decrease and then increase again, all in the 20 to 30
milliseconds it takes the amp to possible change over. If the problem
persists try dropping the power to a very low level then keying the
radio and amp and then increase the power slowly to see if the trip
occurs or not. If there is zero power out when you start and it does not
trip by the time you reach normal drive level then hot switching is a
definite candidate. You may need to examine the transmitter output on a
scope to understand what it is doing at the moment of keying up. What
does the power rise time profile look like?
A bad cable is a bad cable, its length should not be a factor. This is
probably still the most likely problem but there are others to consider
as well.
Martin, HS0ZED
Dan Cotsirilos K9DTC wrote:
> The Kenwood TL-992A provides a jumper with their amp and it measures
> 57 1/2 inches.
>
>
> Dan K9DTC
>
>
>
> ----- Original Message ----- From: "Jim Hargrave" <w5ifp at gvtc.com>
> To: "ICOM Reflector" <icom at mailman.qth.net>
> Sent: Tuesday, December 16, 2008 8:40 AM
> Subject: RE: [ICOM] IC 756 Pro III fuse over-heating?
>
>
>> There is some merit to the length of the coax.
>>
>> Years ago I had a Collins KWM-1 and there was a note that recommended
>> a 21
>> ft length of coax between the KWM-1 and the 30L-1 linear. I don't
>> know the
>> theory on how they arrived at the 21ft. But it does make some sense.
>> I just
>> coil the excess up behind the desk and it makes a good RF choke
>> between my
>> FT-920/IC-735 and the Clipperton-L.amp.
>>
>> * 73's Jim W5IFP *
>>
>>
>>
>>
>> >-----Original Message-----
>> >From: icom-bounces at mailman.qth.net
>> >[mailto:icom-bounces at mailman.qth.net]On Behalf Of Julio Peralta
>> >Sent: Tuesday, December 16, 2008 1:02 PM
>> >To: 'ICOM Reflector'
>> >Subject: RE: [ICOM] IC 756 Pro III fuse over-heating?
>> >
>> >
>> >Try changing the length of the coax between the rig and the input
>> to the
>> >amp. Sometimes the length of that cable will cause the rig to
>> >reduce power
>> >out until it determines the match is OK. I've experienced this
>> myself.
>> >
>> >Julio, W4HY
>> >
>> >-----Original Message-----
>> >From: icom-bounces at mailman.qth.net
>> >[mailto:icom-bounces at mailman.qth.net] On
>> >Behalf Of Phil Krichbaum
>> >Sent: Tuesday, December 16, 2008 5:49 AM
>> >To: ICOM Reflector
>> >Subject: Re: [ICOM] IC 756 Pro III fuse over-heating?
>> >
>> > On Saturday night I had been running along working a bit on
>> 10m CW
>> >and then got a phone call. When I came back on the air some 15
>> minutes
>> >later the Alpha 91B grid safety feature would trip after about two
>> >seconds. I'd not had this problem previously in this contest
>> >during some
>> >good runs or ever before. I have two antennas and one is about
>> >1.3:1 and
>> >the other 1:1 and it didn't mater which antenna I used. I was running
>> >the 756PROIII at less than 50w output. A little further checking
>> and I
>> >noticed when I first started to transmit I was getting less than QRP
>> >output for 2-3 seconds before going to the set power level. This in
>> >itself is a very undesirable feature for contesting not to mention
>> not
>> >having the amp available as it would trip as the power came up! Just
>> >for the heck of it I tried loading on 20m and everything seemed OK. I
>> >went back to 10m and same problem. I bypassed the Alpha and same
>> >problem. I tried a different amp, same problem! I removed the
>> >PROIII and
>> >no problem with another rig and finished the rest of the
>> >contest with it.
>> > I'm using the Icom PS125 that came with my PROII "free" as a
>> >closeout sales promotion (I certainly would not pay that much
>> >for a 25 A
>> >switcher). The PROII got taken out of service permanently with a
>> >lightening surge that apparently came in the neutral side of the
>> power
>> >line. My PROII was off but this PS125 was left on (I've changed that
>> >practice with the PROIII). I assume the PS125 means it is rated at 25
>> >amps and there is a short power cord of about 12 inches with no fuses
>> >and the 6 connector Jones type plug on the end. I suspect if there
>> are
>> >fuses, they are inside the PS125. I need to do some further
>> checking of
>> >the problem as to what bands are affected and at what power levels. A
>> >friend had a similar problem on 6m last summer and it eventually went
>> >way by itself. It seems I should try a different PS if the problem
>> >persists and also take a look at the fuses in the PS125. Perhaps the
>> >PS125 was damaged somehow when the surge came thru it. Damage
>> >to the rig
>> >also came in the rig control cable and CW keying jack from my desktop
>> >that was connected but off. The HD in the desktop was toast. I
>> >have also
>> >picked up some 3 wire 250V 30A knife switches they use in Costa
>> Rica at
>> >the main house feed to shut off power to the house and will
>> >install then
>> >on my 120 and 240 lines into the shack prior to spring so there
>> will be
>> >no connection to the house wiring when I leave the shack assuming I
>> >remember to throw the switch! I've also had my power provider
>> install a
>> >"Meter Treater" at my meter which has some big MOVs in it.
>> > Anyways it seems like I need to check the PROIII with a
>> >different PS
>> >if this problem doesn't fix itself!
>> > 73 Phil N0KE
>> >
>> >Jim Reisert AD1C wrote:
>> >> I've had problems the last couple of weeks with my IC756 Pro III
>> >> shutting down in the middle of transmitting. This weekend I
>> traced it
>> >> to the fuses (which were getting warm to the touch). I notice
>> that
>> >> my SS-30 power supply reports ~ 20 amps being drawn on transmit,
>> and
>> >> the fuses are rated at 20 amps.
>> >>
>> >> ** Is 20 amp draw during full-power transmit normal?
>> >>
>> >> ** Are the fuses supposed to be 20A?
>> >>
>> >> ** Should I replace with 25A fuses, or 20A slow-blow, or is
>> there a
>> >> bigger problem here?
>> >>
>> >> Thanks & 73 - Jim AD1C
>> >>
>> >>
>> >>
>>
>> >------------------------------------------------------------------------
>>
>> >>
>> >>
>> >> No virus found in this incoming message.
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>> >12/15/2008 5:04 PM
>> >>
>> >>
>> >----
>> >Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
>> >Icom Users Net: Sundays, 1700Z, 14.316 MHz
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>> >
>> >
>> >----
>> >Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
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>> ----
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>
> ----
> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
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