[Collins] KWM-2

[Gerald]

On Wed, 2005-08-03 at 21:20 -0400, W4BYG wrote:
> I have obtained a very clean KWM-2 which was not working, but very much 
> worth the effort to repair.  I've never had one before, so I'm just 
> learning about it.
> 
> I first found bad tubes as follows, 3 - 6U8s, 3 - 6AZ8s and 2 - 
> 6BN8s. Several had open filaments, one was shorted.  I've never seen a 
> radio with this many bad tubes, so I suspect a previous owner did 
> something wrong in powering it.

Shorts aren't so likely to come from heater circuit abuse, though
running a heater way up in temperature from over voltage could break
down heater cathode insulation to show up as short in testing. The
standard Collins Power supplies do the proper interconnection of the
four 6.3 volt heater circuits in the radio to run them from 6, 12, or 24
volts. However applying 12 volts (like from a Heathkit power supply)
with the tubes wired for 6 would be hard on heaters.

That is not to say that heaters going open can't happen, its not all
that rare, though normally multiple tubes should have wildly different
failure times.

Tubes overheated can have lost a lot of cathode material and so not work
very well either.
> 
> After applying a siggen, I now have a very weak signal going thru it, 
> but the sensitivity is way down.
> 
> I have a manual for it.  The schematic seems to indicate several jumpers 
> should be in place when operating without any accessories and yet the 
> hookup diagrams and installation instructions show or say nothing, about 
> any jumpers.  It does not have a NB.  The 100kHz calibrator also does 
> not work.

Unless the options of noise blanker, 62S1, or switched receiver muting
(as if used with a separate receiver or transmitter) are used normally
those jacks are jumped together with a piece of wire inside the chassis.
Sometimes I've seen it as a piece of solid bare wire. Instruction with
the optional accessories tell to cut that wire between the jacks.
> 
> When bench testing the rig with only the 516 AC power supply only, are 
> any jumpers necessary to make it functional?
> 
> Anyone else ever experienced a similar multi-tube failure in a KWM-2 or 
> have any helpful suggestions?
> 
> You attention is appreciated.
> Ray, W4BYG
> Naples, FL

Another scenario is that the last owner had multiple radios and picked
the working tubes from this one filling the holes with tubes from other
radios. Or assorted tubes from a flea market, low priced and untested.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer