[Hallicrafters] SR-2000 Tank circuit

[Robert Nickels]

On 5/25/2018 10:00 PM, Jeff wrote:
>  I'm looking at a total rebuild I think.
Hi Jeff,

I'm sure Jim Liles who is the guru of these radios would agree with my 
statement that you are going to have to completely rebuild the PA 
compartment.    The damage and modifications you describe are more 
severe than what I had to deal with, and even after a careful rebuild 
mine still is not 100% right.  For example, even though I kept the 
original PA tank coil and made new spacers for it, the tuning control 
positions do not correspond with the settings in the manual, and even 
though all voltages are correct, I get about the same power output in 
high power as in low (which has be currently stumped).

In my case, the jackass who put CB crystals in place of the 10 meters 
ones had trouble with the bandswitch and rather than disassemble it 
properly he pried the PA subchassis out and bent it, and committed 
numerous other assaults on the radio.   It's worth the effort to do it 
right, but read and study K9AXN's restoration notes and tips first, and 
follow his advice:  http://k9axn.com/

Since the original plastic coil insulators were broken, my solution to 
restore the PA tank coil with minimal disruption was to cut grooves in 
the edge of two pieces of 1/8" thick Garolite (aka phenolic or bakelite) 
that could be inserted into the coil and spread apart to hold the coil 
with proper spacing.   I later wove pieces of "spaghetti" sleeving 
between coil turns to improve spacing.  The garolite assembly supports 
the coil in the proper position and maintains correct spacing, but since 
the knob positions are not the same I assume the coil characteristics 
have changed slightly.    The diameter of the coil and stiffness of the 
wire make this a difficult aspect of the restoration.

It is useful to use an antenna bridge, VNA, or antenna analyzer 
connected to the RF output with a 1500 ohm resistor to ground simulating 
the tube plate impedance to verify that the pi-net will match a 50 ohm 
load on each band (note that you'll have to either bypass the T/R relay 
or force the contacts closed for this power-off measurement).

You don't want to have to do this twice, so I would pay careful 
attention to the spacing of your bandswitch contacts vs. Jim's 
illustrations even if they look OK now, as this is a known failure item.

73, Bob W9RAN