[Collins] FW: KWM-2 driver bias mystery & need K3 - Thanks

[Tim Nichols]

To all who replied with assistance:

Thank you very much. A special thanks to WN4I who very very patiently helped
me trace and ultimately locate the problem. As it turns out, my initial
assessment that the unit had not been tampered with was in error. Someone
had deliberately lifted the B- end of L11 from the circuit and had run it to
ground. This preserved the operating bias on the V8 grid in transmit mode,
but removed the cutoff bias in receive mode.

I can find no Collins documentation whatsoever that this is a factory
update. Indeed, all later models have the cutoff circuit. My original
thinking was that perhaps very early versions were built this way with the
cutoff added later, but now I don't think so. The -70 volt line is right
there where it should be and L11 has obviously been moved.

I am 99.9% certain that, before the sun sets today, I will have returned the
bias circuit to its factory configuration.

Can anyone suggest any reason why I should not and why this may have been
done in the first place? In my distant memory, I remember learning something
about the advantages/disadvantages of keying multiple transmit stages, but
that was sooooo long ago...

Thanks again.

Tim
N2TC

P.S. I'M STILL LOOKING FOR A K3.



-----Original Message-----
From: Tim Nichols [mailto:parkerfly3@comcast.net]
Sent: Sunday, May 05, 2002 12:16 AM
To: collins@mailman.qth.net
Subject: KWM-2 driver bias mystery & need K3


Hi.

I just became a first-time Collins owner (after 36 years of hamming!) with
an E-bay KWM-2 purchase. Having some minor problems getting it on the air
(bad C47 and an open coil on K3 -- not bad at all, really), but the most
curious situation is that my unit (SN 411 - early!) is completely missing
the V8 cutoff bias circuit. It was not removed; It apparently was never
there. The -70v (receive mode only) line is at the appropriate terminal
post, but nothing is connected to it (nothing has been unsoldered, as the
factory red paint on the joint is completely intact). There is no L11, no
C122 and no R103 and one of the terminals that this circuit should be
mounted on (per my 1978 version CCA-downloaded manual) is a "virgin" - it
has never had anything soldered to it at all.

My first question is whether I'm looking at a factory defect or was the V8
bias circuit added after my SN in one of those undocumented production
changes I've been reading about all day?

Second, is this one of those "leave it alone" situations or should I try to
restore the receive mode cutoff bias to the 6CL6? What, if any are the
disadvantages of not keying the driver?

Anyone have any explanations/ideas?

Oh, I also need a K3 replacement and really don't want to pay the Surplus
Electronics price. Anyone have one or know where I can find one or a
suitable substitute?

Thanks to all in advance.


Tim
N2TC