[ICOM] IC 756 Pro III fuse over-heating?
Jim Hargrave
w5ifp at gvtc.com
Tue Dec 16 13:03:03 EST 2008
Martin,
Thanks for the clarification on the Collins recommendation. That was a long
time ago and I have had several "Senior moments" since then. (:->
* 73's Jim W5IFP *
>-----Original Message-----
>From: icom-bounces at mailman.qth.net
>[mailto:icom-bounces at mailman.qth.net]On Behalf Of Martin Sole
>Sent: Tuesday, December 16, 2008 3:54 PM
>To: ICOM Reflector
>Subject: Re: [ICOM] IC 756 Pro III fuse over-heating?
>
>
>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
>>> >>
>>> >>
>>> >----
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>>> >Icom Users Net: Sundays, 1700Z, 14.316 MHz
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>>> >
>>> >
>>> >----
>>> >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
>>> Icom Users Net: Sundays, 1700Z, 14.316 MHz
>>> Icom FAQ: http://www.qsl.net/icom/
>>
>> ----
>> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
>> Icom Users Net: Sundays, 1700Z, 14.316 MHz
>> Icom FAQ: http://www.qsl.net/icom/
>>
>----
>Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
>Icom Users Net: Sundays, 1700Z, 14.316 MHz
>Icom FAQ: http://www.qsl.net/icom/
>No virus found in this incoming message.
>Checked by AVG - http://www.avg.com
>Version: 8.0.176 / Virus Database: 270.9.18/1849 - Release
>Date: 12/16/2008 8:53 AM
>
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