In the HF-2500 the ONLY failures I have had were on 40m and 80m contest
RTTY. On these amps half or three quarters of the turns are shorted on the
toroid on 80m and 40m respectively. To make it worse they attached the
toroid to the front of the pressurized compartment with 5 little holes
bleeding some air out through the insulator and into the middle of the core.
Unfortunately the center of the core is mostly filled with a tecaform rod
and then its capped with another piece so the air path is rather tight:
http://wiki.k1ttt.net/2008%20Maintenance%20and%20Upgrade%20Blog.ashx#hf2500m
ods
This is made worse on the newer ones like in the link above because of the
bigger wire that he changed to. The older toroids that have been ok so far
had a smaller wire that left more room for air to flow inside. One of the
worst ones actually damaged the core material so when I tried to reuse the
core it broke (scroll down to 10/30 on the above page). When you take one
of the melted toroids apart you will notice that the wrapping under the wire
is discolored where the 40m and 80m turns are, this is probably due to the
higher currents in those concentrated over only part of the core, and its at
the top so all the heat rises to there anyway making it that much worse.
If you scroll down to 11/12 you will see the final fix for making it work
well on 80m. The tuning with the improved coil is much better and drive
levels are lower and it is cooler than the original... but of course it only
works on 80m now.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: Carl [mailto:km1h@jeremy.mv.com]
Sent: Sunday, December 23, 2012 15:18
To: Larry Benko; amps@contesting.com
Subject: Re: [Amps] hf-2500
I have a 2500 on the bench right now and there is no sign of overheating.
The owner uses it strictly on 75 SSB which might explain it but the amp
easily puts out 2200W +.
Perhaps different gauge wire was used during production, this one has blue
#12 stranded silver plated Teflon and is a 1996 build. Ive worked on several
others and dont remember wire size but the insulation was black. The skinny
braid used to go to the switch probably doesnt help either. Those were all
earlier models and the toroids were fine from even contest use.
IMO it is a fine CW/SSB amp and like many others should be derated for RTTY.
The transformer isnt that robust anyway.
Carl
KM1H
----- Original Message -----
From: "Larry Benko" <xxw0qe@comcast.net>
To: <amps@contesting.com>
Sent: Saturday, December 22, 2012 11:25 AM
Subject: Re: [Amps] hf-2500
>I used to think that shorted turns on powdered iron toroids was bad until I
>did some tests. My tests were not exhaustive but winding some #12 wire on
>a T200-2 core and shorting out some of the turns to reduce the inductance
>to ~1/2 of the inductance with the turns not shorted showed a moderate
>reduction in unloaded Q and a slight increase in temperature rise but
>nothing hugely significant.
>
> I concluded that if shorting turns on an air core (u=1) was fine and
> shorting turns on a 60Hz transformer (u=10000 or more) was really bad then
> shorting turns on a powdered iron core (u=6 to 10) was much closer to the
> air core case than to the u=10000 case.
>
> Of more importance is that toroidal cores are not easy to cool since they
> usually get connected to the chassis with an insulating washer beneath
> them and air flow not that good. Perhaps this leads to some of the
> reported failures.
>
> 73,
> Larry, W0QE
>
>
> On 12/19/2012 6:16 PM, Jim W7RY wrote:
>> Bill
>>
>> We already had this discussion on here about a year ago. But I 'm going
>> to repeat it.
>>
>> Alpha, Ameritron, Jerry K8RA, myself W7RY, and many others all use
>> toroids with shorted turns on our "L" coil in a PI-L circuit. It works
>> just fine.
>>
>> Do you mean that a shorted turn toroid for the PI network is not good?
>> Or are you making a general statement that they don't work at all in an
>> amplifier?
>>
>>
>> 73
>> Jim W7RY
>>
>>
>> -----Original Message----- From: Bill Turner
>> Sent: Wednesday, December 19, 2012 12:26 PM
>> To: Amps
>> Subject: Re: [Amps] hf-2500
>>
>> ORIGINAL MESSAGE:
>> On Wed, 19 Dec 2012 17:15:03 +0000, K1TTT wrote:
>>
>>> Agreed, that is why I have modified one to do only 40-10m... removing
>>> the
>>> 80m and 160m coils from the toroid removes the shorted turns problem and
>>> makes it run very nicely doing rtty on 40m.
>>
>> REPLY:
>> As David found out, shorted turns on a toroid or any iron core coil
>> are a no-no unless you do some very careful design and testing, which
>> Command obviously did not do. Shorted turns on an air core coil are
>> usually no problem. A word to the wise.
>>
>> 73, Bill W6WRT
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