Hello....
I've just completed converting a Heathkit HL2200 amp, stripping all the tank
out as well as the input ckt. I left the existing plate tuning cap in and am
using it as the load cap. The plate tuning cap, now where the load cap was, is
a 100pf variable. This was all down according to an April '08 article on the
same. The input is a T network and it by the way works perfectly with a 1:1
SWR.
The suppressors are as the article says - nichrome ribbon, 1/4" wide formed
into a U with 3 2W carbon resistors in parallel at its base. All soldered using
silver solder for the nichrome. All seemed to go fine.
After installing the tank components I used a dip meter to try and see if the
tank coil was set right. I first confirmed the meter setting by checking it on
a radio set to 50.1Mhz... and then tried to insert its coil into the limited
area of the tank coil... it measured 60Mhz! So I squeezed the turns together -
it dropped to 54Mhz... I decided to leave it as is and wait until I could power
it up.
When I first tried to run it I was only able to get about 60W out with about
20W drive and the plate tune cap was at its minimum setting... I believe this
indicates that indeed the coil is too much inductance (does it not?) and so I
pulled the turns apart as best I could. This allowed me to get about 350W out
(with about 840W in) with maybe 35W drive (I didn't measure this by itself -
just going by where I had the power set on the 746Pro). But very quickly,
before I could increase the input, it started to smell! The carbon resistors
were getting VERY hot.
I quickly removed the cover (obviously made sure the HV was 0) and with a
thermal gun checked the temperature of the suppression resistors - one set was
at about 160 deg and the other set about 125 degrees. One resistor in the one
set appears to be higher than the others - but this might just be the
difficulty in checking with a thermal gun.
So my question is: WHY are these getting hot?! I would have thought, if the
article said use these resistors, that it would work fine. The U nichrome wire
is almost identical to the one they have pictured (mechanical picture for
scale) in the article. Note I couldn't find the 150 ohm resistors they said - I
used 3 180 ohm resistors - but I find it impossible to believe that it would
work with 50 ohms eqv and not 60 ohms eqv nor that the 60 ohm eqv should
dissipate MORE power, not less.
I'm also a bit troubled by the efficiency... but until I get this resolved I'm
not going to worry too much about this - however if someone things it is
related I'd love to hear it.
Thanks for your time.
Gary
K9RX
[the amp was bought from someone that said it had been converted to 6 meters
replacing the 10 position and the other bands still there... it didn't work on
6 which didn't make me happy so I decided to rip it all out and just make it a
6 only amp... note however it would put out about 1150W on 20 meters before the
change.]
ps: Also note that when I remove the cover (the HV short is disabled) I get
550W out with about 800W in but now the load cap is at its maximum value!
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