[Collins] 32S3 loading control

[Dr. Gerald N. Johnson]

On Wed, 2008-07-23 at 20:30 -0700, Robert Jefferis wrote:
> Smith,
> 
> Adjustment of C80 is problematic, and is not likely to yield acceptable
> results without compromising the load setting on the other segments that C80
> influences.

That's guaranteed. That compromising the load settings on the other
bands. These loader trimmers were set so a 50 ohm load on the ham bands
loads properly with the load control knob pointing at the 50 ohm mark on
its dial.

> The root of the problem is S5 and the taps on L15. While the
> exciter tuning is capable of tuning any frequency in the specified range of
> the 32S-3, the emission tuning and load lever are not, because of the
> limited L15 taps. This problem also shows up on 17M. On 17M you can
> sacrifice the 21.0 band and add a tap for 17M between 1 and 2 turns lower on
> L15. I have never been interested in 30M, but the schematic shows that the
> unused 3C segment of S5 may allow you to add a tap to L15, again between 1
> and 2 turns lower than the 14 MHz tap, and get good results. This mod will
> not be for the feint of heart. It is risky to S5, and you obviously have to
> remove the PA cage. If you pursue a "proper" fix, be sure to use the same
> size buss wire, and be sure to flatten the end soldered to the coil (L15)
> just as the factory did to avoid a melt-down of the joint. There is a lot of
> RF current at that point.

All of that works. When the loading capacitor ends up at full
capacitance, the transmitter would respond to a T connector on the RF
output with a shunt fixed capacitor. Go for a low drift ceramic rated
for several KV of voltage (to get power dissipating volume), 100 or 200
pf. That's simpler than attacking the radio and its setting.

When going to 30 or 17 from a higher band, you have a condition of being
low on inductance, so it takes more capacitance for both tuning and
loading (so if your tuner for that band reflected 100 ohms instead of 50
the tuning capacitor on the 32S would probably be in a better position)
which leads to a higher loaded Q and more circulating current in the
tank circuit.

The flattening of the switch wire at the coil also prevent turn to turn
shorts or arcs.
> 
> Several outboard kluge fixes have been published. One of these may be OK.
> 
> V/R
> Bob KF6BC
> 
> -----Original Message-----
> From: collins-bounces at mailman.qth.net
> [mailto:collins-bounces at mailman.qth.net] On Behalf Of w9hak
> Sent: Wednesday, July 23, 2008 3:14 PM
> To: Gerald Johnson; CRA Reflector
> Subject: [Collins] 32S3 loading control
> 
> My 32S3 loading control capacitor is fully meshed when loaded up on 30 
> meters. In looking at the schematic it seems that increasing the 
> capacity of C-80, 100-500 pf, would help move the loading capacitor from 
> the fully meshed position. I have been living with this issue for some 
> time. I have to reduce the drive to maintain proper plate current.
> 
> C-80 isn't easy to reach and I suspect it was set at Collins during 
> construction. Haven't found any information on setting up the loading 
> assembly on this transmitter. Therefore I have been hesitant in tuning 
> these capacitors. Any insight in the area would be greatly appreciated.

I'm sure its set to make the load capacitor show 50 ohms when tuned for
maximum output (and proper loading) on that band.
> 
> Smith Bradford
> W9HAK
> 
> 
> 

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