[Collins] kwm-2 fan and outputs

Dr. Gerald N. Johnson geraldj at weather.net
Sun Oct 10 21:30:28 EDT 2010


I don't know of the fan kit.

6146B are decent tubes, just they have more capacitance than 6146 and 
6146A so the neutralization circuit has to be changed. I would have 
thought there would be a service bulletin or service information letter 
about that, but I can't find any. Indeed the revision 7 Rockwell KWM-2 
manual that lists many of the revisions doesn't admit the PA 
neutralization trimmer was ever changed. IT WAS!

6146B are rated at more output than 6146 or 6146A. I'm of the opinion 
that the Collins output tank (especially the load padders) can't carry 
the extra current when tuned to load the 6146B to full ratings and will 
have short lives. They are very hard to get to for replacement and not a 
common everyday computer part so are hard to purchase these days. For 
that reason, I suggest running the 6146B or W at 6146 ratings which will 
put the extra capability into a much greater tube longevity.

A PA hard to tune can be a sign of poor neutralization or of high screen 
voltage. While the radios are rated at 285 volts for the low B+, some 
516F-2, especially with 125 volts applied to the primary rated at 115 
volts will run more like 325 which makes the off resonance PA plate 
current peg the meter. It would be better to run lowered low B+ by using 
a tube rectifier instead of a solid state rectifier and by lowering the 
line voltage with a bucking transformer or a variac. This will also get 
the heater voltage back down to rated and improve the longevity of all 
the tubes.

I suppose it would be a good project for the winter for me to produce a 
service bulletin on converting the PA neutralization circuit in 32S and 
KWM-2 transmitters for 6146B. I see in the Rev 7 parts list that the air 
trimmer range is described as 1.7 to 50 while the schematic shows 1.7 to 
8.4 which is the true range for the E.F. Johnson trimmer specified. It 
probably takes a mounting insulator and a bit of sheet metal work to 
make room for it. I might even have some of that particular capacitor 
squirreled away, if I can find those things during or after moving.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association

On 10/10/2010 2:52 PM, yash at aol.com wrote:
>
> Hi I decided to get another KWM-2 after a few years absence from
> Collins gear.When I had my KWM-2a, I bought a fan kit ,2 small fans
> that fit with no rework of the rig. I looked now and can't find a
> darn thing about the kit. Still being made? or a contact, about them?
> Also have a question about outputs. The new rig will need new
> outputs...low out and hard to tune. The rig came with a brand new
> pair of RCA 6146B/8293A ,boxed tubes,but something in the back of my
> mind says these are not a good version of the 6146 to use in the
> KWM-2. Right or wrong? if not what is the perfered version for the
> rig? thanks for any help on this. 73's dale wt4t
>
>
>


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